improved crud heat transfer model for dryout on …

The Pennsylvania State University

The Graduate School

Department of Mechanical and Nuclear Engineering

IMPROVED CRUD HEAT TRANSFER MODEL FOR DRYOUT ON FUEL PIN

SURFACES AT PWR OPERATING CONDITIONS

A Dissertation in

Nuclear Engineering

by

Guoqiang Wang

© 2009 Guoqiang Wang

Submitted in Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

May 2009

ii

*Signatures are on file in the Graduate School

The dissertation of Guoqiang Wang was reviewed and approved* by the following:

Seungjin Kim Assistant Professor of Mechanical and Nuclear Engineering Dissertation Co-Advisor Co-Chair of Committee

Fan-Bill Cheung Professor of Mechanical and Nuclear Engineering Dissertation Co-Advisor Co-Chair of Committee

Arthur Motta Professor of Nuclear Engineering and Materials Science and Engineering

Kostadin N. Ivanov Distinguished Professor of Nuclear Engineering

Digby D. Macdonald Distinguished Professor of Materials Science and Engineering

Michael Y. Young, Special Member Chief Engineer of Westinghouse Electric Company LLC

William A. Byers, Special Member Fellow Engineer of Westinghouse Electric Company LLC

Zeses Karoutas, Special Member Manager of FPMDT of Westinghouse Electric Company LLC

Robert L. Oelrich, Jr., Special Member Manager of FRTHD of Westinghouse Electric Company LLC

Jack S. Brenizer, Jr. J. “Lee” Everett Professor of Mechanical and Nuclear Engineering Chair of Nuclear Engineering

Lawrence E. Hochreiter (deceased) Professor of Mechanical and Nuclear Engineering Special Signatory

iii

ABSTRACT

Researchers have performed many studies to understand crud formation on fuel rod

cladding surfaces, which have been observed in Pressurized Water Reactors (PWRs) as a result of

sub-cooled nucleate boiling and oxidation/reduction reactions. Crud deposits with high

concentration of boron species will result in an axial offset anomaly (AOA), usually named Crud

Induced Power Shift (CIPS) due to the effect of the boron on the power shape distribution. If the

crud deposit is thick enough at high heat flux level, it may cause fuel rod surface dryout and

accelerate cladding corrosion. This study examines and measures the crud thermal conductivity at

PWR operating conditions, which is one of the most important parameters for simulating CIPS

and thick crud dryout phenomena.

To better understand crud formation and measure crud thermal conductivity on the fuel

rod cladding surfaces at pressurized water reactor operating conditions, a single rod crud

Thermal-Hydraulic test facility was built at the Westinghouse Science and Technology Center

(STC) in October 2005. Since then, a number of updates have been made to the test facility,

which was named the Westinghouse Advanced Loop Tester (WALT).

Recently, a four regime theory and methodology has been proposed for crud thermal

conductivity study and measurement. Meanwhile, a number of preliminary tests have been

performed in the WALT loop, which was utilized to successfully generate crud and measure its

thermal parameters as a function of crud thickness and fluid conditions. In the WALT test loop,

crud is deposited on the heater rod surface, then the crud characteristics are confirmed to be

similar to these observed in the PWRs and the crud from the WALT loop has been determined to

be representative of crud observed in PWRs. After the WALT loop crud thermal conductivity is

iv

calculated based on other direct measurements, it can be used to evaluate cladding surface

temperatures at various crud thicknesses and thermal-hydraulic conditions.

In this dissertation, the method for crud thermal conductivity measurement and some

preliminary test results from the WALT loop is presented and discussed. Furthermore, since the

WALT system is also being utilized by Westinghouse to perform crud dry-out testing, these test

results are also used to validate models used in the thermal-hydraulic codes and to support the

industry goal of zero fuel failures by 2010 and beyond, established by commercial nuclear

industry executives at an Institute of Nuclear Power Operations (INPO) meeting.

v

TABLE OF CONTENTS

LIST OF FIGURES ............................................................................................................ vii

LIST OF TABLES.............................................................................................................. xi

NOMENCLATURE ........................................................................................................... xii

ACKNOWLEDGEMENTS ................................................................................................ xviii

DEDICATION ................................................................................................................... xix

Chapter 1 INTRODUCTION ............................................................................................. 1

1.1 Effects of Sub-cooled Nucleate Boiling ................................................................. 3 1.1.1 Basic Mechanism of Sub-cooled Nucleate Boil ........................................... 3 1.1.2 Effects of Sub-cooled Nucleate Boiling on Crud Deposition and AOA ........ 4

1.2 Corrosion Products and Fuel Crud Formation ........................................................ 6 1.2.1 Corrosion Products From Non-Fuel Surfaces............................................... 6 1.2.2 Corrosion Products From Fuel Surfaces....................................................... 7 1.2.3 Corrosion Product Circulation ..................................................................... 8 1.2.4 Fuel Crud Formation and Its Characteristic.................................................. 14

1.2.4.1 The Characteristic of Crud Formation ............................................... 14 1.2.4.2 Crud Observations in PWR Cores ..................................................... 15

1.3 Electrochemistry Reactions, Corrosion, and Fuel Crud Formation.......................... 19 1.4 Boron Hideout in Crud Layer Inducing AOA......................................................... 21

Chapter 2 BACKGROUND AND LITERATURE REVIEW.............................................. 22

2.1 First Existing Porous Crud Model (Model-1) ......................................................... 22 2.2 Wick Boiling Heat Transfer Model (Model-2) ....................................................... 24 2.3 Crud Deposit Model (Model-3).............................................................................. 24 2.4 Uhle’s Model (Model-4) ........................................................................................ 27 2.5 Current State-of-Art Models to Address Crud Problems......................................... 28

2.5.1 BOA Model (Model-5)................................................................................ 28 2.5.2 Independent Review or Derivation of Current BOA Conservation

Equations ..................................................................................................... 30 2.5.2.1 Momentum Conservation Equations for Current BOA Theory........... 30 2.5.2.2 Energy Conservation Equations for Current BOA Theory ................. 38 2.5.2.3 Mass Conservation Equations for Current BOA Theory .................... 42 2.5.2.4 Boundary Conditions and Porous Crud Properties ............................. 42

Chapter 3 THEORETICAL MODEL AND METHODOLOGY ......................................... 44

3.1 The Improved Crud Heat Transfer Model .............................................................. 44 3.2 Four-Regime Theoretical Model and Crud Thermal Conductivity Measurement

Methodology ....................................................................................................... 45 3.2.1 Four-Regime Theoretical Model in Crud ..................................................... 46

3.2.1.1 Calculations and Identifications of Four-Regime in Crud .................. 48

vi

3.2.1.2 Simulations of Four-Regime in Crud at WALT Loop Operations ...... 49 3.2.1.3 Applications of Four-Regime in Crud at PWR Operations................. 51

3.2.2 Crud Thermal Conductivity Measurement Methodology.............................. 53 3.2.2.1 Formulations of the Heat Flux and Temperatures .............................. 53 3.2.2.1.1 Heat Flux Formulations in the Heater Rod...................................... 54 3.2.2.1.2 Formulations of Cladding Outer Surface Temperature.................... 55 3.2.2.1.3 Formulations of Crud Outer Surface Temperature .......................... 58 3.2.2.1.4 Formulations between Crud Outer Surface and Fluid

Temperatures........................................................................................ 58 3.2.2.2 Methods to Solve Crud Thermal Conductivity Equations .................. 59 3.2.2.2.1 Mathematic Solutions .................................................................... 60 3.2.2.2.2 Perturbation Method to Solve Crud Thermal Conductivity

Equations.............................................................................................. 61 3.2.2.2.3 Changing Geometry Method to Solve Crud Thermal

Conductivity Equations......................................................................... 65 3.2.2.2.4 Direct Solution Method Using Existing Correlations ...................... 66

3.3 Summary of Theoretical Model and Method .......................................................... 68

Chapter 4 WALT LOOP DESIGN/BUILDUP AND EXPERIMENTAL STUDY .............. 70

4.1 WALT Loop Design/Buildup and Update .............................................................. 70 4.1.1 The First Completed WALT Loop Design Description ................................ 70

4.1.1.1 Background Information ................................................................... 71 4.1.1.2 Description of the WALT Loop ........................................................ 71 4.1.1.3 The Heater Rod and the Test Section in WALT Loop........................ 76

4.1.2 WALT Loop Update ................................................................................... 82 4.2 Experimental Procedures ....................................................................................... 88

4.2.1 WALT Loop Operations.............................................................................. 88 4.2.2 Measurement of Heater Rod Deposit Thickness and Structure ..................... 91

4.2.2.1 Depth from Defocus (D.F.D.) Method............................................... 91 4.2.2.2 Scanning Electron Microscope (SEM) Method.................................. 92 4.2.2.2.1 Porosity and Thickness by Cross Sectioning................................... 92 4.2.2.2.2 Elemental Content.......................................................................... 94

4.2.3 XRD Exams................................................................................................ 96 4.2.4 Data Reductions .......................................................................................... 98

4.3 Selections of Test Cases ........................................................................................ 98

Chapter 5 EXPERIMENTAL RESULTS AND BENCHMARK......................................... 100

5.1 Determination of Crud-regimes.............................................................................. 100 5.2 Crud Thermal Conductivity Calculations for Different Crud-regimes ..................... 102

5.2.1 Selected Experimental Data or Results ........................................................ 103 5.2.1.1 SEM Results for Selected Cases........................................................ 103 5.2.1.2 XRD Results for Selected Cases ....................................................... 117 5.2.1.3 Cladding Electric Resistance Measurement Results........................... 125 5.2.1.4 Selected WALT Test Data Reduction................................................ 130 5.2.1.4.1 Data Reduction for Heater Rod #22B ............................................. 131

vii

5.2.1.4.2 Data Reduction for Heater Rod #30................................................ 133 5.2.1.4.3 Data Reductions for Heater Rods #34, #39, #43, and #44 ............... 135

5.2.2 Overall Crud Thermal Conductivity Calculations and Results...................... 140 5.2.2.1 Overall Crud Thermal Conductivity for Heater Rod #22B ................. 141 5.2.2.2 Overall Crud Thermal Conductivity for Heater Rod #30.................... 144 5.2.2.3 Overall Crud Thermal Conductivity for Heater Rods #34, #39, #43,

and #44................................................................................................. 144 5.2.3 Crud Thermal Conductivity Result Analyses ............................................... 148

5.2.3.1 Effect of Crud Porosity – Crud-regime II .......................................... 148 5.2.3.2 Effect of Chemistry Component – Crud-regime IV............................ 151 5.2.3.3 Effect of Crud Thickness – Crud-regime III ...................................... 152 5.2.3.4 Error Analysis................................................................................... 154 5.2.3.4.1 R-squared Calculation and Analysis ............................................... 154 5.2.3.4.2 Relative Error Analysis .................................................................. 155 5.2.3.5 Crud Thermal Conductivity Results .................................................. 158

5.3 Crud Structure Comparisons between Plant and WALT data.................................. 160 5.4 Crud Thermal Conductivity Sensitivity Study with BOA Computer Code .............. 171

5.4.1 VIPRE Modeling of the WALT Loop.......................................................... 171 5.4.2 BOA Modeling of the WALT Loop............................................................. 173 5.4.3 VIPRE/BOA Calculations and Comparisons with Experimental Results ...... 175 5.4.4 Sensitivity Study on Cladding Temperature Differences and Crud

Thermal Conductivity................................................................................... 178

Chapter 6 CONCLUSIONS AND RECOMMENDATIONS .............................................. 182

6.1 Conclusions........................................................................................................... 182 6.2 Recommendations and Comments for Future Work ............................................... 186

BIBLIOGRAPHY .............................................................................................................. 188

Appendix A Formulations of Heat Flux and Temperatures in a Hollow Cylinder ................ 196

A.1 Formulation Derivation of Cladding Outer Surface Temperature.......................... 196 A.2 Formulation Derivation of Crud Outer Surface Temperature ................................ 200

Appendix B EDTA ............................................................................................................ 203

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LIST OF FIGURES

Figure 1-1: Factors Contributing to Crud Formation............................................................ 2

Figure 1-2 Nickel (Ni)/Iron (Fe) Solubilities in PWR Primary Coolant at 650 ppm B10 [2]. . 10

Figure 1-3 Solubility of Magnetite versus Water Temperature and pH [3]. .......................... 11

Figure 1-4 Calculated Fe Solubility in PWR Primary Coolant at 650 ppm B10 [2]. ............... 12

Figure 1-5 High Magnification SEM of a Crud Flake Obtained from Plant C Cycle 8 [2]. ... 17

Figure 1-6 Cross Sectional View of a Plant A Cycle N Crud Flake...................................... 18

Figure 2-1 The Porous Crud Model by Macbeth (AEEW-R 711)......................................... 23

Figure 2-2 Typical Thick Axial Offset Anomaly (AOA) Crud in Cross Section. .................. 25

Figure 2-3 Diagram showing definitions and directions for Darcy's law.............................. 32

Figure 2-4 Forces on a Vapor Bubble or Vapor in a Pore..................................................... 36

Figure 3-1 Proposed Four-regime Theoretical Model in Crud Chimney. .............................. 47

Figure 3-2 A Sample Partial Boiling Curve from WALT Operation..................................... 50

Figure 3-3a WALT Loop Heater Rod Cross Section View .................................................. 56

Figure 3-3b WALT Loop Heater Rod Side View................................................................. 57

Figure 3-4 WALT Test Case with Condition A . ................................................................. 63

Figure 3-5 WALT Test Case with Condition A′ . ................................................................ 64

Figure 4-1 Picture of the Completed WALT........................................................................ 72

Figure 4-2 Layout of the WALT Loop. ............................................................................... 74

Figure 4-3 Heater Rod of the WALT Loop and Westinghouse PWR Fuel Assembly. .......... 77

Figure 4-4 Heater Rod Design of the WALT....................................................................... 81

Figure 4-5 Layout of the Updated WALT Loop .................................................................. 83

Figure 4-6 Chemical Addition-up System ........................................................................... 85

Figure 4-7 The Updated Heater Rod for WALT. ................................................................. 87

Figure 4-8 A SEM Machine of SUPRA 40 at Westinghouse................................................ 93

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Figure 4-9 A Sample SEM Image to Define Pore Area (green) for Heater Rod #22B........... 95

Figure 4-10 A SCINTAG XDS2000 X-ray diffraction system at Westinghouse................... 97

Figure 5-1 A Sample Boiling Curve for Determine Crud-regimes........................................ 101

Figure 5-2 A SEM Image for Heater Rod Test #22B (750x). ............................................... 105

Figure 5-3 EDS Results for Heater Rod Test #22B (750x)................................................... 106

Figure 5-4 A SEM Image for Heater Rod Test #30 (750x)................................................... 107

Figure 5-5 EDS Results for Heater Rod Test #30 (750x). .................................................... 108





Figure 5-10 A SEM Image for Heater Rod Test #43 (750x)................................................. 113

Figure 5-11 EDS Results for Heater Rod Test #43 (750x). .................................................. 114

Figure 5-12 A SEM Image for Heater Rod Test #44 (750x)................................................. 115

Figure 5-13 EDS Results for Heater Rod Test #44 (750x). .................................................. 116

Figure 5-14 XRD Pattern Obtained from Crud Formed on Heater Rod Test #22B. .............. 119

Figure 5-15 XRD Pattern Obtained from Crud Formed on Heater Rod Test #30. ................. 120





Figure 5-20 Resistance vs. Temperature (Rod OD of 0.95cm).. ........................................... 127

Figure 5-21: Electric Resistivity vs. Temperature for Zirlo Tubes........................................ 129

Figure 5-22 Boiling Curve for Test with Heater Rod #22B. ................................................. 142

Figure 5-23 Boiling Curve for Test with Heater Rod #30..................................................... 145

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Figure 5-24 Overall Crud Thermal Conductivity (Kd) vs. Porosity. ..................................... 149

Figure 5-25 Overall Crud Thermal Conductivity (Kd) vs. Crud Thickness........................... 153

Figure 5-26 Overall Crud Thermal Conductivity (with error) vs. Porosity............................ 156

Figure 5-27 Overall Crud Thermal Conductivity (with error) vs. Crud Thickness. ............... 157

Figure 5-28 Chimney Density in Rod # 10 Crud Flakes (WALT Data). ............................... 163

Figure 5-29 Boiling Chimney Diameter Distribution (WALT Data). ................................... 164

Figure 5-30 A Scanning Electron Microscope (SEM) Picture for WALT Rod# 40 (100X)... 165

Figure 5-31 A SEM Picture for WALT Rod# 40 (500X). .................................................... 166

Figure 5-32 A SEM Picture for WALT Rod# 40 (1000X). .................................................. 167

Figure 5-33 A SEM Picture for WALT Rod# 40 (2500X). .................................................. 168

Figure 5-34 Comparison of Crud Structures from Plant (left) and WALT Loop (right). ....... 169

Figure 5-35 VIPRE-01/BOA Calculation Overview Diagram. ............................................. 174

Figure 5-36 Cladding Delta_T Comparison. ........................................................................ 177

Figure 5-37 Calculated Cladding Delta_T and Dry Crud (%)............................................... 179

Figure 5-38 Cladding Delta_T vs Crud Thermal Conductivity Factor. ................................. 181

Figure A-1 A Hollow Cylinder Heater Rod Design. ............................................................ 197

Figure A-2 A Hollow Cylinder Heater Rod with Crud Layer. .............................................. 201

Figure B-1 A 3D chemistry model of the EDTA structure.................................................. 203

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LIST OF TABLES

Table 4-1 Key Parameters of the Westinghouse WALT Loop.............................................. 79

Table 4-2 Typical PWR Operating Conditions. ................................................................... 80

Table 4-3 Selected Cases of the WALT Crud Tests ............................................................. 99

Table 5-1 Electric Resistance Measurement Results for Heater Rod OD of 0.95cm. ............ 126

Table 5-2 Selected Data Points for Heater Rod #22B .......................................................... 132

Table 5-3 Selected Data Points for Heater Rod #30. ............................................................ 134

Table 5-4 Selected Data Points for Heater Rod #34 ............................................................. 136


Table 5-6 Selected Data Points for Heater Rod #43 ............................................................. 138


Table 5-8 Overall Crud Thermal Conductivity Calculations for Heater Rod #22B ............... 143

Table 5-9 Overall Crud Thermal Conductivity Calculations for Heater Rod #30.................. 146

Table 5-10 Overall Crud Thermal Conductivity Calculations for Heater Rod #34, 39, 43, and 44 ......................................................................................................................... 147

Table 5-11 Table 5-11 A Sample Calculation for Crud Thermal Conductivity ( crudk ). ........ 159

Table 5-12 Plant Boiling Chimney Analysis Data................................................................ 161

Table 5-13 WALT Heater Rod #40 Chimney Analysis Data................................................ 162

Table 5-14 Operating Conditions for Selected WALT Data................................................. 172

Table 5-15 Data from Selected WALT Tests....................................................................... 176

xii

NOMENCLATURE

A The cross-sectional area to flow ( 2m )

0A Cross-sectional area of the heater rod tube ( 2cm )

poresA The cross-sectional area for pores

lA φα ⋅−⋅= )1(AAl ( 2m )

vA φα ⋅⋅= AAv ( 2m )

ilC Concentration of species i in the liquid at location x ⎟⎠

⎞⎜⎝

⎛3m

mol

ivC Concentration of species i in the vapor at location x ⎟⎠

⎞⎜⎝

⎛3m

mol

iC Equilibrium concentration of species i

pfC Liquid saturated specific heats at constant pressure ( KkgJ ⋅/ )

pgC Vapor saturated specific heats at constant pressure ( KkgJ ⋅/ )

C Concentration (amount of substance/length3, i.e. 3m

mol)

eD Equivalent diameter (m)

ilD Diffusion coefficient for species i in the liquid ⎟⎟⎠

⎞⎜⎜⎝

⎛

s

m2

ivD Diffusion coefficient for species i in the vapor ⎟⎟⎠

⎞⎜⎜⎝

⎛

s

m2

dP Pressure drop ( Pa ) over porous deposit distance of dx (m)

xiii

D Diffusion coefficient or diffusivity (length2/time, i.e. s

m2

)

comE Net energy into the crud deposit by convection (W )

fgE Concentration conversion factor from liquid to vapor

)(rf Probability density function of deposit pore radii

G Mass flux variable ( )/( 2 smkg ⋅ )

lG Local mass flux of liquid in the porous deposit ( )/( 2 smkg ⋅ )

vG Local mass flux of vapor in the porous deposit ( )/( 2 smkg ⋅ )

h Convective heat transfer coefficient at the crud layer outer surface (watts/m2-K)

lH Liquid enthalpy ( kgJ / )

fH Liquid saturated enthalpy ( kgJ / )

vH Vapor enthalpy ( kgJ / )

gH Vapor saturated enthalpy ( kgJ / )

I Measured electric current through the heater rod cladding ( amps )

J Diffusion flux (amount of substance/length2/time, i.e. sm

mol

⋅2)

k The thermal conductivity of the heater rod cladding ( KmW ⋅/ )

dk The overall thermal conductivity of crud structure ( KmW ⋅/ )

fluidk Fluid thermal conductivity ( KmW ⋅/ )

lk Liquid thermal conductivity ( KmW ⋅/ )

crudk The thermal conductivity of the solid crud ( KmW ⋅/ )

xiv

vk Vapor thermal conductivity ( KmW ⋅/ )

HL The heater rod heated length (cm)

l Subscript for water

NNu Nusselt Number, dimensionless

NRe Reynolds number, dimensionless

NPr Prandtl number, dimensionless

P WALT system pressure ( aP )

lP Pressure of the liquid acting on the bubble from the outside ( aP )

vP Pressure of the vapor within the bubble ( aP )

Q Average heater rod power (W)

q ′′ Heat flux at the heater rod outer surface (W/m2)

'''q Volume heat flux generated in the cladding by electric current (W/m3)

crudq Conduction heat flux through the crud solid part (W/m2)

vq Vapor conduction heat flux in the pores (W/m2)

lq Liquid conduction heat flux in the pores (W/m2)

2r Crud layer inner surface radius (m)

3r Crud layer outer surface radius (m)

elecR Measured heater rod electric resistance ( ohms )

1r The inner radius of the heater rod cladding (m)

2r The outer radius of the heater rod cladding (m)

R Bubble radius (m)

xv

R Critical pore radius defining a boundary between the liquid and vapor regions (m)

maxR Maximum pore radius (m)

2T Crud layer inner surface temperature (°C ), which is assumed the same as the

calculated heater rod outer surface temperature

3T Crud layer outer surface temperature (°C )

2T The calculated heater rod outer surface temperature (°C )

1T The thermocouple measured heater rod inner surface temperature (°C )

4T Water temperature in the heater rod channel (° C )

lT Liquid temperature (°C )

crudT Local crud temperature (°C )

satT Saturated temperature (°C )

vT Vapor temperature (°C )

wallT Cladding wall temperatures (°C )

U Measured voltage along the heater rod (volt )

u Fluid velocity (m/s)

V Volume flow rate (m3/s)

TV The total or bulk volume of porous material, including the solid and void

components

cladV The cladding volume (m3)

FV The volume of fluid-space (i.e., water or vapor space)

xvi

ilW Total mass flow of the species i in the liquid ⎟⎠

⎞⎜⎝

⎛

s

mol

ivW Total mass flow of the species i in the vapor ⎟⎠

⎞⎜⎝

⎛

s

mol

lW Total mass flow rate of liquid in the porous deposit ( skg / )

vW Total mass flow rate of vapor in the porous deposit ( skg / )

x Position or length (m)

xvii

Greek Letters

α Fraction of vapor in pores is defined aspores

v

A

A

κ Permeability to transmit fluids (darcy ( D ), or mD )

φ Porosity, defined asA

A

V

V pores

T

F = assuming homogeneous pores in the medium

ρ Fluid density ( 3/ mkg )

0ρ Electric resistivity (measured in ohm inches, Ω-cm),

μ Fluid viscosity ( sPa ⋅ )

Ω Electric resistivity (ohm)

v Subscript for vapor

σ Surface tension ( mN / )

τ Tortuosity factor, a property of pores being tortuous or twisted

vv Vapor kinematics viscosity (m2/s, or Stoke (St))

lv Water kinematics viscosity (m2/s, or Stoke (St), 1 St = 10-4 m2/s)

xviii

ACKNOWLEDGEMENTS

I would like to express my deepest appreciation to my former dissertation adviser, the

late Dr. Lawrence E. Hochreiter, whose reassurance stood as a great wall of strength and support.

The late Professor Lawrence E. Hochreiter is remembered with great honor and utmost respect as

a dutiful teacher, an honest researcher, and an invaluable patron.

I also extend my very sincere thanks to my current dissertation co-advisers and all my

doctoral committee members and honored committee members, Dr. Seungjin Kim, Dr. Fan-Bill

Cheung, Dr. Arthur Motta, Dr. Kostadin N. Ivanov, Dr. Digby D. Macdonald, Dr. William A.

Byers, Mr. Michael Y. Young, Mr. Zeses Karoutas, Mr. Robert L. Oelrich, Jr., Dr. Jack S.

Brenizer, Jr., Mr. Brian R. Beebe, Mr. Jeff Deshon, and Mr. George K. Roberts for their time,

their constructive and invaluable guidance, and encouragement.

My special thanks are extended to Electric Power Research Institute (EPRI) and

Westinghouse Electric Company LLC Science and Technology Department, Core Engineering

Department, Fuel Engineering Services Product Management Department, and Production

Engineering Department for funding my doctoral research and making their resources available. I

wish to express my greatest thanks to Mr. W. F. Profota and Mr. M. G. Peck in Westinghouse

Electric Company LLC for helping us to build and maintain the WALT loop normal operation.

The Penn State University libraries and the Department of Mechanical and Nuclear Engineering

have provided me prompt and professional support in honoring all my requests.

At this time of accomplishment, I respectfully express my warmest and most sincere

thanks to my family, my dear friends, and my colleagues and management, whose love and/or

undying support have greatly helped me throughout my graduate study journey.

xix

Dedicated to

My most beloved father and mother, Changsuo Wang and Dazhuo Shu, who have

been teaching me the rules of life and virtues of simple living, and also my most beloved

wife, Mei Chen, and two most beloved children, Lily C. Wang and Max C. Wang, who

have been supporting and understanding my research work with a lot of patience.

Chapter 1

INTRODUCTION

Crud is defined as a coating or an incrustation of filth or refuse in the current

English dictionary [72]. CRUD is also an acronym for Chalk River Unidentified Deposits

(CRUD). In the nuclear industry, crud or crud deposition is related to heat transfer

equipment and is typically defined as the deposition of unwanted material or impurities

on heat exchanger tube surfaces or on nuclear fuel rod cladding of Light Water Reactors

(LWRs). It is understood that crud deposit may cause potential heat transfer degradation

and may also capture boron species from coolant in the primary system of Pressurized

Water Reactors (PWRs).

In this chapter, background information on crud deposits in PWRs is presented.

The key factors contributing to the root cause of potential heat transfer degradation or

dryout are also discussed. At PWR operating conditions, crud deposits with high

concentration of boron species will result in axial offset anomaly (AOA), also identified

as crud induced power shift (CIPS). If the crud is thick enough at high heat flux level, it

may lead to fuel rod surface heat transfer degradation and dryout. These phenomena are

due to the following four interrelated conditions (Figure 1-1).

(1) Sub-cooled nucleate boiling,

(2) Sufficient corrosion products deposition,

(3) Oxidation / Reduction or Electrochemistry Reactions, and

(4) Soluble Boron in coolant.

2

(I) Crud Deposits Due to Subcooled Boiling

Coolant Corrosion Products

Subcooled Boiling

Oxidation / Reduction

Soluble Boron in Coolant

(II) Crud Deposit Due to Oxidation / Reduction

(III)Boron Hideout in Crud AOA

Coolant and Corrosion Products

Figure 1-1 Factors Contributing to Crud Formation

3

Figure 1-1 illustrates the interrelationships among the four interrelated conditions, which

are discussed in Sections 1.1 through 1.4 of this chapter.

1.1 Effects of Sub-cooled Nucleate Boiling

1.1.1 Basic Mechanism of Sub-cooled Nucleate Boiling

The basic mechanism of sub-cooled nucleate boiling on clean surfaces can be

reasonably characterized [1]. Boiling processes on crud surfaces, however, are more

complex and are not as well understood, although many of the same basic principles [1]

can be applied. Under PWR operating conditions, the coolant at the reactor core inlet is

highly sub-cooled, typically by about 100° F (55.6° C). The heat transfer from the fuel

cladding to the coolant is initially achieved by forced convection to single-phase water.

The coolant temperature increases as the water flows toward the top of the reactor core.

With a high heat flux from the fuel rod wall to water, the cladding surface temperature

can be above the saturation temperature (superheated) such that the coolant temperature

immediately near the fuel cladding wall (i.e. the solid surface of the heat source) will also

be slightly above the saturation temperature.

The liquid adjacent to the wall is also superheated, even though the average bulk

coolant temperature (or, the average coolant temperature across the flow channel at the

same elevation) is sub-cooled. “Bubbles will begin to form at certain preferential sites on

the superheated cladding surface called nucleation sites” [2], which are minor surface

imperfections such as scratches, or grinding marks, or oxidations/corrosions that provide

indentations on the surface where dissolved gases can be trapped. Bubbles tend to form at

4

these locations first, hence the term on-set of nucleate boiling, or local boiling.

Initially, “bubbles are generated only on the localized cladding areas, while forced

convection heat transfer continues on the rest of the fuel cladding surface. At higher heat

fluxes and /or if the coolant is closer to the saturation temperature, more nucleation sites

become active and bubbles are generated over a larger part of the surface. Eventually,

more and more surface can be covered with bubbles. If the bulk coolant is still less than

the saturation temperature, this is called fully developed sub-cooled nucleate boiling.

Bubbles formed in this heat transfer regime will condense locally, either by sticking out

from the superheated boundary layer into the sub-cooled liquid, or by detaching from the

surface, floating into the main sub-cooled coolant stream, and condensing there.” [2]

Under normal operation conditions in a PWR, local sub-cooled nucleate boiling occurs at

the top grid spans of some assemblies.

1.1.2 Effects of Sub-cooled Nucleate Boiling on Crud Deposition and AOA

Sub-cooled nucleate boiling on fuel rod surface is one of the primary mechanisms

for crud deposition on top of the grid span in PWRs. The high concentration boron

hideout in the crud on the top grid span region, and boron will absorb neutrons and lead

to a shift in the reactor core power shape resulting in bottom skewed axial power

distribution. This phenomenon is called axial offset anomaly (AOA) or Crud Induced

Power Shift (CIPS). Earlier fuel inspection results [2] supported the hypothesis that

AOA-related crud deposition was caused by sub-cooled nucleate boiling. “Since these

inspections, more detailed crud scrape inspections have been conducted at both AOA and

5

non-AOA affected PWRs.” [2] These inspections generally support the supposition that

higher sub-cooled nucleate boiling leads to enhanced crud deposition, and therefore, an

axial power shape shift in the reactor core.

A 3D thermal-hydraulic analysis using the VIPRE-W sub-channel computer code

has been proposed to calculate the cladding surface steaming rate (kg/s-m2) to

characterize the sub-cooled boiling condition of a core [2]. Steaming rate is a local

parameter that will vary throughout the reactor core. Its magnitude depends on the local

heat flux, the local coolant temperature, and the local value of the forced convection heat

transfer coefficient. “Only the relatively high power fuel assemblies in a PWR will

exhibit sub-cooled nucleate boiling (i.e. steaming rate greater than zero). At any given

location within the core, the steaming rate value will also vary with burn-up as the power

distribution changes. Therefore, calculations at a number of burn-up steps throughout a

fuel cycle are necessary to adequately characterize the amount of steaming rate at sub-

cooled nucleate boiling in a given reactor. In terms of thermal hydraulic parameters,

steaming rate correlates the best with the surface deposition rate of crud formation.”[2]

When crud is becoming thicker and thicker (typically, 3 mils (76.2 microns) or greater),

dryout in the crud layer on a fuel cladding surface may occur. With crud dryout, the

cladding surface temperature increases and cladding corrosion is accelerated due to

higher cladding temperature. These processes can accelerate and eventually may cause

cladding failure.

Core management and fuel mechanical design features also affect sub-cooled

nucleate boiling. For example, the incorporated intermediate flow mixing grid (IFMs)

designs, which improves the local convection heat transfer [36] and decreases the local

6

mass evaporation rate, may prevent the conditions believed to have contributed to the

crud buildup that led to and/or dryout (causing cladding failures) during Plant A Cycle N

[2]. Crud deposited on the clad serves as the substrate for concentrating boron on top grid

span region that leads to the reactor core power shape bottom skewed.

1.2 Corrosion Products and Fuel Crud Formation

1.2.1 Corrosion Products From Non-Fuel Surfaces

“The main source of corrosion products that deposit on fuel surfaces is the

corrosion of steam generator (SG) tubes. The steam generator tubes comprise

approximately 65% of the Reactor Coolant System (RCS) surface area in Westinghouse

and Combustion Engineering designed PWRs and nearly 75% in Babcock & Wilcox

PWRs. The steam generator tube materials used in PWRs today are Inconel 600, Inconel

690, and Incoloy 800. It is generally accepted that corrosion rates under PWR primary

coolant conditions for these three alloys have the following order, Inconel 600 exhibiting

the highest corrosion rate[2]: Inconel 600 > Inconel 690 > Incoloy 800. The lower nickel

content in Inconel 690 and Incoloy 800, along with higher chromium levels is believed to

be responsible for the increased corrosion resistance” [2].

“Corrosion of materials is a process where the exposed surface of an alloy to

primary coolant is transformed to an oxide that is more thermodynamically stable. As the

corrosion process continues, the rate of corrosion slows down if the oxide is protective.

Therefore, the corrosion rate of steam generator tubes is higher in the first Replace Steam

Generator cycle than in subsequent cycles. Over time, the corrosion rate will become

7

indiscernible from one cycle to the next, assuming all conditions remain the same.

Corrosion product release accompanies the corrosion process and occurs by dissolution

into the coolant. The rate at which the metal ion enters the coolant is dependent on the

diffusion coefficient for the particular metal and the oxide thickness and composition, as

well as how close to saturation the metal(s) is in the coolant” [2].

1.2.2 Corrosion Products From Fuel Surfaces

Crud deposited on once and twice burned fuel assemblies has been evaluated in

the past several years as a contributing source of crud for axial offset anomaly (AOA)

affected feed assemblies. As described in Reference 2, crud on burned fuel assemblies

can become a source of crud to fresh fuel during subsequent operating cycles. “Whether

or not once and twice burned assemblies will "give up" nickel and iron crud in the next

cycle depends largely on the relative power of that assembly during the subsequent

cycle(s) of operation. To illustrate this point, two Plant B high-powered assemblies

having nearly the same power during Cycle 9 were reinserted into the Cycle 10 core. One

assembly was reinserted into a peripheral core location (low-power position) and the

other assembly was reinserted into a central high-powered position. Crud scrapes from

the two assemblies were taken during the cycle reload process. The crud measured on the

twice-burned centrally located assembly was significantly heavier than on the

peripherally located low-powered assembly” [2].

As has been observed in other cases[2], the relative power and/or core location

exhibited in the second cycle of operation for the reinserted assemblies typify what can

8

occur to crud on the reinserted fuel assemblies. “Specifically, if the fuel in the second

cycle of operation has a relative power factor of approximately one or above, crud

thickness will remain similar or perhaps increase slightly. If the relative power in the

second cycle is below one (sub-cooled nucleate boiling will likely be low or zero), crud

will not be rigorously retained allowing it to be either partially dissolved or eroded. This

released material becomes available to redeposit on system surfaces, and notably, on high

-powered feed fuel assemblies” [2]. This further substantiates the important role of sub-

cooled nucleate boiling, and demonstrates that crud from burned fuel assemblies is a

source of crud to higher power feed fuel assemblies.

1.2.3 Corrosion Product Circulation

From a system-wide perspective, the reactor coolant system represents a forced-

convective and non-isothermal system where corrosion product transport, in either

soluble or particulate form, which is dictated by release and deposition mechanisms.

Parameters that impact soluble species release and deposition are principally temperature,

pH, steaming rate, and local electrochemical reaction rates. This latter parameter driven

by the coolant hydrogen concentration. At different parts of the reactor primary system,

tendencies for release and deposition may be higher or lower. Corrosion product transport

also occurs by way of particles released by fluid shear forces (system flow) and system

perturbations (e.g. power transients, pH changes, etc).

9

Circulating corrosion products in the reactor coolant exist either in dissolved

(soluble) or particulate form. Reference 2 documented nickel and iron solubilities in

equilibrium with NiFe2O4 across the reactor coolant temperature range. Figure 1-2 [2]

shows calculations of Ni and Fe solubilities depicted graphically under a mid-cycle

coolant conditions (i.e. 650 ppm Boron).

As can be seen from Figure 1-2, an increase in pH value from 6.5 to 7.0 (at 560

ºF, or 293 ºC) causes a reduction in nickel and iron solubility in the cooler regions of the

RCS. The effect of changing pH value from 7.0 to 7.6 at the same temperature of 560 ºF

(293 ºC) is clearly less for iron and negligible for nickel.

From the summary of the experimental results by Burrill and Shaddick [3],

solubility of Fe versus water temperature and pH curves is selected and given in Figure 1-

3, which shows the trends of the iron solubility curves at higher pH values (i.e. pH of 9.7

– 10.2).

Other calculations using different solubility data have predicted higher coolant

iron concentrations (Fig. 1-4). Figure 1-4 [2] shows the variation in iron concentration

across the core. For example, the calculated solubility of iron increases for pH value of

7.6 as temperature increases. The iron solubility slowly decreases for pH value of 7.0 as

temperature increases. At normal operation conditions of a PWR, pH values are typically

controlled to be around 7.0.

The nickel species driving solubility are dependent on the coolant hydrogen

concentration [2], In Reference 2, the author predicts that Ni will deposit in the lower

spans (e.g. at a clad wall temperature below 322ºC), where hydrogen concentration is

relatively higher.

10

(T(F) = 1.8T(K)-459.67)

Figure 1-2 Nickel (Ni)/Iron (Fe) Solubilities in PWR Primary Coolant at 650 ppm B10 [2]

11

Figure 1-3 Solubility of Magnetite versus Water Temperature and pH [3]

12

(T(F) = 1.8T(C)+32)

Figure 1-4 Calculated Fe Solubility in PWR Primary Coolant at 650 ppm B10 [2]

13

A coolant sample system, usually a small continuous flow sub-system attached to

the primary system, is used for monitoring or collecting corrosion products. High

temperature sampling systems have been installed at two nuclear plants [2]. Continuous

flow reactor coolant is sampled from the hot legs through small pore silver membrane

filters at temperatures around 225 ºC (437 ºF). This apparatus provides better information

on steady-state and transient corrosion products during the operating cycle, than typical

“batch” sample systems. The coolant samples from these systems show the following:

“Chromium-nickel-ferrites are a major component of circulating crud particles in

the Reactor Coolant System (RCS). The end-of-cycle ferrites were similar to magnetite in

structure (low substitution of other elements). Ni particles were also found to be an

important part of RCS circulating corrosion product chemistry. No NiO has been

observed from the particulate samples at these two stations. The presence of elemental

carbon was noted from the Plant E particulate samples. The carbon presence, though not

known with certainty, likely comes from a reduction of the acetate anion from zinc

injection” [2]. In summary, it is observed that corrosion products circulating in the

reactor coolant systems can be measured in different ways, e.g. sampling during normal

operation and reactor shutdown, provided the sampling system is properly designed.

14

1.2.4 Fuel Crud Formation and Its Characteristics

1.2.4.1 The Characteristic of Crud Formation

In order to understand the characteristic and formation of crud, some relevant

experimental results are cited and documented in this section. Collier and Pulling [4]

have studied the deposition of silica from unsaturated solutions in a low-pressure steam-

heated vertical tube evaporator. In the saturated nucleate boiling region, deposition

appeared in the form of annular rings around a large number of discrete sites on the

surface. An explanation for the form of these deposits can be given in terms of an

examination of the micro-structure of the boiling process. Vapor bubbles grow at active

nucleation sites leaving, as they grow, a very thin ‘micro-layer’ of liquid beneath the

bubble. This thin layer is totally evaporated leaving the solid content of the micro-layer

deposited on the surface. The bubble detaches and unsaturated liquid contacts the surface

again, removing some of the deposit. A new bubble forms and the deposit process is

repeated. This is why the deposition appeared in the form of annular rings and looked like

a porous layer.

As discussed in Section 1.1.1, under PWR operation conditions, bubbles formed

in the sub-cooled nucleate boiling regime will condense locally, either by sticking out

from the fuel rod surface into the sub-cooled liquid, or by detaching from the surface,

floating into the main sub-cooled coolant stream, and condensing there. The crud

formation under PWR operation conditions is similar to the process discussed in the

above paragraph. So, it is expected that the crud on the fuel pin surface is formed in

porous structure.

15

1.2.4.2 Crud Observations in PWR Cores

In order to better understand corrosion product deposits on fuel surfaces, the

nuclear industry undertook a significant amount of effort in the past few years. A list of

general observations and crud features documented in Reference 2 are provided below.

1. Crud mass is proportional to the degree of sub-cooled nucleate boiling. “Since

sub-cooled nucleate boiling does not occur until the upper spans of the assembly,

crud is typically heavier in spans 5 and 6. Figure 1-5 is a high magnification SEM

of a crud flake obtained from upper span of Plant C Cycle-8. It depicts the effect

of sub-cooled nucleate boiling and creation of boiling chimneys within the

deposit”.

2. “The nickel-to-iron ratio in crud from upper spans increases with rod power and

sub-cooled nucleate boiling”. This may be explained by either of the following: “a)

Crud tends to be thicker in regions of sub-cooled nucleate boiling and

concentrates soluble species such as lithium (as the crud thickens). As pH rises

within the deposit, the solubility of iron rises and can preclude precipitation of

iron-based deposits and/or dissolve existing iron within the deposit. Since nickel

solubility is essentially independent of pH at clad surface temperatures (e.g. Fig.

1-2), it will remain deposited on the clad, thus raising the observed nickel-to-iron

ratio. b) Alternatively, the observed high nickel-to-iron ratio in high sub-cooled

nucleate boiling deposits may merely be due to the presence of a less soluble form

of nickel under shutdown chemistry environments resulting in the high measured

16

ratios. It is also observed that porosities within crud are in some cases lower near

the clad and higher near the coolant interface. In other cases, the porosity is

uniform across the crud deposit”.

3. Ni has been measured in thinner deposits from low power feed rods and from

twice and thrice burned rods. NiO is more prevalent in deposits ongoing sub-

cooled nucleate boiling. “Thin crud has more chromium than thicker crud. Thick

crud from rods undergoing significant sub-cooled nucleate boiling can have fully

substituted nickel ferrite (Fe2+ does not exist), whereas thinner crud contains

partially substituted nickel ferrite (contains both Fe2+ and Fe3+)”.

Figure 1-6 is a cross sectional view of a Plant A Cycle N crud flakes. It is noted

that three layers present in the flakes. “The center "lighter" layer was comprised

principally of ZrO2. Detail investigation shows that the crud from Plant F Cycle 9 has

similar appearance to the crud from Plant A Cycle N (Figure 1-6). Plant F experienced

the severest axial offset anomaly (AOA) case yet recorded during Cycle 9. During the

analysis of this crud, an oxide not previously identified in any PWR was measured [2].

Bonaccordite, Ni2FeBO5, was measured as the predominant oxide next to the clad wall

and had a unique needle or rod-like structure”.

17

Figure 1-5 High Magnification SEM of a Crud Flake Obtained from Plant C ycle 8 [2]

18

Figure 1-6 Cross Sectional View of a Plant A Cycle N Crud Flake

19

Like Plant A, “a central layer dominated by ZrO2 was measured. Also of

significance in the Plant F crud was the equal amount of NiFe2O4 and NiO present. Both

were on the order of about 10 wt%, which is relatively low for NiFe2O4 and fairly high

for NiO. The presence of bonaccordite and unique ZrO2 layer in the crud structure are

developed by high rate of water evaporation and electrochemistry reactions. Considering

the boron content in the bonaccordite, this oxide must have factored into the observed

AOA during the operating cycle” [2].

1.3 Electrochemistry Reactions, Corrosion, and Fuel Crud Formation

It has been observed that almost all types of corrosion can be explained in terms

of electrochemistry reaction, or oxidation-reduction reactions. In the porous crud layer

the accumulation of radiolysis products, such as H2, O2, and H2O2, occur with relative

long life times. These products provide the some contribution to the corrosion process

and to the nickel-ferrite oxide crud deposition on the fuel clad surface [25, 27]. When the

oxygen concentration in the film fulfills stoichiometry, the zirconium oxide is no longer a

passive layer to protect zircaloy clad and grows as white porous layer, which can be

described by a wick boiling and diffusion model [6].

Corrosion due to radiation effects is a function of fast neutron fluence, oxygen

concentration in the coolant, absolute temperature at the coolant and cladding interface,

the condition of the alloy surface, and the metallurgical state [5]. The reduction in the

thermal conductivity of zirconium oxide will increase the metal-oxide interface

temperature, which increases potential of anode and cathode to corrode the metal surface.

20

Radiolysis, H2O2, which has the longest life-time of radiolysis products and has a large

ionization trail, provides the maximum contribution to the corrosion process [7].

Corrosion weight gains and oxide film observations show that the average surface

roughness is larger on the gamma-ray irradiated surface than on the non-irradiated

surface [8]. Thus, this increases the nucleation site density for further boiling and

corrosion on the zircaloy cladding. More nucleation sites may also induce larger

subcooled nucleate boiling region when the coolant conditions are proper. The

relationship between oxidation/reduction and subcooled boiling is shown in Figure 1-1.

The deposition probability sue to oxidation/reduction is also shown in Figure 1-1.

Currently, all the depositions are included in the subcooled boiling process in the BOA

computer code [12, 34]. This is reasonable since the BOA model is benchmarked against

plant data.

On the fuel surface with increased nucleation sites, subcooled nucleate boiling

becomes more pronounced as a result of non-wetting surface or deposit [1]. As discussed

in Section 1.2.4.1, in the subcooled nucleate boiling region under PWR operation

conditions, vapor bubbles grow at active nucleation sites leaving, as they grow, a very

thin ‘micro-layer’ of liquid beneath the bubble. This thin layer is totally evaporated

leaving the solid content (i.e. corrosion products) of the micro-layer deposited on the

surface. The bubble detaches and water contacts the surface again, removing some of the

deposit. A new bubble forms and the deposit process is repeated. The crud is formed

during this subcooled nucleate boiling process. It is expected that the crud on the fuel pin

surface is formed in porous body.

21

1.4 Boron Hideout in Crud Layer Inducing AOA

Analytical studies of mass transfer in porous materials overlaying on boiling

surfaces show that concentrations of solute can build within crud. “Once a sufficient crud

thickness is achieved, the ratio of crud surface to bulk concentrations of a soluble

chemical species can increase as an exponential function of the surface boiling heat flux”

[2]. This gives more changes for the chemical species to adhere to the crud surface.

“Thus, sub-cooled boiling duty (or, steaming rate) can result in crud deposition

containing coolant additives, such as lithium and boron” [2]. It was suggested [9] that the

most likely boron hideout process is via lithium meta-borate (LiBO2) precipitation within

the deposit. “If Boron hideout in crud layer is high enough, axial offset anomaly (AOA)

will occur. It is noticed that LiBO2 has retrograde solubility with respect to temperature

(e.g. solubility drops as temperature is increased and vise versa), this hypothesis is

supported by observations of lithium dissolved from crud to reactor coolant during power

reductions in plants symptomatic of AOA. Plants with AOA cycles have also observed

lithium deposition in crud (or hideout) after reactor returning to full power (raised

temperature and resumption of sub-cooled nucleate boiling)” [2].

Chapter 2

BACKGROUND AND LITERATURE REVIEW

As discussed in Chapter 1, corrosion and crud initiation on Zircaloy fuel cladding

under the PWR operating conditions are very complex process, which depend on many

factors including electrochemistry reactions, heat flux, radiation, pH value, and water

chemistry in the primary coolant. In the past, crud formation investigations in different

fields have been conducted by different researchers considering the effects of some or

most of these factors listed herein. Some of the key existing crud models and/or

investigation results are summarized in this chapter.

2.1 First Existing Porous Crud Model (Model-1)

Per Reference 1, the first existing porous crud model (named “Model-1” by the

author) is proposed by Macbeth (AEEW-R 711) [19] in 1971. This porous crud model is

given in Figure 2-1, which shows the process of boiling heat transfer upon a heating

surface (with heat flux of Φ) overlaid by a porous crud (with thickness of h). It is

assumed that liquid is drawn into the crud through smaller size of pores (d1) by surface

tension and the vapor is ejected via the larger diameter pores (d2). In his paper, Macbeth

presents a detailed description and analytical model of the wick boiling process that is

considered more likely to be taking place where porous magnetite deposits are found.

23

Figure 2-1 The Porous Crud Model by Macbeth (AEEW-R 711)

24

2.2 Wick Boiling Heat Transfer Model (Model-2)

In 1987, Kovalev [20] published his paper on “Liquid Boiling on Porous

Surface”, which further investigated the porous deposit/crud model first presented by

Macbeth (AEEW-R 711) [19]. Kovalev’s model can be named existing “Model-2” (by

the author). There are also a number of other existing heat transfer models [21, 22, 23]

for investigating boiling phenomenon in porous media. The common base of these

existing models is wick boiling.

In Kovalev’s paper [20], “available data on boiling heat transfer from surface with

various porous coatings is analyzed. For the region of developed nucleate boiling a

physical model is suggested on which basis a mathematical description of the boiling

process is given. Heat transfer is described by a conduction equation involving a term

which takes into account heat absorption due to liquid evaporation from the surface of

menisci located inside the porous structure. A numerical experiment is performed by

employing the model suggested. The influence of the porous layer conductivity on heat

transfer is revealed. An analysis of the laws governing heat transfer is conducted with the

aid of the similarity theory. A method to classify porous structures is suggested and

correlations are obtained”.

2.3 Crud Deposit Model (Model-3)

Crud deposit modeling (named “Model-3” by the author) has been investigated by

different researchers. For crud deposition, a chimney structure of the crud layer (Fig. 2-2)

has been observed and a material analysis of the deposit indicates the presence of Fe, Ni,

and Zr in the form of non-stoichiometric nickel ferrite (FexNi3-xO4)[5].

25

ZrO 2

Ni-rich needles

Chimney

LiBO 2 zone

ZrO 2 precipitation

Typical thick AOA crud in cross section

Typical Thin Crud Top Down View

Figure 2-2 Typical Thick Axial Offset Anomaly (AOA) Crud in Cross Section

Flow

26

The porous crud layer on the zircaloy cladding surface have been observed mainly

on the subcooled boiling length of a fuel rod where high wall temperature exists [26]. In

the porous crud layer the accumulation of radiolysis products, such as H2, O2, and H2O2,

occur that have relative long life times. These products provide some contribution to the

corrosion process and to the nickel-ferrite oxide crud deposition on the fuel clad surface

[27]. The effects of radiolysis of liquid water, which somewhat affect cladding corrosion

and solutes within the primary coolant system, have been examined in terms of pH,

temperature, and Linear Heat Transfer Rate [28].

Reference 28 also discusses the effect of mass transfer, especially diffusion, on

the concentration distribution of the radiolytic products of H2 and O2. Corrosion of

Zircaloy cladding and crud initiation under LWRs operating conditions are very complex

processes that depend on many factors including electrochemistry (oxidation - reduction)

reactions, temperature, heat flux, radiation (especially at higher burnups), and water

chemistry (e.g. pH values, etc) in the primary coolant [29].

Deposition process of Ni2+ and Co2+ ions on a heated surface has been studied and

is divided into two stages [30]. The first stage is the deposition process of hydroxide

precipitate on the heated surface by microlayer evaporation (subcooled boiling) as

discussed in Chapter 1. The second is settlement by conversion of hydroxide into oxides

such as NiO, NiFe2O4, CoO, and CoFe2O4. The effective deposition coefficients of Ni2+

and Co2+ ions, without supplying Fe crud (simulated in the experiment[30]), are smaller

than that of α-Fe2O3 because of the higher solubility of those hydroxides at a low

concentration condition. Their effective deposition coefficients increase with the

27

simulated Fe crud concentration [30], because the Ni and Co hydroxide react with the

simulated Fe crud to produce insoluble NiFe2O4 and CoFe2O4 on the heated surface.

In addition, a crud particle/wall interaction and deposition model, which is based

on Gerassimov’s [31] modification of Beal’s deposition model in turbulent flow [32], in a

pressurized water reactor primary system has been presented in Reference 33. More

recently, the deposition process was viewed as a multi-step process from the bulk coolant

to the wall. This model was developed for deposition on steam generator secondary sides

[37]. Some other crud deposition models were cited in Uhle’s Ph. D. dissertation [13].

2.4 Uhle’s Model (Model-4)

There are a number of crud deposit models (Section 2.3) and heat transfer models

(Sections 2.1 and 2.2) for boiling in porous media, which are based on wick boiling. Most

of these models are separated between the crud deposit models and the heat transfer

models. Uhle’s model presented in her Ph. D. dissertation [13] is an adaptation of the

Kovalev model, the porous models were coupled with heat transfer models.

In Uhle’s Ph. D. dissertation, the boiling heat transfer characteristics of steam

generator tube fouling deposits were identified. A boiling heat transfer model was

developed and the model accuracy was determined by comparing the calculated and

experimental results. Magnetite deposits were fabricated in the laboratory and were

characterized. Then, measurements were performed for heat transfer and the effects of

deposit parameters, which include pore size, porosity, deposit thickness, mass flux, and

heat flux. Uhle’s model prediction is consistent with the experimental results. The

28

difference is about ±17.5%.

Data from Uhle’s model are consistent with that of the fouled steam generator

tubes pulled from CANDU steam generators. The conditions measured in Uhle’s

dissertation were similar to those of US and Canadian steam generators. Thus, further

work is necessary for investigating the deposit characteristics formed under PWR

operating conditions.

2.5 Current State-of-art Models to Address Crud Problems

2.5.1 BOA Model (Model-5)

The Boron-induced Offset Anomaly (BOA) computer code from Electric Power

Research Institute (EPRI) [34] was based on the Westinghouse Build-up of Boron (BOB)

computer code, which is documented in Reference 35. As stated in Reference 14, Uhle’s

dissertation [13], which is based on the Russian literatures for boiling in porous media is

the basis or centerpiece of the methodology in BOA computer code. Uhle’s dissertation

provided a state-of-the-art application of the Kovalev model, which is one of the most

current models with experimental results. In Uhle’s experiment, the porous crud layer

was uniquely manufactured. These manufactured crud layers are similar to those found

on steam generator or fuel rod surface, which was discussed in Chapter 1.

The model used in BOA code [35] for the concentration of species for the boiling

deposits is based on a diffusion model according to Fick’s law as discussed in detail later.

29

However, as discussed in Section 2.3 (Model-3), corrosion of Zircaloy/Zirlo1 cladding

and crud initiation under LWRs operating conditions are very complex processes that

depend on many factors including electrochemistry (oxidation - reduction) reactions,

temperature, heat flux, radiation (especially at higher burnups), and water chemistry (e.g.

pH values, etc) in the primary coolant. Deposition process of ions (e.g. Ni2+, etc) in the

primary coolant system on a heated surface can be divided into two stages. The first stage

is the deposition process of hydroxide precipitate on the heated surface by microlayer

evaporation (subcooled boiling). The second is settlement by conversion of hydroxide

into oxides such as NiO, and NiFe2O4. The effective deposition coefficient of ions (e.g.

Ni2+, etc) at the PWR operating conditions is one of the key parameters for crud thickness

calculations and/or dryout investigations.

The BOA code [34] has considered the following mechanisms for crud deposition

modeling,

• Boiling and non-boiling forms of deposition occur on the core.

• Corrosion products may re-deposit or be re-released from the steam generator

tubes as well as the fuel cladding surface in the core.

• The coolant contains particulate and dissolved forms of the corrosion product.

The amount in dissolved form depends on coolant pH and temperature. The

amount deposited in each form depends on separate mass transfer coefficients and

concentration gradients.

• The release rate of corrosion products into the coolant depends on coolant pH and

the material of the steam generator tubes.

1 ZIRLO is a trademark of Westinghouse Electric Company LLC.

30

The proposed improved heat transfer modeling for dryout investigation will

consider some of these mechanisms and other relevant parameters (e.g. crud thermal

conductivity, etc.) in order to better predict conditions when crud dryout occurs. This tool

is useful to help operating Pressurized Water Reactors (PWRs) to meet the “0 by 2010”2

requirement set by commercial nuclear industry executives at an executive meeting of

Institute of Nuclear Power Operations (INPO).

2.5.2 Independent Review or Derivation of Current BOA Conservation Equations

The Kovalev’s hydrodynamic model (Reference 20) for evaporation employs

capillary forces for driving liquid and vapor streams in and out the porous deposit

respectively. This model was applied by Uhle (Reference 13) to simulate boiling process

in crud deposit on secondary side of PWR steam generators. Mass, momentum, and

energy transfer for a control volume of porous deposit were studied extensively for the

existing BOA model development, which was based on Kovalev’s and Uhle’s models,

and improvements made by Westinghouse (Reference 12). Mass, momentum, and energy

conservation equations in current BOA are reviewed and described in this section.

2.5.2.1 Momentum Conservation Equations for Current BOA Theory

In fluid dynamics, Darcy's law is a phenomenologically derived momentum

conservation equation that describes the flow of a fluid through a porous medium. The

2 Commercial nuclear industry executives at an executive meeting of Institute of Nuclear Power Operations (INPO) require zero fuel failure by the year of 2010 and beyond.

31

law was formulated by Henry Darcy based on the results of experiments on the flow of

water through beds of sand. It has since been derived from the Navier-Stokes equations

via homogenization generically. Hence, Darcy's law is a generalized relationship for flow

in porous media. Since its discovery, Darcy's law has been found valid for any

Newtonian fluid. Likewise, while it was established under saturated flow conditions,

Darcy's law may be adjusted to account for unsaturated and multiphase flow.

Darcy's law is a simple proportional relationship between the instantaneous

discharge rate through a porous medium, the viscosity of the fluid, and the pressure drop

over a given distance. Darcy’s law is expressed as the following (Figure 2-3 and Equation

2-1).

dx

dPAV

μκ−= , or

0=+A

V

dx

dP

κμ

(2-1)

where, κ is the permeability to transmit fluids (commonly symbolized as κ, which is a

measure of the ability of a material (typically, porous medium) to transmit fluids). A

common unit for permeability is the darcy ( D ), or more commonly the millidarcy ( mD )

(1 darcy ≈ 10-12 2m = 1.076x10-11 ft2).

32

Figure 2-3 Diagram showing definitions and directions for Darcy's law

33

If the fluid (liquid or vapor) velocity changes in the porous deposit, modified

Darcy’s law with an additional term of spatial acceleration shall be applied in order to get

the momentum conservation equation. As discussed earlier, one can also derive the

momentum conservation equation directly from the Navier-Stokes equations via

homogenization. Either way will lead to the following momentum conservation equation.

0=++A

V

dx

duu

dx

dP

κμρ (2-2)

Applying Equation (2-2) to both water and vapor in porous deposit leads to the

following.

0=++ll

lllll

l

A

V

dx

duu

dx

dP

κμρ (2-3)

0=++vv

vvvvv

v

A

V

dx

duu

dx

dP

κμρ (2-4)

where, φα ⋅−⋅= )1(AAl ( 2m ),

φα ⋅⋅= AAv ( 2m ),

A

A

V

V pores

T

F ==φ (if homogeneous pores in medium), which is defined as porosity

( FV is the volume of fluid-space (i.e. water or vapor space), TV is the total or bulk

34

volume of porous material, including the solid and void components, and poresA is

the cross-sectional area for pores.),

α = fraction of vapor in pores = pores

v

A

A,

Before Equations (2-3) and (2-4) are merged, the following variables or equations

are defined or summarized.

l

l

l

ll

l

ll

G

A

W

A

Vu

ρρ === /

(2-5)

v

v

v

vv

v

vv

G

A

W

A

Vu

ρρ === /

(2-6)

where, l

ll A

WG = = local mass flux of liquid in the porous deposit ( )/( 2 smkg ⋅ ),

v

vv A

WG = = local mass flux of vapor in the porous deposit ( )/( 2 smkg ⋅ ),

At steady state conditions, the total mass flow of liquid in the porous deposit

equals the total mass flow of vapor out of the deposit at any location x with length of dx

as shown in Figure 2-3.

vl WW =− , or

vvll AGAG =− (2-7)

Dividing by A on each side of Equation (2-7),

35

A

AG

A

AG vvll =− (2-8)

Equation (3-8) can be re-written as,

GGG vl ≡=−− αφφα )1( (2-9)

where, =α fraction of vapor filled pores, and defined in Equation (2-10) below.

∫==max

)(R

Rpores

v drrfA

Aα (2-10)

where, =)(rf the deposit pore radii are assumed to be distributed according to this

density function.

From Equation (2-10), the following equation can be obtained,

)(RfdR

d −=α, or

dx

dRRf

dx

d)(−=α

(2-11)

In addition, consider a bubble in an equilibrium condition or vapor section in

pores (as shown in Figure 2-4), the following force balance equation exists,

36

Pv

Pl

σ σ

R

(b) Forces on a Vapor Bubble in Liquid

(a) Forces on the Vapor Section in a Pore

Pl

Pv R

σ σ

Vapor in a pore

Vapor/water interface

Crud pore wall

Figure 2-4 Forces on a Vapor Bubble or Vapor in a Pore

37

σππ RPPR lv 2)(2 =− (2-12)

Equation (2-12) can be simplified and re-written as,

R

PP lv

σ2=− , or

( )

dx

dR

Rdx

dP

dx

dP

dx

PPd lvv ⋅=−=−

2

2σ (2-13)

Subtracting Equation (2-3) from Equation (2-4) leads to the following equation,

0=−+−+−ll

ll

vv

vvlll

vvv

lv

A

V

A

V

dx

duu

dx

duu

dx

dP

dx

dP

κμ

κμρρ (2-14)

Equation (2-14) indicates that the capillary driving force is balanced against the

acceleration due to phase change and vapor fraction change (i.e. dxdG and dx

dα ) and the

resistance through the porous body. Substituting Equations (2-5), (2-6), (2-9), (2-11), and

(2-13) into Equation (2-14) leads to the following,

( )

( ) ( ) φκαν

καν

φραρα

φραρασ

G

dx

dGG

RfG

Rdx

dR

v

v

l

l

vl

vl

×⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅+

⋅−+⋅×⎟⎟

⎠

⎞⎜⎜⎝

⎛

⋅−

⋅−

=⎥⎥⎦

⎤

⎢⎢⎣

⎡ ⋅×⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅+

⋅−−×

1

1

1

1

)(1

1

12

222

2

2

332

(2-15)

38

As Uhle pointed out in her dissertation (Reference 13), the capillary driving force

must be sufficient to overcome the acceleration as well as the viscous resistance.

Therefore, the term for phase change (i.e. multiplying dxdG ) must be positive and will

represent one of the necessary conditions for solution of the equation set. Essentially, the

mass flux gradient of dxdG is calculated from the energy equation. Then, the

corresponding change of R is calculated from Equation (2-15) above.

2.5.2.2 Energy Conservation Equations for Current BOA Theory

Heat is transferred from the fuel cladding wall into the crud deposit. At the top

grid span, subcooled boiling exists and vapor is generated. As discussed before, the flow

within the crud deposit is driven by surface tension forces on stationary vapor-liquid

interfaces. To solve the energy equation, the deposit/liquid/vapor matrix is viewed as a

homogeneous porous body through which heat is transferred by both convection and

conduction. At steady state conditions, the energy transported into the deposit equals the

energy transferred out of the deposit. If heat transfer from the fuel cladding wall to the

deposit surface is defined as positive direction, the energy balance for a crud layer

between x-Δx and x (x is closer to outside crud surface than x-Δx) was written as the

following.

39

The net energy into the crud deposit by conduction:

xxcrudv

lxcrudvl

xxcrudcrudvvllxcrudcrudvvll

qAqA

qAqAqAqA

qAqAqAqAqAqA

Δ−

Δ−

−++−+−++−−=

+++++−

])1(

)1([])1()1([

][][

φαφφαφαφφα (2-16)

The net energy into the crud deposit by convection:

xxvvllxvvllcom HWHWHWHWE Δ−+−+= ][][ (2-17)

Apply Equations (2-8) and/or (2-9) to Term (2-17), the net energy into the deposit

by convection becomes,

xxvlxvlcom HHGAHHGAE Δ−−−−= ][][ (2-18)

To combine the term in the right hand side of Equation (2-16) and Term (2-18),

the energy conservation equation is obtained as the following,

0][][

])1()1[(])1()1[(

=−−−+−++−+−++−−

Δ−

Δ−

xxvlxvl

xxcrudvlxcrudvl

HHGHHG

qqqqqq φαφφαφαφφα (2-19)

40

In Equation (2-19), vl HandH can be approximately expressed as the following,

gsatvpgv

fsatlpfl

HTTCH

HTTCH

+−≈

+−≈

)(

)( (2-20)

Also in Equation (2-19), the conduction heat is assumed to follow Fourier’s law,

which is:

dx

dTkq

dx

dTkq

dx

dTkq

crudcrudcrud

vvv

lll

−=

−=

−=

(2-21)

At subcooled boiling conditions, the deposit/vapor/liquid matrix temperature is

close to Tsat. Thus, it is reasonable to assume that the crud deposit, the liquid, and the

vapor are all at the same temperature of Td within control volume from x-Δx to x.

dcrudvl TTTT === (2-22)

41

Substitute Equations (2-20), (2-21), and (2-22) into Equation (2-19), the following

energy conservation equation is obtained.

0])()([

])()([

)])(1()()()1[(

)])(1()()()1[(

=−−−+−−

−−−+−+⎭⎬⎫

⎩⎨⎧ −−+−+−−+

⎭⎬⎫

⎩⎨⎧ −−+−+−−−

Δ−

Δ−

xxgsatdpgfsatdpf

xgsatdpgfsatdpf

xx

dcrudvl

x

dcrudvl

HTTCHTTCG

HTTCHTTCG

dx

dTkkk

dx

dTkkk

φαφφα

φαφφα

(2-23)

Define the overall thermal conductivity of dk as the following.

crudvld kkkk )1())1(( φααφ −++−= (2-24)

where, crudk represents the solid portion of the deposit, dk is for the overall structure.

Using Equation (2-24) to simplify Equation (2-23) as the following,

0])()([

])()([

=+−−−−

+−−−−⎭⎬⎫

⎩⎨⎧−

⎭⎬⎫

⎩⎨⎧

Δ−

Δ−

xxfgsatdpfsatdpg

xfgsatdpfsatdpg

xx

dd

x

dd

HTTCTTCG

HTTCTTCG

dx

dTk

dx

dTk

(2-25)

42

2.5.2.3 Mass Conservation Equations for Current BOA Theory

The basic mass conservation equations are given in Equations (2-7) through (2-9).

Besides, subcooled boiling in a porous crud deposit will tend to concentrate any

chemicals dissolved in the liquid. It is assumed that the species flows by diffusion

according to Fick’s law. As we know, Fick's first law is used in steady-state diffusion, i.e.,

when the concentration within the diffusion volume does not change with respect to time

( outin JJ = ). Since this part is not utilized in the dissertation, there is no detail discussed

in this section.

2.5.2.4 Porous Crud Properties

The mass, momentum, and energy conservation equations from Sections 2.5.2.1

through 2.5.2.3 are utilized to solve deposit temperature, flow, and pressure distributions.

To this end, boundary conditions and porous crud properties are needed. The porous crud

properties are discussed in this section.

The following properties were established for the porous deposit in the BOA

model.

• Porosityφ

• Permeability κ

• Minimum pore size minR

• Maximum pore size maxR

43

• Distribution of pore size )(rf

• The thermal conductivity of the solid crud crudk

• Tortuosity factor τ

This dissertation work is focused on obtaining the thermal conductivity of the solid

crud crudk , which is calculated based on the Westinghouse Advanced Loop Tester

(WALT) experimental data at PWR operating conditions.

Chapter 3

THEORETICAL MODEL AND METHODOLOGY

A theoretical model and a methodology are developed to measure crud thermal

conductivity at different PWR operating conditions. The calculated crud thermal

conductivity based on other direct measurements is a key factor to improve the heat

transfer model in the BOA computer code.

3.1 The Improved Crud Heat Transfer Model

As discussed in Chapter 2, crud thermal conductivity is a complex function of

many factors, including but not limited to heat flux, fluid conditions, and crud porosity in

the crud. Equation (2-24) defines the overall thermal conductivity of dk , which is

calculated based on other direct measurements and is a key factor to improve the heat

transfer model in BOA code. Equation (2-24) is re-written as the following,

crudvld kkkk )1()1( φφααφ −++−= (3-1)

On the right hand side of Equation (3-1), the first two terms contain liquid and

vapor thermal conductivities, which are a function of fluid conditions and crud porosity.

At crud dryout conditions, the first term is zero and the second term is relatively small in

45

Equation (3-1). After simplification, only the third term remains in Equation (3-1), which

becomes:

crudd kk )1( φ−≈ ; at crud dryout conditions (3-2)

Equation (3-2) is the simplified overall crud thermal conductivity at dryout

conditions. As described in the next section, a new theoretical model and a methodology

are developed to measure crud thermal conductivity at different PWR operating

conditions. In other words, the overall thermal conductivity of dk in Equations (3-1) is

calculated based on other direct measurements at different fluid conditions.

3.2 Four-Regime Theoretical Model and Crud Thermal Conductivity Measurement

Methodology

Recently, preliminary WALT loop experiments have been performed. Boiling

heat transfer on porous crud heater rod surfaces with vapor chimney (or pores) was

investigated experimentally to determine the effects of the size and/or porosity of the

vapor chimney on the boiling heat transfer. Observations from the WALT tests showed

that bubbles escaping from the porous chimney (i.e. pores in crud deposit) enhanced the

heat transfer, comparing with porous chimney without vapor passing through. As shown

in Figure 3-1, a four-regime theoretical model for crud chimney or pores are identified

and proposed by the author. Accordingly, a methodology to measure crud thermal

conductivity is discussed in Section 3.2.2.

46

3.2.1 Four-Regime Theoretical Model in Crud

The four-regime theoretical model in crud is the creation of a Data-to-Conditions

crud thermal conductivity matrix, which is important in the benchmarking of crud heat

transfer models. Without such benchmarking, we would have an incomplete crud heat

transfer model. This crud thermal conductivity matrix created and documented in this

dissertation can be utilized in the improved crud heat transfer model in the future version

of BOA computer code.

This four-regime crud theoretical model is proposed based on WALT loop crud

deposition observation in the first phase of the experiments. On the other hand, this

model is used to guide the second phase WALT loop experiments for crud thermal

conductivity measurement at different PWR operating conditions. This four-regime crud

theoretical model includes Flooding Model, Mixture Model, Dryout Model, and Particle

Model, which are described or defined as the following. Figure 3-1 shows the physical

conditions of these four models.

(1) Flooding model is for simulating conditions of liquid flooding in crud chimney or

pores. This condition is typically for relatively lower power operating conditions in PWR

(Eq. (3-1)).

(2) Mixture model is for conditions of vapor and liquid mixed in crud chimney or pores.

This situation is for normal high power operating conditions in PWR (Eq. (3-1)).

(3) Dryout model is for simulating crud dryout gradually from the bottom to the top of

crud layer at higher power operating conditions in PWR (Eq. (3-2)).

(4) Particle model is for crud pores filled or partially filled with solid particles

( dcrud kk = ).

47

Water in all Crud Chimney

Crud

Vapor in Crud Chimney Crud

Vapor in all Crud Chimney

Crud

Particles in Crud Chimney

Crud

Water in Crud Chimney

(a) Flooding model: liquid flooding in crud chimney or pores (lower power operating conditions)

(b) Mixture model: vapor and liquid mixed in crud chimney or pores (normal high power operating

conditions)

(c) Dryout model: crud dryout gradually from the bottom to the top of crud layer (higher power operating conditions)

(d) Particle model: crud pores filled or partially filled with solid particles

Figure 3-1 Proposed Four-regime Theoretical Model in Crud Chimney

48

3.2.1.1 Calculations and Identifications of Four-Regime in Crud

Flow conditions inside crud are of great importance for measuring crud thermal

conductivity in various cases. As the most important characteristic of porous crud, the

flow regime in crud not only characterizes the flow condition in an explicit way, but also

determines the measurement model in each measuring method for crud thermal

conductivity. Figure 3-2 shows how to determine the four crud regime conditions.

Based on the calculations for sub-cooled boiling rate and identification of solid

particles filled in crud pores on the heater rod of the WALT loop, features reflecting the

characteristics of crud conditions are determined. These methods are suitable for the

identification of the four-regimes inside crud, i.e. liquid flooding, vapor and liquid mixture,

crud pore dryout, and crud pores filled or partially filled with solid particles.

Based on the calculated boiling rate results, the first three regimes in crud can be

determined. The boiling rate calculations method and detail procedure are discussed in

the next chapter. The fourth regime in crud is identified using Scanning Electron

Microscope (SEM) and/or X-ray Diffraction (XRD). The SEM is a microscope that uses

electrons rather than light to form an image. The SEM has a large depth of field, which

allows a large amount of the sample to be in focus at one time. The SEM also produces

images of high resolution, which means that closely spaced features can be examined at a

high magnification for crud structure. X-ray diffraction (XRD) is a versatile, non-

destructive technique that reveals detailed information about the chemical composition

and crystallographic structure of crud or other natural and manufactured materials. Both

SEM and XRD are discussed in detail in the next chapter.

49

3.2.1.2 Simulations of Four-Regime in Crud at WALT Loop Operations

The WALT system provides a complete state-of-art tool for crud deposition at

PWR operating conditions. This system offers the nuclear industry, for the first time, a

reliable and important operating test system for modeling and simulating PWR operating

conditions with and without crud depositions.

The first three regimes in crud, i.e. liquid flooding, vapor and liquid mixture, crud

pore dryout, is simulated by adjusting the WALT operating power levels from low to

high. In other words, the fluid conditions inside the crud pores can be simulated when the

heater rod power level is increased following a boiling curve as shown in Figure 3-2.

Alternative ways, such as adjusting inlet temperature, flow rate, or system pressure, may

be used to reach different fluid conditions in crud. However, those alternative ways are

not suggested because the PWR normal operating conditions are typically not changing

for parameters of inlet temperature, flow rate, or system pressure.

The fourth regime in crud is simulated by running the WALT loop in relatively

low power. By doing this, the crud is formed without pores or with small pores. The

second way to simulate the fourth regime in crud is to inject zinc or other solid particles,

such as silica, after crud is formed during normal WALT loop operation.

In summary, the four-regimes in crud can be simulated in the WALT loop by

either changing the heater rod operating power and/or adding different chemical solutions.

The WALT loop design, setup, and operating procedure are discussed in detail in Chapter

4.

50

Figure 3-2 A Sample Partial Boiling Curve from WALT Operation

A Sample Partial Boiling Curve at WALT(Pr = 15.6 MPa, G = 0.301 kg/s-cm^2, T inlet = 332 Deg-C, OD = 0.86 cm)

332.0

334.0

336.0

338.0

340.0

342.0

344.0

346.0

348.0

350.0

352.0

0.010.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

100.0

110.0

Heat Flux, W/cm^2

Cla

d T

empe

ratu

re, D

eg-C

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Boi

ling

Rat

e, k

g/s-

m^2

Clad Temp.

Boiling Rate

51

3.2.1.3 Applications of Four-Regime in Crud at PWR Operations

Pressurized Water Reactors (PWRs) generate nuclear power through the use of

nuclear fuel assemblies housed in a reactor vessel. The fuel assemblies are comprised of

elongated hollow metallic tubular fuel rods, which contain pellets of enriched uranium

dioxide material. The hollow metallic tubular part of the fuel rods, commonly known in

the nuclear industry as cladding, prevent the escape of materials, such as uranium dioxide

and fission gasses, from the interior of the fuel rod to reactor primary system.

The impurity material which collects on the exterior of the cladding is commonly

known as Chalk River Unidentified Deposit or “crud”. Although crud is a non-

homogenous material, crud has been found to be generally made of several elemental

components. The major components of crud can include, for example, iron, nickel, cobalt,

zinc, silica, chrome and manganese. As nuclear plant fuel performance is influenced by

crud deposition during normal plant operation as well as sequence and economics of

refueling and maintenance outages, it is necessary and important to analyze fuel

assemblies for the presence of crud to determine the nature and amount of the deposits.

To perform analysis of crud deposited on the fuel assemblies, samples must be

taken by mechanically scraping the exterior of the fuel rods from the nuclear power

plants. As discussed in the last two sections, SEM and/or XRD machines are used to

exam the plant crud. Crud deposits that are attached to the nuclear fuel rod cladding are

either tenacious with less pores or loss with more pores. These crud types are matching

with or covered by the theory of the four-regimes in crud discussed previously in this

chapter.

52

The nuclear power plant is typically operating at a constant power level. However,

as time going on, the fuel burnup is increasing. The fresh fuel assemblies, which have

higher pin powers at the beginning of the operating cycle, are having lower pin powers at

a later time of the operating cycle due to burnup increasing. The crud conditions due to

the local power changes are covered by the first three regimes in crud presented earlier in

this chapter. There is a need for an analysis method for crud deposits [47, 48], which will

adequately categorize crud obtained from scraped fuel rods from the nuclear power plants.

The method [47, 48] also provides for sorting the crud flakes into particle

fractions, and analyzing the crud with a number of analytical tools including a scanning

electron microscope (SEM). The method also provides steps for analysis of a crud flake

cross section on a nuclear fuel rod, determining a morphology of crystals of the flake,

determining a size of the crystals of the flake, and correlating elemental distributions of

the flake at various locations on the flake. All these information and the plant operating

conditions, including power history, can be utilized to determine the crud structure and

conditions when the fuel assemblies are in the reactor core.

The four-regime theory discussed previously can be lined up with the crud

information obtained in the plant crud analysis. Using the methodology presented in the

next section, crud thermal conductivity measurement can be performed and the calculated

crud thermal conductivity based on other direct measurements can be used in relevant

computer codes for plant crud simulation or application.

53

3.2.2 Crud Thermal Conductivity Measurement Methodology

Based on the four-regime theory and simulations of four-regime in crud at WALT

loop discussed in the precious sections, crud thermal conductivities can be calculated

based on other direct measurements as the following.

(1) Crud thermal conductivity and cladding temperature at nominal power operating

conditions for the first and second regimes in crud.

(2) Crud thermal conductivity and cladding temperature at crud bottom dryout

conditions for the third regime in crud.

(3) Crud thermal conductivity and porosity with zinc addition or other impurities in

the PWR primary system, which is for the fourth regime in crud.

The crud thermal conductivity measurement methodology and steps are discussed

in the following sub-sections.

3.2.2.1 Formulations of the Heat Flux and Temperatures

The formulations of the heater rod heat flux, cladding temperatures, crud layer

temperatures, and the fluid temperatures are derived and presented in this section. The

derivations are based on the basic laws of Joule's Law, Ohm's Law, Fourier’s Law, and

Newton's law of cooling. Some of the detail derivations are provided in the appendix.

54

3.2.2.1.1 Heat Flux Formulations in the Heater Rod

The heater rod in the WALT loop is heated by direct current. In direct current

resistive conductor, e.g. the heater rod in WALT, instantaneous electrical power is

calculated using Joule's Law,

IUQ = (3-1)

where, Q is the electric power, U is the potential difference or voltage on the two

ends of the heater rod, and I is the electric current through the heater rod.

Joule's law can be combined with Ohm's law ( elecIRU = ) to produce the

following equation:

elecRIQ 2= (3-2)

where, elecR is the electric resistance.

As shown in Figure 3-3, volume heat flux generated through the heater rod

cladding of the WALT loop by electric current is calculated using Equations (3-1) and (3-

2) as the following.

( ) ( ) HH

elec

clad Lrr

UI

Lrr

RI

V

Qq

×−××=

×−×==

21

22

21

22

2

'''ππ

(3-3)

55

In Equation (3-3), '''q is the volume heat flux generated in the cladding by electric

current. Q is the average heater rod power. cladV is the volume of cladding. HL is the heated

length of heater rod. I , elecR , and U are measured electric current through the heater rod

cladding, measured heater rod electric resistance, and measured voltage along the heater rod

respectively. 1r and 2r is the cladding inner and outer surface radius respectively as shown in

Figure 3-3.

3.2.2.1.2 Formulations of Cladding Outer Surface Temperature

The following equation is derived from Fourier’s Law (See Appendix A.1) and used to

calculate the heater rod cladding outer surface temperature. The inner cladding surface

temperature is measured using thermocouples embedded inside the heater rod.

⎥⎦

⎤⎢⎣

⎡+−⎟⎟

⎠

⎞⎜⎜⎝

⎛×−= 2

22

12

12112 ln2

4'''

rrr

rr

k

qTT (3-4)

In Equation (3-4), 1T is the thermocouple measured heater rod inner surface temperature.

2T is the calculated heater rod outer surface temperature. 1r and 2r are the inner and outer

radius of the heater rod cladding respectively. k is the thermal conductivity of the heater rod

cladding. '''q is the volume heat flux generated in the cladding by electric current, which is

calculated using Equation (3-3).

56

Figure 3-3a WALT Loop Heater Rod Cross Section View

, 0≥φ

57

Figure 3-3b WALT Loop Heater Rod Side View

, 0≥φ

58

3.2.2.1.3 Formulations of Crud Outer Surface Temperature

The following equation is derived from Fourier’s Law (See Appendix A.2) and used to

calculate the crud outer surface temperature.

2

323 ln

2 r

r

kL

QTT

dH

×−=π

( )2

32

12

22 ln

2 r

r

k

rrqT

d

×−′′′

−= (3-5)

In Equation (3-5), 3T is the crud layer outer surface temperature. 2T is the crud layer

inner surface temperature, which is assumed the same as the calculated heater rod outer surface

temperature as in Equation (3-4). 3r is crud layer outer surface radius. 2r is the crud layer inner

surface radius, which is the same value as in Equation (3-4). Q is the average heater rod power,

which is the same as in Equation (3-3). HL is the heater rod heated length, which is the same as

in Equation (3-3). And dk is the overall thermal conductivity of the crud.

3.2.2.1.4 Formulations between Crud Outer Surface and Fluid Temperatures

It is convective heat transfer from the crud outer surface to fluid, the crud outer surface

and water temperatures follow the following equation, which is based on the Newton's law of

cooling.

hrL

QTT

H 334 2π

−=( )

hr

rrqT

3

21

22

3 2

−′′′−= (3-6)

59

In Equation (3-6), 3T is the crud layer outer surface temperature, which is the same as in

Equation (3-5). 4T is the water temperature in the heater rod surrounding channel, which is

calculated based on the inlet and outlet RTD temperature measurement. 3r is the crud layer outer

surface radius as given in Equation (3-5). Q is the average heater rod power, which is the same

as in Equation (3-3). HL is the heater rod heated length as presented in Equation (3-3). h is the

convective heat transfer coefficient at the outer surface of the crud layer.

3.2.2.2 Methods to Solve Crud Thermal Conductivity Equations

From Equations (3-4) through (3-6), the following equation is obtained:

hrL

Q

r

r

kL

Qrr

r

rr

k

qTT

HdH 32

322

21

2

12114 2

ln2

ln24

'''

ππ−×−

⎥⎥⎦

⎤

⎢⎢⎣

⎡+−⎟⎟

⎠

⎞⎜⎜⎝

⎛×−= , or

( ) ( ) ( )hr

rrq

r

r

k

rrqrr

r

rr

k

qTT

d 3

21

22

2

32

12

221

22

2

12114 2

ln2

ln24

''' −′′′−×−′′′

−⎥⎥⎦

⎤

⎢⎢⎣

⎡−+⎟⎟

⎠

⎞⎜⎜⎝

⎛×−= (3-7)

There are two unknowns in Equation (3-7), kcrud, and h. Two independent specific

equations can be obtained by applying Equation (3-7) to either different WALT loop operating

conditions or different heater rod inner diameters. Then, these two independent specific equations

can be used to solve the overall crud thermal conductivity (kd) at the associated WALT operating

conditions.

When applying Equation (3-7) to different operating conditions, it is called “perturbation

method”. When using different geometries, it is called “changing geometry method”. Both

methods are discussed in the following two sub-sections.

60

3.2.2.2.1 Mathematic Solutions

To solve Equation (3-7), we define the following terms as:

41 TTt −=Δ

⎥⎦

⎤⎢⎣

⎡+−⎟⎟

⎠

⎞⎜⎜⎝

⎛×= 2

22

12

121 ln2

4

'''rr

r

rr

qA

2

3ln2 r

r

L

QB

H

×=π

32 rL

QC

Hπ=

Thus, Equation (3-7) becomes,

h

Ck

Bk

Atdcladding

111 ×+×+×=Δ (3-8)

Consider two WALT test cases as shown in Figures 3-4 and 3-5, which have two

different test conditions A (with coefficients of 1A , 1B , 1C ) and A′ (with coefficients of

2A , 2B , 2C ). The following specific equations can be obtained from Eq. (3-8).

h

Ck

Bk

Atdcladding

1111111 ×+×+×=Δ (3-9)

h

Ck

Bk

Atdcladding

1112222 ×+×+×=Δ (3-10)

61

From Equations (3-9) and (3-10), we can solve for dk as the following by

assuming claddingk is known.

claddingd kCBCB

CACA

CBCB

CtCt

k

11

1221

1221

1221

1221 ×−−−

−Δ−Δ= (3-11)

where, 01221 ≠Δ−Δ CtCt , or

01221 ≠− CBCB , or

01221 ≠− CACA .

This method of obtaining the crud thermal conductivities is an important part of

the dissertation. To perform a detailed error analysis is very important for future

applications. It is suggested that more data points be fit with a regression equation in

order to obtain more representative crud thermal conductivity results. Some sample error

analysis is presented in Chapter 5.

3.2.2.2.2 Perturbation Method to Solve Crud Thermal Conductivity Equations

In order to get independent specific equations from Equation (3-7), operating conditions

can be slightly changed. This is called “perturbation method”. Using this method, the state of the

operating parameters can be perturbed during the WALT loop operation.

Heater rod power perturbation: qqq ′′′Δ±′′′→′′′

where, q ′′′Δ is small.

62

Two sets of equations are obtained at fluid temperature conditions of q ′′′ and qq ′′′Δ±′′′ .

Accordingly, as shown in Figures 3-4 and 3-5, the same geometry data and other operating

parameters are measured and recorded in the WALT loop data acquisition system. After getting

all the relevant data, only two unknowns, kd, and h, are left in the two independent equations. It is

reasonable to assume that the changes of kd, and h are small in these two equations. Thus, these

two unknowns, kd, and h, are kept the same in the two equations. Finally, kd, and h can be solved

from the two independent equations, e.g. Equations (3-9) and (3-10) to Equation (3-11), by

assuming claddingk is known. The calculated crud thermal conductivity results are provided in

Chapter 5.

63

Figure 3-4 WALT Test Case with Condition A

64

Figure 3-5 WALT Test Case with Condition A′

65

3.2.2.2.3 Changing Geometry Method to Solve Crud Thermal Conductivity

Equations

Different from those discussed in the last section, alternative ways may be used to get

independent specific equations from Equation (3-7) by changing the heater rod geometry, which

is called “changing geometry method”. Under this method, the heater rod diameters or crud outer

surface geometry can be manufactured or grown somewhat differently for different WALT loop

operation cases.

Heater rod outer diameter: 222 rrr Δ−→

where, 2rΔ is small.

Two sets of equations are obtained at different heater rod outer diameters of 2r and

22 rr Δ± . Other geometry data and operating conditions can be retained exactly the same for

these two cases (as illustrated in Figures 3-4 and 3-5), which are measured and recorded in the

WALT loop data acquisition system. As discussed previously, only two unknowns, kd, and h, are

left in the two independent equations. And these two unknowns, kd, and h, can be solved from the

two equations, e.g. Equations (3-9) and (3-10).

This is an ideal case for future applications because only the heater rod outer diameter is

changed. Since all other geometry data and operating conditions are maintained the same (by

growing proper thickness of crud layer, e.g. 333 rrr Δ−→ ) for two WALT operation cases, kd,

and h can be controlled identical in the two experimental runs on WALT loop. Detail discussions

and solutions are provided in Chapter 5.

66

3.2.2.2.4 Direct Solution Method Using Existing Correlations

Meanwhile, another reasonable option is to use Dittus Boelter and/or Thom

correlations to calculate the heat transfer coefficient, h, at forced convection and

subcooled boiling conditions respectively. The calculated heat transfer coefficient of h

can be substituted to Equation (3-7) or (3-12) to solve the crud thermal conductivity of

kcrud. Using this method, the calculated overall crud thermal conductivity of dk should

be consistent with or very close to those calculated using the first method (i.e. using two

specific equations of Equations (3-9) and (3-10)). This method is consistent with the logic

used in the EPRI BOA code, which is utilized in Chapter 5. However, when the flow or

operating conditions are outside the applicable ranges of the Dittus Boelter or Thom

correlations, the method mentioned previously is suggested for the calculation of dk .

Equation (3-7) can be rearranged to solve dk directly,

h

C

k

At

Bk

cladding

d

−−Δ= (3-12)

For forced convection heat transfer, h is calculated using Dittus-Boelter Equation

for pipe flow [55].

( ) ( )n

mbmbfluid

eNu NN

k

hDN Pr

8.0Re023.0=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛= (3-13)

67

The conditions on Equation (3-13) are:

(a) Fluid properties evaluated at the arithmetic mean bulk (mb) temperature:

2

21 bbmb

TTT

+= (3-14)

(b) Reynolds number greater than 10,000.

(c) Prandtl number between 0.7 and 100.

(d) n = 0.4 for heating and 0.3 for cooling.

(e) L/D > 60.

For nucleate boiling, Thom correlation is utilized to calculate the crud outer

surface temperature.

( ) 2

072.0

1260

⎥⎦

⎤⎢⎣

⎡ −×=′′ satwall TTe

qP

(3-15)

where, q ′′ is the heat flux at the heater rod outer surface (Btu/hr-ft2). P is the WALT

system pressure (psia). wallT and satT are cladding wall and saturation temperatures (°F),

respectively. SI units must be converted into English units for using Equation (3-15).

Then, the heater transfer coefficient, h, is calculated using Newton's law of

cooling. It has been noted that wall superheats in sub-cooled boiling region are not very

high and, consequently, it is frequently deemed adequate to extend the heat transfer

correlation from nucleate boiling region down into the sub-cooled boiling region. To do

this is slightly more conservative [55] because of the slight overestimates of the wall

temperatures by using Thom correlation.

68

3.3 Summary of Theoretical Model and Method

In the previous sections, the four-regime theory, simulations of the four-regime in

crud at WALT loop, and applications of the four regimes at PWR operating conditions

have been discussed. The crud thermal conductivity measurement method has been

presented as well. The four regime theory works as a guideline for the WALT loop

experiments and the crud thermal conductivity calculations. Meanwhile, the PWR typical

operating conditions are the base lines for the WALT loop tests.

Finally, the crud thermal conductivity can be calculated based on the WALT data

using equations and steps provided in Section 3.2.2.2. In general, all the following

regimes and conditions can be covered for crud thermal conductivity measurements.

a. Crud thermal conductivity at nominal or typical PWR operating conditions,

which is related to the first and second regimes in the four-regime theory in

crud.

b. Crud thermal conductivity at high PWR transient power conditions, which is

related to the crud dryout regime in the four-regime theory in crud.

c. Crud thermal conductivity and porosity with impurities in the PWR primary

system that may fill the crud pores, which is the fourth regime in the four-

regime theory in crud.

69

In summary, the theoretical model and methodology developed and documented

in this chapter can be utilized to measure crud thermal conductivity at different PWR

operating conditions. The calculated crud thermal conductivity based on other direct

measurements is a key factor to improve the heat transfer model in the BOA computer

code. A benchmarking summary work is presented at the end of Chapter 5.

70

Chapter 4

WALT LOOP DESIGN/BUILDUP AND EXPERIMENTAL STUDY

The crud formation and crud thermal conductivity are investigated experimentally.

To this end, one of the main efforts in this dissertation is to build the Westinghouse

Advanced Loop Tester (WALT) as a co-worker of the team. The WALT loop design,

buildup, and update as well as the procedure of experimental study are discussed in this

chapter.

4.1 WALT Loop Design/Buildup and Update

The preliminary design for Westinghouse Advanced Loop Tester (WALT) was

initiated in late 2003 and early 2004. Since then, a lot of efforts are made on the WALT

loop build up and updates. The complete WALT loop design description and update are

presented in the following sub-sections.

4.1.1 The First Completed WALT Loop Design Description

The first version of the Westinghouse Advanced Loop Tester (WALT) loop was

completed in October 2005. The first completed WALT loop design description and

some background information are presented below.

71

4.1.1.1 Background Information

Researchers have performed many studies [13, 19, 20, 21, 49] to try to understand

crud formation on the fuel pin clad surfaces, which is observed in pressurized water

reactors (PWR) as a result of sub-cooled nucleate boiling and precipitation reactions.

Crud deposits, if present in combination with high concentration of boron species, will

result in axial offset anomaly (AOA), and/or, if the deposits are thick enough at high heat

flux level, may cause fuel rod surface dryout.

The Westinghouse Advanced Loop Tester (WALT) at the George Westinghouse

Science and Technology Center (STC) is being used to examine the effects of crud

formation on fuel pin cladding. The STC crud test loop with a single heated rod is

utilized for convection and sub-cooled boiling tests with and without crud deposition.

This test loop can also be used to evaluate how plant chemistry changes may affect crud

deposition under PWR operating conditions.

4.1.1.2 Description of the WALT Loop

The WALT loop was completed in October 2005 based on the WALT loop design

review meeting [42] and other information and suggestions [15, 16]. A picture of the first

completed test loop is shown in Figure 4-1. This test facility has the capability of

performing sub-cooled boiling experiments at PWR reactor operating conditions [49].

Additional experiments are currently being performed at STC for some other projects.

72

Figure 4-1 Picture of the Completed WALT

73

The Westinghouse Advanced Loop Tester (WALT) at STC has a uniformly

heated single rod and is being used to simulate a fuel rod in the upper region of a

Pressurized Water Reactor (PWR). The WALT test loop has been designed to meet

current and future requirements for measuring crud deposition and dryout [41, 42], as

well as evaluating of plant chemistry effects on crud deposition on PWR fuel pins.

Consistent with Figure 4-1, a detail diagram of the WALT test loop is shown in

Figure 4-2, which includes heater rod, valves, pumps, heat exchangers, make-up tank,

flow meter, level indicator, pressure control system, and protection system. All

thermocouples are isolated from the power supply. Thermocouples, flow meters, pressure

gauge, water level indicators for the make-up tank, current and voltage meters are

reset/calibrated and recorded for each test.

Since the WALT loop was completed in October 2005 at the Westinghouse

Science and Technology Center (STC), a number of recent upgrades [43, 44] of the test

loop have increased the pressure and thermal stabilities. The temperature conditions of

the fluid in the test loop are mainly based on the heater rod (tube) fixed by a built-in end

on top of the autoclave and the pre-heater from outside of the autoclave. The power

supply of the heater rod is controlled by adjusting the DC current, which are calculated

based on the heater rod resistance and dimension for certain power level using the revised

Excel spreadsheet [17]. The sub-cooled boiling conditions can be achieved by increasing

the electronic power to the heater rod (tube).

74

Figure 4-2 Layout of the WALT Loop

75

The fluid conditions in the test loop (Figure 4-2) are also controlled by the pump

flow, pre-heater, the heat exchanger with air cooling (Figure 4-2, [41]), and the system

pressure controlled by the pressure control system with a large make-up tank. The flow

rate is measured by a flow meter. A nitrogen line and a safety relieve line are used to

prevent the system over pressure. The system pressure is measured by a pressure gauge

connected to the Nitrogen line. The thermocouples on the heater tube and RTDs in the

inner-tube of the autoclave are connected to the recorder through the top end of the

autoclave by passing the thermocouple or RTD wires through seals.

A rapid power shutdown system has been installed in the test facility, based on

the measured heater rod cladding temperature, pressure, power (current), inlet

temperature, and flow rate, that would immediately trip the power supply to the heater

rod. For example, if the heater rod surface temperature is approximately 86 to 122 °F

superheated at specified pressure (e.g. 2250 psia, or15.5 MPa), the power shutdown

system would also immediately trip the Rectifier DC power supply. If either the system

pressure is too high/low (e.g. (1974 psai) 13.6 MPa ≤ Pressure ≤ 20.7 MPa (3005 psia) )

or the flow rate is too low (e.g. Flow ≤ 3.15 10-4 m3/s, 1.112x10-2ft3/s), the Rectifier DC

power supply to the heater rod is also shutdown. The general idea is to maintain the

integrity of the test loop and to preserve the heater rod surface for post test examination.

As shown in Figure 4-2, the test loop pressure is controlled by a small pressure

control system and an independent pressurizing pump. The heat is removed from the loop

by a heat exchanger with an air-blown cooler. The autoclave in the WALT loop is

surrounded by electrical heaters that control the temperature of the coolant entering the

inner chimney tube. High temperature coolant is surrounding the chimney tube. The

76

coolant temperature difference between inside and outside the chimney tube is only few

degree Fahrenheit. Due to this small coolant temperature and the chimney tube-in-tube

design with an air gap, the heat loss from inside the chimney tube to the outside is

negligible. A RTD is utilized to measure the coolant temperature at the inner tube

entrance inside the autoclave.

The main pump can be powered off and the air-blown cooler fan turned on during

natural circulation tests. The top right corner in Figure 4-2 shows the main heat

exchanger, which is located at a higher elevation than the heater rod section in order to

have natural circulation flow. The natural circulation flow rate can be adjusted by closing

or opening Valve-1 in the main loop. In addition, the flow can be accurately measured by

means of the flow meter in the main loop. The power distribution is uniform in the axial

direction in the heater rod. The heater rods are constructed from standard PWR fuel

cladding tubes. The WALT loop can simulate the typical PWR operating conditions.

4.1.1.3 The Heater Rod and the Test Section in WALT Loop

The heater rod of the WALT loop is shown in Figure 4-3(a). The total heated

length for the original design is 11.1 inches. The heater rod in WALT loop is used to

model one of the top grid spans of a PWR fuel assembly as shown in Figure 4-3(b). As

can be seen in Figure 4-2, a large diameter pipe is used as the inlet to the autoclave in

order to allow for better mixing of water flowing into the inner chimney tube that

surrounds the heated rod.

77

(a) Heater Rod in WALT Test Loop

(b) Westinghouse PWR Fuel Assembly

Figure 4-3 Heater Rod of the WALT Loop and

Westinghouse PWR Fuel Assembly

78

The inner tube is a tube-in-tube design with an air gap. The air gap reduces the

heat loss from the water in the flow channel to the bulk autoclave solution. As shown in

Figure 4-2, there are two thermocouples for measuring the inner tube inlet and outlet

temperatures. The WALT can be operated with either forced flow or natural circulation.

The heater rod is rated for a thermal power of 31.8 kW (108,533 Btu/hr). The

maximum operating pressure is 20 MPa (3000 psia). The maximum linear velocity rate

can be as high as 6 m/s (19.7 ft/s) in the WALT loop. Key parameter ranges of the

WALT loop are summarized in Table 4-1.

The power distribution is uniform in the axial direction in the heater rod. The

heater rods are constructed from standard PWR fuel cladding. The WALT loop will

simulate the typical PWR operating conditions given in Table 4-2.

The heater rod detail design is shown in Figure 4-4. There are four thermal wells

built in the heater rod. Four thermocouples are inserted to the thermal wells to measure

the heater rod cladding temperatures.

As shown in Figure 4-4, the original heater rod design for the WALT loop has

several unique features, which are summarized below:

1. uniform axial power shape distribution;

2. movable thermocouples to measure the inside wall temperatures axially;

3. heated length is long enough to model the PWR fuel assembly top grid span;

4. short copper rods are inserted into the heater rod ends to reduce the electric

resistance;

5. Ceramic material filled the inside of the heated length section of the heater rod to

physically support the rod against system pressure.

79

Parameters

Upper Limits

Or Design Values

System pressure, MPa 20.0 (3000 psia)

DC power supply, kW 100.0 (341300 Btu/hr)

Heat exchanger, kW 31.8 (108,533 Btu/hr)

Flow, m/s 6.0 (19.7 ft/s)

Inlet temperature, 0C <Tsat

Axial Power Shape Uniform

Heater rod material A zirconium alloy cladding tube filled

with ceramic material

Heater rod length, m 0.28 (11.1 inches)

Heater rod OD, mm 9.5 (0.374 inches)

Heater rod ID, mm 8.6 (0.329 inches)

Test section shroud size, mm ID of 16.7 (0.6575 inches)

Test section flow area, mm2 149.0 (0.231 in2)

Table 4-1 Key Parameters of the Westinghouse WALT Loop

80

Parameters Normal Values

Reactor System Pressure, MPa 15.5 (2250 psia)

Reactor Core Power, MWth 3411.0 (1.16x1010Btu/hr)

Core Inlet Flow, m/s 4.5 (14.7636 ft/s)

Core Inlet Temperature, °C 290.5 (555 °F)

Axial Power Shape Non-Uniform

Fuel Rod Cladding Material zirconium alloy

Fuel Rod Heated Length, m 3.66 (144 inches)

Fuel Rod OD, mm 9.5 (0.374 inches)

Fuel Rod ID, mm 8.6 (0.329 inches)

Table 4-2 Typical PWR Operating Conditions

81

Figure 4-4 Heater Rod Design of the WALT

82

Before the WALT loop is updated, over ten rods have been built for the WALT

loop project. Electric resistance measurements have been performed on each rod before

the heater rods are installed in the WALT loop so that the rod heat flux can be accurately

determined.

4.1.2 WALT Loop Update

To understand crud formation on the fuel cladding surfaces, many studies have

been performed in pressurized water reactors (PWRs) or in test simulations. Crud layers

are believed to form on fuel pin surfaces as a result of sub-cooled nucleate boiling and/or

precipitation reactions combined with particle deposition. The Westinghouse Advanced

Loop Tester (WALT) at the Westinghouse Science and Technology Department in

Pittsburgh, PA, has been used to examine the effects of crud formation on PWR fuel pin

cladding. This facility is designed to operate at typical PWR conditions. The WALT loop

operates with a single heated rod. It was built in October 2005 [50] and was updated in

2007.

A diagram of the updated WALT loop is provided in Figure 4-5. This updated test

facility has been utilized to perform sub-cooled boiling thermal hydraulic experiments as

well as to evaluate how plant chemistry changes may affect crud deposition under PWR

operating conditions [30, 50, 51, 52, 53]. By studying the WALT loop generated crud

morphology and its effect on cladding temperature at different power levels a better

understanding of dryout and the CIPS or AOA phenomena can be obtained.

83

Figure 4-5 Layout of the Updated WALT Loop

84

Figure 4-5 describes the WALT loop and the updated test equipment used. Figure

4-6 shows photographs of the chemical addition system.

A number of recent upgrades of the WALT loop have increased the pressure

stability and thermal stability during normal operation. These updates, as shown in

Figures 4-5 and 4-6, include the following items.

(a) System pressure stabilization,

(b) Pressurizer pump improvement,

(c) Chemical addition-up system,

(d) Heater rod update with He pressurization,

(e) More completed protection system.

After the above improvements, the WALT test loop is well suited to meet current

and future test requirements for measuring crud deposition and dry-out, as well as

evaluation of plant chemistry effects on crud deposition under PWR operating conditions.

A major improvement to the loop was active pressure control. The pressure in the

loop is set by the balance of flows into and out of the system. The flow into the system is

accomplished with a charging pump which operates at a constant flow rate of typically 80

ml/min. The original pump was a single-piston Whitey pump which imposed a

sinusoidal pressure variation with a frequency of 1 Hz. The original pump was replaced

with a Varian SD-1 dual piston microprocessor-controlled pump with a flow noise of less

than 0.3 percent. The original mechanical back-pressure regulator was replaced with an

active control system which measures pressure and then adjusts the setting on an air-

loaded backpressure regulator several times per second.

85

Figure 4-6 Chemical Addition-up System

86

This new pressurizing pump together with the boron and lithium make-up system

is utilized to inject simulated PWR coolant from the large storage tank as shown in

Figure 4-6. A typical concentration of 1000 ppm for boron and 2.2 ppm for lithium are

used to simulate the chemistry conditions of normal PWR operating conditions.

The WALT loop has a more comprehensive protection system now. In addition to

the protection parameters mentioned previously, the updated protection system for the

WALT loop has the following items.

(a) High chimney tube inlet temperature setpoint,

(b) Power supply voltage setpoint,

(c) Heat exchanger fan speed setpoint,

(d) Protection system ON/OFF switch alert light.

The heater rod was updated to use helium pressurization instead of using a solid

ceramic material. The internal rod pressure can either be set manually to a single value or

it can be set to follow the system pressure. Equalization of the tube pressure with the

system pressure is accomplished with a high pressure bladder in a 3.8 liter pressure vessel.

A typical updated heater rod design is shown in Figure 4-7.

87

Figure 4-7 The Updated Heater Rod for WALT

15.2" (38.6cm)

OD=0.374" (0.95cm)

ID=0.329" (0.84cm)

1.75" (4.4cm) 1.75" (4.4cm)

Zr-4 Plug with FourSmall Holes Near

the EdgeZr-4 Plug

Ceramic Materialand He

Pressurized

Ø 0.374" (0.95cm) Ø 0.374" (0.95cm)

Thermal wellHoles X 4

(Symmetrically)

11.7" (29.7cm)

Ø 0.2" (0.51cm)

88

4.2 Experimental Procedures

In order to measure the crud thermal conductivity, a number of steps are required

to perform the experiments, to record the WALT loop operating conditions, to measure

the crud thickness and porosity, and to measure the crud chemical components. The brief

procedures for these measurements are discussed in the following sections.

4.2.1 WALT Loop Operations

As discussed in Section 4.1.1.2, an immediate power shutdown system has been

installed in the WALT loop test facility. This protect system is activated based on the pre-

set limits against the measured values for heater rod cladding temperature, system

pressure, heater rod power (current), fluid inlet temperature, and flow rate. As expected,

the WALT loop is typically at normal operating conditions.

In all cases, the general idea is to maintain the integrity of the test loop and to

preserve the heater rod surface for post test examination. The following operating

procedure is recommended for the WALT loop normal operation.

(1) Prepare and make the heater rod (Figure 4-7).

(2) Assemble the heater rod with the test section (Part (a) of Figure 4-3).

(3) Put the test section to the WALT loop and seal the loop and put insulation

material on piping and all equipments as shown in Figure 4-1.

(4) Connect all the thermocouples and Resistance Temperature Detectors (RTD)

with the data acquisition system, then connect the power supply cable from

89

the rectifier power supply to the heater rod.

(5) Start the data acquisition computer system.

(6) Pressurize the heater rod to 10.3 MPa (gage pressure), or 1500 psig.

(7) Fulfill the WALT loop system with water and start the main pump and the

pressurizing pump. As shown in Figure 4-2, the test loop pressure is

controlled by a small pressure control system and an independent pressurizing

pump.

(8) The autoclave in the WALT loop is surrounded by electrical heaters that

control the temperature of the coolant entering the inner chimney tube. Start

the pre-heater (as shown in Figure 4-5) to heat up the test loop to desired high

temperature. A relative low heater rod power (i.e. ≤ 2.5 kW (8533 Btu/hr))

can be turned on in order to speed up the system heat up.

(9) When the WALT loop system is reaching steady state, turn off the heater rod

power temporally in order to celebrate the thermocouples at isothermal

conditions.

(10) Re-start the heater rod power step-by-step in order to get a boiling curve at

different power levels. If a boiling curve is not desired, turn on the heater rod

power and then gradually increase the power to the desired power level for

steady state operation. The heat is removed from the loop by a heat exchanger

with an air-blown cooler. The speed of the air-blown fan can be adjusted for

different power levels of the heater rod.

(11) Turn on the valves to initiate the boron and lithium supply, which is to

simulate the chemistry conditions at normal PWR operation.

90

(12) When WALT loop is in steady state operation, inject crud solution

periodically in order to generate crud layer on the heater rod surface.

(13) Inject zinc from a separate tank, if zinc addition is simulated.

(14) Inject crud solution with EDTA (Appendix B). After seeing the cladding

temperature reaching a desired value, stop crud injection.

(15) Keep running the WALT loop for a desired time.

(16) A boiling cure can be obtained at the end of the WALT loop operation.

(17) Shutdown the heater rod supply, then shutdown the main pump

immediately.

(18) Quickly blow down all water from the WALT system in order to preserve

the heater and crud layer.

(19) Let the WALT loop cooled down over night.

(20) Open up the test section and carefully disassemble the heater rod for the

test section. Then, this heater rod is ready for SEM and XRD exams.

(21) In addition, attentions are needed for the following items.

• The main pump can be powered off and the air-blown cooler fan turned on

during natural circulation tests. The top right corner in Figure 4-2 shows the

main heat exchanger, which is located at a higher elevation than the heater rod

section in order to have natural circulation flow. The natural circulation flow

rate can be adjusted by closing or opening Valve-1 in the main loop.

• Main pump cooling shall be turned on before the start the main pump.

• The rectifier circuit shall be insolated from ground.

• The heater rod shall be preserved for SEM and XRD exams.

91

4.2.2 Measurement of Heater Rod Deposit Thickness and Structure

The crud thickness and structure measurements are very important for reactor normal

operations, since a localized thick layer of crud may cause fuel rod overheat and cladding rupture.

The purpose of the WALT loop tests in Science and Technology Center (STC) of Westinghouse

is to simulate plant crud, and the WALT loop test results can be used to benchmark the crud

dryout model. The measurement method for crud deposit thickness can be metallographic (i.e. by

optical flake measurement using microscopes or metallograph) or removal of crud from the fuel

rod surface (i.e. by weight and density measurements [18]). For the WALT crud test, a SEM

machine or Depth from Defocus (D.F.D.) method is used to measure the crud thickness [15]. The

method descriptions of SEM and D. F. D. in Sections 4.2.2.1 and 4.2.2.2 are mainly from

discussions with and/or writings of Dr. W. A. Byers.

4.2.2.1 Depth from Defocus (D.F.D.) Method

Deposit thickness was measured in most cases by the focus micrometer method, which is

also known as the D.F.D. (Depth from Defocus) method. In this method, a thin scratch was first

made in the heater rod deposit. Care was taken to scratch down to the level of the zirconium

oxide, and to not flake away the outer deposit layers. The rod was placed under a microscope and

the objective was set to produce a moderately high magnification, typically 200X. The focus

micrometer was then moved, and the height at which the top of the deposit and the bottom of the

scratch are in focuses. The number of micrometer units between the top and bottom of the deposit

was converted to depth through reference to a thickness standard. By making multiple scratches

in the deposit, multiple thickness measurements could be made covering the entire length of the

heater rod.

92

The above process was done both manually and using image analysis to determine focus.

The system used to do the image analysis was a Keyence VHX-600 with a VH-Z20R lens.

4.2.2.2 Scanning Electron Microscope (SEM) Method

All deposits are cross sectioned to examine their internal structure and to confirm the

thickness measured in Section 4.2.2.1 using scanning electron microscope (SEM) as shown in

Figure 4-8.

4.2.2.2.1 Porosity and Thickness by Cross Sectioning

Samples for internal structure determination are taken five inches from the rod top weld.

This is the same location that rod internal temperature measurements are made during operation.

In a few cases, a deposit sample could be flaked from the heater rod using a razor blade or by

slightly bending the heater rod. When an intact, undamaged flake was obtained it was mounted in

epoxy, using a magnetic field to orient the flakes. The flake was sectioned by first grinding the

epoxy mount, and then polishing the mount with a series of silicon carbide papers, ending at 1200

grit. The finish polish was done with a 0.5-micron diamond paste.

When an intact sample of the deposit could not be obtained by the above procedure, the

entire rod was cross sectioned along with the deposit. Before the rod was cut, the deposit was

sprayed with Krylon to keep the deposit intact. A 0.5 inch section was then cut from the rod

using an abrasive cut-off wheel. The rod segment with deposit was then mounted in epoxy,

ground, and polished in the same manner as the individual flake samples.

The epoxy used for mounting was Loctite Hysol Resin RE2038. It was cured with

Loctite Hardener HD3404 at a temperature of 125°F for a period of at least 12 hours.

93

Figure 4-8 A SEM Machine of SUPRA 40 at Westinghouse

94

The cross sectioned deposits are examined with a scanning electron microscope (SEM) as

shown in Figure 4-8. Two different electron microscopes are used in this work, a Carl Zeiss

SUPRA 40 SEM and an ASPEX Personal SEM. Images are collected primarily in the

backscattered SEM mode, which gave better contrast between pore and solid deposit regions.

The detector gain and offset are selected so that there was contrast between epoxy, nickel ferrite

particles, and zirconium oxide particles.

After the SEM images or pictures are obtained, crud porosity is measured. The SEM

image is imported into PAXIT (a computer program for image analysis). Then, representative

region of interest (ROI) was defined. And intensity threshold is selected to distinguish between

pore and particle. Finally, the porosity is calculated for ROI as pore area / total area. A sample

picture is provided in Figure 4-9. The porosity is 0.44 in this example.

It is reasonable to conclude that a random cross section pore area ratio is representative

for the 3-D porosity. Especially, when the same method is utilized in porosity measurement for

crud from nuclear power plants and in porosity calculation in the BOA computer code, the bias

are canceled out. As long as the method is consistent is all case, this is reasonable to use.

4.2.2.2.2 Elemental Content

Elemental concentrations are also measured during the SEM analysis using EDS (Energy

Dispersive x-ray Spectroscopy). The electron beam was restored over a broad area of the deposit

(typically 50 x 50 microns) while collecting an X-ray spectrum for the EDS analysis.

95

Figure 4-9 A Sample SEM Image to Define Pore Area (green) for Heater Rod #22B

96

4.2.3 XRD Exams

Powder X-ray diffraction (XRD) was used to identify the structure of the crystalline

species in the simulated crud. A SCINTAG XDS2000 X-ray diffraction system (shown in Figure

4-10) was used to obtain the data from the crud deposit on the WALT heater rod. A high purity

germanium energy dispersive x-ray detector restricted the response to only Cu K X-rays.

Typically, the angular range between 10 and 90 degrees 2-theta was examined. Phases are

identified by comparison of the experimental diffraction patterns to the ICDD PDF-2 diffraction

library [54]. Elemental data from the SEM/EDS analyses are used in selection of matching

phases.

97

Figure 4-10 A SCINTAG XDS2000 X-ray diffraction system at Westinghouse

98

Samples are prepared for XRD by scraping the heater rod with a razor blade and

collecting the removed deposits on wax paper. The rod was scraped at a location centered 3 cm

below the tube center unless noted otherwise. The area scraped was roughly 0.7 x 3 cm. The

scraped powder was then sprinkled across a cello-grease-covered glass slide. The cello-grease

was obtained from Fisher Scientific, Cat. No. C602-100. The glass slide was then mounted on a

magnetic sample holder and leveled.

The calibration of the XRD system was checked before and after the deposit samples by

comparison to metallic silicon standard reference material 640B from NIST, the National Institute

of Standards and Technology.

4.2.4 Data Reductions

All data from Sections 4.2.1 through 4.2.3 are analyzed. These data include data

recorded in the WALT data acquisition system, crud structure data from SEM exams, and

the structure of the crystalline species in the simulated crud from XRD exams. All these

relevant data are analyzed together in Chapter 5 for the selected test cases.

4.3 Selections of Test Cases

Based on preliminary studies, the following cases listed in Table 4-3 are selected

for detail analyses in the next chapter.

99

Heater Rod # Crud Thickness (Microns)(1) Porosity(2)

22B 57.3 0.44

30 75.0 0.44

34 42.7 0.39

39 29.9 0.25

43 53.7 0.45

44 29.7 0.35

Notes: (1) The local crud thickness is accurately measured (error < 1%). However, the

crud thickness varies axially and azimuthally due to the fluid condition differences. The

crud thickness listed in this table is the azimuthally averaged crud thickness at the axial

location, where the thermocouples are located. (2) It is assumed that the percent pore area

is the same as the percent pore volume on a random cross section of randomly distributed

particles [73]. The same assumption is utilized in the BOA computer code and the plant

crud data treatment.

Table 4-3 Selected Cases of the WALT Crud Tests

Chapter 5

EXPERIMENTAL RESULTS AND BENCHMARK

In this chapter, the crud-regime determination is discussed first. Crud thermal

conductivity is calculated based on the WALT loop operating condition data, SEM data,

and XRD data. Crud structure comparisons between plant and WALT data is presented to

demonstrate that crud from the WALT loop is representative of the PWR plant crud.

Finally, a sensitivity study is performed using the EPRI BOA computer code.

5.1 Determination of Crud-regimes

The crud-regimes are determined using calculated boiling rate, measured data (i.e.

cladding temperature), as well as post-test Scanning Electron Microscope (SEM) exam.

The first three regimes in crud, i.e. liquid flooding, vapor and liquid mixture, crud pore

dryout, are determined by following the calculated boiling duty levels from none, low

value, to high value. As shown in Figure 5-1, the fluid conditions inside the crud pores is

in the regime of liquid flooding when the calculated boiling duty is zero. However, if the

calculated boiling duty is above zero, but not too high (e.g. ≤600 lbm/hr-ft2, or ≤0.814

kg/s-m2), the fluid conditions inside the crud pores is in the regime of vapor and liquid

mixture.

101

(Crud-regime IV is determined by the SEM exam.)

Figure 5-1 A Sample Boiling Curve for Determine Crud-regimes

A Sample Part ial Boiling Curve at WALT(Pr = 15.6 MPa, G = 0.301 kg/s-cm^2, T inlet = 332 Deg-C, OD = 0.86 cm)

332.0

334.0

336.0

338.0

340.0

342.0

344.0

346.0

348.0

350.0

352.0

0.010

.020.0 30

.040

.050.0 60.0 70

.080.0 90.0

100.0

110.0

Heat Flux, W/cm^2

Cla

d T

empe

ratu

re,

Deg

-C

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Boi

ling

Rat

e, k

g/s

-m^2

Clad Temp.

Boiling Rate

Crud-regime III

Crud-regime II

Crud-regime I

102

When the crud thickness are high enough (e.g. >3 mils, or >76.2 microns) and the

calculated boiling rate is high (e.g. >600 lbm/hr-ft2, or >0.814 kg/s-m2), the fluid

conditions inside the crud pores are in the regime of crud pore dryout. The crud thickness

is measured and confirmed using SEM after the WALT loop test. For the fourth crud-

regime, post-test SEM exam is utilized to measure the crud structure and porosity, which

is used to determined the forth crud-regime. The fourth crud-regime may be created by

zinc injection or other partials, such as silica, after or during crud is formed during

normal WALT loop or plant operations.

5.2 Crud Thermal Conductivity Calculations for Different Crud-regimes

For different crud-regimes, the crud thermal conductivity is calculated based on

the WALT loop operating data, i.e. heater rod power, flow rate, fluid inlet temperature,

and system pressure. Meanwhile, the heater rod and chimney geometries, crud structure

and crud thickness from the SEM exam are also utilized in the crud thermal conductivity

calculations.

In addition, the cladding electric resistance is measured in order to support the

crud thermal conductivity calculations. Powder X-ray diffraction (XRD) is used to

identify the structure of the crystalline species in the simulated crud. The XRD results are

used to identify the crud chemical components, which is a important factor in the crud

thermal conductivity analysis. In the following sections, the selected WALT data,

measured electric resistance results, SEM and XRD results are provided first. Then, crud

thermal conductivity calculations, result analyses, and uncertainty analyses are followed.

103

5.2.1 Selected Experimental Data or Results

In this section, the WALT experimental data, measured cladding electric

resistance results, SEM and XRD exam results are provided for the selected cases.

5.2.1.1 SEM Results for Selected Cases

All crud layers for the selected cases were cross sectioned to examine their

internal structure and to measure the thickness using scanning electron microscope

(SEM) as shown in Figure 4-8. Crud samples were taken from the axial location where

the internal thermal couples are located, i.e. about five inches from the top weld of the

heater rod. In some cases, a deposit sample is flaked from the heater rod outer surface

using a razor blade an/or by slightly bending the heater rod.

In some other cases, the entire rod is cross sectioned along with the deposit. As

discussed in Chapter 4, before the rod is cut, the deposit is sprayed with Krylon in order

to keep the deposit intact. In both methods, the rod segments with deposit or the flaked

samples were then mounted in epoxy, grounded, and polished for use in the SEM exam.

All the final polished samples were examined with a scanning electron

microscope (SEM). Images were collected primarily in the backscattered SEM mode,

which gave better contrast between pore and solid deposit regions. Elemental

concentrations were also measured during the SEM analysis using Energy Dispersive x-

ray Spectroscopy (EDS). The SEM images and EDS results for the selected case are

provided in Figures 2 through 13.

104

Figures 5-2, 5-4, 5-6, 5-8, 5-10, and 5-12 are SEM images for crud layers from

heater rod test #22B, 30, 34, 39, 43, and 44. From the six figures, we can see that the crud

is fairly uniform distributed for a short axial length. Using the scales shown in these six

figures, the crud thickness is measured for all the six cases.

In Figures 5-2, 5-4, 5-6, 5-8, 5-10, and 5-12, there is a box marked with “EDS”,

which indicates that the boxed area is performed EDS analyses. All the EDS analyses for

these six cases are shown in Figures 5-3, 5-5, 5-7, 5-9, 5-11, and 5-13. From these EDS

pictures, we can conclude that the main chemistry components in crud from the six

selected heater rods are Fe and Ni. Very little other elements are shown in Figures 5-3, 5-

5, 5-7, 5-9, 5-11, and 5-13. These are consistent with what are injected in the WALT test

loop for generating crud deposit.

105

Figure 5-2 A SEM Image for Heater Rod Test #22B (750x)

106

Figure 5-3 EDS Results for Heater Rod Test #22B (750x)

107

Figure 5-4 A SEM Image for Heater Rod Test #30 (750x)

108

Figure 5-5 EDS Results for Heater Rod Test #30 (750x)

109


110


111


112


113


114


115


116


117

5.2.1.2 XRD Results for Selected Cases

Powder X-ray diffraction (XRD) is used to identify the structure of the crystalline species

in the simulated crud. A SCINTAG XDS2000 X-ray diffraction system (shown in Figure 4-10) is

used to measure the structure of the crystalline species from the crud deposit on the WALT heater

rod. Typically, the angular range between 10 and 90 degrees 2-theta is examined. Phases were

identified by comparison of the experimental diffraction patterns to the ICDD PDF-2 diffraction

library [54], which is based on the room temperature (i.g. 68 °F, or 20 °C) and atmosphere

pressure (~14.7 psi, or 1.013 bars). Elemental data from the SEM/EDS analyses from Figures 2

through 13 were used in selection of matching phases.

Crud layer samples from the WALT loop heater rod outer surface were prepared by

scraping the heater rod with a razor blade and collecting the removed deposits on a wax paper.

The area scraped is roughly 0.7 x 3 cm. The scraped powder is then sprinkled across a cello-

grease-covered glass slide. The glass slide is then mounted on a magnetic sample holder and

leveled.

The calibration of the XRD system is checked at room temperature (i.g. 68 °F, or 20 °C)

and atmosphere pressure (~14.7 psi, or 1.013 bars) before and after the crud layer samples are

measured by comparison to metallic silicon standard reference material 640B from NIST, the

National Institute of Standards and Technology.

The measured XRD results for the selected cases are provided in Figures 14

through 19. These results are utilized as chemistry conditions in Section 5.2.3 for crud

thermal conductivity results analyses.

Figure 5-14 is the X-ray Diffraction pattern obtained from crud formed on heater

rod #22B. This figure further demonstrates that the main chemistry components are

118

Nickel Iron Oxide, which is consistent with the EDS results discussed earlier. However,

in this figure, the blue line for Nickel Iron Oxide is just off the measured peak slightly.

There are two possible reasons, XRD machine calibration error and/or NiFe2O4 (in the

library) vs. Ni0.9Fe2.1O4 (from the WALT loop).


rod #30. This figure shows that the main chemistry components are Nickel Iron Oxide

and Iron Oxide, which is consistent with the EDS results discussed earlier. In this figure,

the big peak on the left hand side is cello-grease using for mounting material. And carbon

is from the pump bearing. Graphite is usually oriented such that one crystal plane is

preferred and only one peak is observed as shown in Figure 5-15.


rod #34. This figure shows that the main chemistry components are Nickel Iron Oxide

which is consistent with the EDS results discussed earlier. This figure is relatively clean

with a peak on the left hand side due to the exist of cello-grease mounting material.

Figure 5-17 shows that the main chemistry components are Nickel Iron Oxide for

crud from heater rod #39. This is consistent with the EDS results discussed earlier. This

figure also shows some zinc oxide and small amount of copper oxide.

Figures 5-18 and 5-19 show that the main chemistry components are Nickel Iron

Oxide for crud from heater rods #43 and #44. This is consistent with the EDS results

discussed earlier. These two figures both indicate a small amount of copper oxide. It is

noted that the “Iron Oxide” mark in Figure 5-18 is for reference only. Iron Oxide doesn’t

present in the measured results. On the other hand, cello-grease mounting material is

shown in Figure 5-19.

119

Figure 5-14 XRD Pattern Obtained from Crud Formed on Heater Rod Test #22B

120

Figure 5-15 XRD Pattern Obtained from Crud Formed on Heater Rod Test #30

121


122


123


124


125

5.2.1.3 Cladding Electric Resistance Measurement Results

Since the heater rod is used to model the top grid span of a PWR fuel assembly,

the heater rod operates at high pressure and high temperature. The heater rod electric

resistance is used for calculating the electric power applied to the heater rod. The electric

resistance is a function of temperature. Therefore, it is critical to measure the electric

resistance of the heater rod as a function of temperature.

A Zirlo tube is measured in an oven from low to high temperatures. The measured

results are presented in Table 5-1. As shown in Table 5-1, the heater rod electric

resistances has been measured at different temperatures, which cover the expected range

of heater rod cladding temperatures at PWR operating conditions. Based on Ohm’s Law

( elecIRU = ) and data shown in Table 5-1, the electric resistance is calculated for a heater

rod with a 0.374 inches (0.95cm) OD and length of 11.7 inches (29.7cm). The calculated

electric resistance results are shown in Figure 5-20, which are utilized in the experimental

data reductions.

When the temperature of the conductor increases, the collisions between electrons

and ions increase. Thus, as the heater rod heats up because of electricity flowing through

it (or by the coolant), the resistance will increase as shown in Figure 5-20.

Figure 5-20 is limited to the heater rod geometry used for the electric resistance

measurement, i.e. the heater rod length, cladding ID and OD. If the heater geometry is

different for different test cases, the electrical resistivity is calculated. For the same

material, the same electrical resistivity is applicable for calculating electric resistance for

heater rods with different geometries.

126

TEMPERATURE

(°C)

CURRENT

(A)

MEASURED

VOLTAGE

(V)

23.0 5.0 0.0589

265.0 5.0 0.0865

291.8 5.0 0.0902

338.2 5.0 0.0941

356.2 5.0 0.0957

405.9 5.0 0.1024

* Note: It is suggested that the electric resistance be measured at both high temperature

and high current conditions (e.g. at the WALT operating conditions) for utilizations in

future calculations and analyses (if practical).

Table 5-1: Electric Resistance Measurement Results for Heater Rod OD of 0.95cm*

127

Resistivity vs Temperature

y = 1.98030E-08x + 9.95719E-06

1.0E-05

1.2E-05

1.4E-05

1.6E-05

1.8E-05

2.0E-05

0 100 200 300 400 500

Temperature (Deg-C)

Res

ista

nce

(Ohm

-cm

)

Figure 5-20 Resistance vs. Temperature (Rod OD of 0.95cm).

128

Electrical resistivity (also known as specific electrical resistance) is a measure of

how strongly a material opposes the flow of electric current. The electrical resistivity ρ

of a material is given by,

H

elec L

AR 0

0 ⋅=ρ (5-1)

where,

0ρ is the electric resistivity (measured in ohm inches, Ω-cm),

elecR is the electrical resistance of a uniform specimen of the Zirlo heater rod tube

(measured in ohms, Ω),

HL is the length of the heater rod (measured in cm),

0A is the cross-sectional area of the heater rod tube (measured in cm2).

Using Equation (5-1), electric resistivity for Zirlo tubes has been calculated based

on data from Figure 5-20. The calculated electric resistivity as function of temperature is

presented in Figure 5-21, which can be used to calculate electric resistance using

Equation (5-2) for Zirlo heater rods with different geometries, i.e. different heater rod ID,

OD, or length. This is used in the different experiental tests in the WALT loop.

( )220

4

IDOD

LR H

elec −⋅×⋅=

πρ (5-2)

129

Resistance vs Temperature

y = 2.36861E-05x + 1.19096E-02

0.010

0.012

0.014

0.016

0.018

0.020

0.022

0 100 200 300 400 500

Temperature (Deg-C)

Res

ista

nce

(Ohm

)

Figure 5-21: Electric Resistivity vs. Temperature for Zirlo Tubes

130

5.2.1.4 Selected WALT Test Data Reduction

In applications where the raw data are already digital, i.e. for the WALT loop test

data, data reduction may consist simply of such operations as editing, scaling, Excel

coding, sorting, collating, and tabular summarization.

More specifically, the data reduction process is applied to readings or

measurements involving random errors. These are the indeterminate errors inherent in the

process of assigning values to flow, heater rod power, inlet temperature, and system

pressure. In such cases, before data may be coded and summarized, the most probable or

stable value of a quantity must be determined, such as for a stabilized WALT loop system

pressure. Provided the errors are normally distributed, the most probable (or central)

value of a set of measurements may be given by the arithmetic mean or, in the more

general case, by the weighted mean. When WALT loop system pressure and other

operating conditions are stabilized, selected stable data points are utilized for crud

thermal conductivity calculations.

During the WALT loop experimental tests, a boiling curve is generated by

varying the heater rod power levels (i.e. by adjusting D.C. current) after the crud

deposition reaches a desired thickness. This is for measuring the heater rod inner surface

temperature with limited crud deposition. The WALT operating conditions as well as the

heater rod inner surface temperature have been recorded in the data acquisition system.

For selected each case, data reduction has been performed in this section. The data

reduction results are presented for those cases listed in Table 4-3. These data are used in

the crud thermal conductivity calculations in the following sections.

131

5.2.1.4.1 Data Reduction for Heater Rod #22B

Historically, there were twenty nine heater rods, which were designed based on

Figure 4-4. Inside these heater rods, ceramic casting material is used to support the heater

rod cladding to stand high pressure from outside the rod. Later, the heater rod design is

changed based Figure 4-7. In the new design, the heater rod is pressurized inside with

Argon, which is the most frequently used the noble gases. Argon's full shell makes heater

rod inner surface stable and resistant to bonding with other elements. It is important to

note that the first nine heater rods were not numbered and none of the first twenty nine

tests were totally successfully with crud depositions. Heater rod #21 is the first rod with

Argon pressurized inside.

Heater rod # 22 is the same design as heater rod #21. Heater rod #22B is rebuilt

with new end plugs, which is now slightly shorter because of previous damage. Besides

Argon noble gases, FiberFax insulation material is used inside the heater rod tube to

support the four thermal wells and four thermal couples in positions.

The WALT test run with heater rod #22B is the first completed successful

experiment with relative thick crud deposit. This test is started at 11:27am on September

26, 2006. When the fluid condition is stable, the heater rod is powered on at 1066

seconds from the starting time. Totally, six injections were made to the WALT loop

during the rod #22B test. After the crud deposition reaches a desired thickness, a boiling

curve is generated by measuring the heater rod inner surface temperature with limited

crud deposition. All the operating conditions were recorded in the WALT data

acquisition system. After data reduction, selected data points are provided in Table 5-2.

132

Table 5-2 Selected Data Points for Heater Rod #22B

Tclad Measured Inner Inner Inner Inner Flow Flow Meas. Meas. Meas. Measured RecordAverage Resistance Tclad1 Tclad2 Tclad3 Tclad4 Tin Tout Pres. Current Flow Voltage Time

(°C) (ohms)* (°C) (°C) (°C) (°C) (°C) (°C) (MPa) (amps) (m3/hr) (V) (hr)

346 0.0201 346 346 346 346 323 324 15.5 200.7 2.44 4.32 25.87

352 0.0203 352 352 353 352 322 324 15.5 254.4 2.44 5.49 25.99

356 0.0203 356 357 356 356 323 325 15.5 315.6 2.44 6.82 25.85

359 0.0204 358 359 359 359 323 325 15.5 352.5 2.43 7.64 25.91

362 0.0205 361 363 362 361 325 328 15.5 403.6 2.43 8.76 25.59

365 0.0205 364 366 365 364 322 325 15.5 452.7 2.43 9.84 25.17

374 0.0208 373 377 376 372 323 327 15.5 550.8 2.42 12.07 25.01

381 0.0209 380 384 383 378 324 328 15.5 601.7 2.42 13.24 24.95

386 0.0211 384 389 388 382 323 328 15.5 630.0 2.41 13.92 24.93

* Note: The electric resistance is calculated based the equation given on the top of Figure 5-21 using the averaged cladding temperature from the first column of this table. The averaged cladding temperature is based on columns 3-6. All other data are from the WALT data acquisition system.

133

5.2.1.4.2 Data Reduction for Heater Rod #30

Heater rod # 30 is the same design as heater rod #22B. Besides Argon noble gases,

FiberFax insulation material is used inside the heater rod tube to support the four thermal

wells and four thermal couples in positions. The thermal couple locations are 6 inches

from the top of the heater rod.

The WALT test run with heater rod #30 is the sixth completed good experiment

with relative thick crud deposit and zinc addition. This test is started on February 2, 2007.

When the fluid condition is stable, the heater rod is powered on. Meanwhile, the original

mechanical back-pressure regulator is replaced with an active control system which

measures pressure and then adjusts the setting on an air-loaded backpressure regulator

several times per second.

This time, the really PWR typical operating conditions were simulated. This new

pressurizing pump together with the boron and lithium make-up system is utilized to

inject simulated PWR coolant from the large storage tank as shown in Figure 4-6. A

typical concentration of 1000 ppm for boron and 2.2 ppm for lithium are used to simulate

the chemistry conditions of normal PWR operating conditions. Totally, six injections

were made to the WALT loop during the rod #30 test. After the crud deposition reaches a

desired thickness, zinc injection is initiated from a separate tank. A boiling curve is

generated at the end of this experiment. The operating conditions were recorded in the

WALT data acquisition system. After data reduction, selected data points are provided in

Table 5-3.

134

Table 5-3 Selected Data Points for Heater Rod #30



405 0.0215 411 411 417 382 328 331 15.6 629.9 2.60 14.38 63.11

401 0.0214 406 405 411 380 334 336 15.6 603.6 2.60 13.74 63.20

393 0.0212 398 397 402 376 335 337 15.6 561.6 2.61 12.72 63.24

384 0.0210 387 387 391 370 334 336 15.6 503.5 2.61 11.32 63.30

370 0.0207 373 373 376 360 331 332 15.0 450.9 2.61 10.04 63.39

369 0.0207 371 371 374 360 330 330 15.6 400.6 2.61 8.91 63.50

363 0.0205 364 364 366 356 329 329 15.6 349.3 2.62 7.74 63.57

358 0.0204 359 360 361 353 330 329 15.6 302.3 2.62 6.67 63.70

351 0.0202 346 354 354 349 329 328 15.6 248.8 2.62 5.46 63.75

348 0.0202 349 350 350 344 329 328 15.7 197.4 2.61 4.32 63.83

344 0.0201 344 344 344 344 329 328 14.8 147.8 2.62 3.22 63.87

339 0.0199 339 339 339 337 329 328 14.3 95.6 2.63 2.07 63.92


135

5.2.1.4.3 Data Reductions for Heater Rods #34, #39, #43, and #44

Heater rods #34, #39, #43, and #44 are the same design as heater rod #30. Both

Argon noble gases and FiberFax insulation material were used inside the heater rod tube

to support the four thermal wells and four thermal couples in positions. The thermal

couple locations are 6 inches from the top of heater rods #34, #39, #43, and #44.

The WALT test run with heater rods #34, #39, #43, and #44 are the 8th, 12th, 13th,

and 14th completed good experiment with relative thick crud deposit. These four tests

were completed from April to early August, 2007. When the fluid condition is stable, the

heater rod is powered on. The WALT tests for heater rods #34 and #39 have 6 crud

injections. While, seven crud injections were made in the other two WALT runs.

Again, the really PWR typical operating conditions were simulated. This new

pressurizing pump together with the boron and lithium make-up system is utilized to

inject simulated PWR coolant from the large storage tank as shown in Figure 4-6. A

typical concentration of 1000 ppm for boron and 2.2 ppm for lithium are used to simulate

the chemistry conditions of normal PWR operating conditions.

No zinc addition is made for the WALT test with heater rod #34. However, After

the crud deposition reaches a desired thickness, 60 ppb zinc concentration is injected to

the run with heater rod #39. On the other hand, 60 ppb zinc and 7.5 ppm SiO2 were used

in the run for heater rod #43. Similarly, 40 ppb zinc and 5.0 ppm SiO2 were injected in

the run for heater rod #44. The operating conditions were recorded in the WALT data

acquisition system until the operating conditions are stabilized. The selected data points

are provided in Tables 5-4 through 5-7 respectively.

136




372 0.0207 372 372 374 373 329 332 15.6 630.2 3.18 13.83 4.62372 0.0207 372 372 374 373 331 334 15.6 629.9 3.18 13.82 7.33372 0.0207 372 372 374 373 330 333 15.6 629.8 3.18 13.83 10.04372 0.0207 372 372 374 373 331 334 15.6 629.6 3.18 13.86 14.13372 0.0207 372 372 374 373 332 335 15.6 629.5 3.17 13.86 18.99372 0.0207 372 372 373 372 333 336 15.6 629.3 3.12 13.89 24.14375 0.0208 375 374 376 375 332 336 15.6 629.2 3.14 13.90 24.21373 0.0207 373 373 374 373 331 334 15.6 629.7 3.14 13.90 24.92373 0.0207 373 373 374 373 328 331 15.6 629.7 3.14 13.91 26.95373 0.0207 373 372 374 373 330 333 15.0 629.8 3.14 13.89 31.13375 0.0208 375 375 376 374 330 333 15.6 629.7 3.14 13.93 31.20375 0.0208 375 375 374 374 331 334 15.6 629.9 3.14 13.95 32.00375 0.0208 375 375 374 374 332 335 15.6 629.8 3.13 13.97 35.11375 0.0208 375 375 374 374 329 333 15.6 629.5 3.13 13.95 40.55375 0.0208 375 375 374 374 331 335 15.5 629.5 3.13 13.97 47.74378 0.0209 378 378 376 376 332 335 15.6 629.5 3.12 13.98 47.91378 0.0209 378 379 375 375 331 334 15.6 629.4 3.11 14.01 48.52379 0.0209 379 379 375 375 330 334 15.6 629.8 3.12 14.02 51.72380 0.0209 380 381 376 376 331 334 15.8 629.8 3.11 14.06 54.81381 0.0209 381 382 376 377 331 334 15.6 629.8 3.12 14.04 54.86384 0.0210 384 385 376 377 332 335 15.6 629.8 3.12 14.09 57.16385 0.0210 385 386 376 377 335 338 15.6 630.3 3.10 14.11 59.59385 0.0210 385 386 375 377 330 333 15.6 629.7 3.12 14.10 66.00386 0.0210 386 387 376 377 330 333 15.6 629.2 3.11 14.12 71.63


137


Tclad Measured Inner Inner Inner Inner Flow Flow Meas. Meas. Meas. Measured Record

Average Resistance Tclad1 Tclad2 Tclad3 Tclad4 Tin Tout Pres. Current Flow Voltage Time


390 0.0212 390 387 389 387 330 334 15.5 629.9 3.48 15.35 0.01391 0.0212 391 387 389 387 330 333 15.5 630.1 3.49 15.36 2.50391 0.0212 391 388 389 388 330 334 15.5 630.3 3.49 15.37 5.00391 0.0212 391 388 390 388 330 333 15.5 630.0 3.48 15.38 6.25391 0.0212 391 388 390 388 330 334 15.5 630.2 3.48 15.38 7.50392 0.0212 392 389 390 388 330 334 15.5 630.1 3.49 15.38 8.75392 0.0212 392 389 390 388 330 333 15.5 630.3 3.48 15.39 10.00392 0.0212 392 389 391 389 330 334 15.5 630.5 3.48 15.39 11.25393 0.0212 393 390 391 389 330 333 15.5 629.9 3.48 15.40 12.50393 0.0212 393 390 391 389 330 334 15.5 629.9 3.48 15.40 13.75393 0.0212 393 389 391 389 330 334 15.5 630.8 3.48 15.40 15.00393 0.0212 393 390 391 389 330 334 15.4 630.7 3.48 15.42 16.25393 0.0212 393 390 392 390 330 334 15.4 630.9 3.48 15.42 17.50393 0.0212 393 390 392 390 330 334 15.5 630.7 3.46 15.43 18.75393 0.0212 393 390 392 390 330 334 15.4 630.9 3.48 15.44 20.00394 0.0212 394 390 392 390 330 334 15.4 630.8 3.48 15.45 21.25394 0.0212 394 391 392 390 330 334 15.4 631.1 3.49 15.46 22.50393 0.0212 393 390 392 390 330 334 15.4 631.6 3.49 15.46 23.75394 0.0212 394 391 393 391 330 334 15.5 630.6 3.47 15.44 25.00394 0.0212 394 391 393 391 330 334 15.5 630.2 3.48 15.51 26.25394 0.0212 394 391 393 391 330 334 15.5 630.1 3.48 15.49 27.50394 0.0212 394 391 393 391 330 333 15.5 630.1 3.47 15.48 28.75394 0.0212 394 391 393 391 330 334 15.5 630.0 3.49 15.47 30.00394 0.0212 394 391 393 391 330 334 15.5 629.8 3.47 15.49 31.25394 0.0212 394 391 393 392 330 334 15.5 630.4 3.48 15.47 32.50394 0.0212 394 391 393 392 330 333 15.5 630.3 3.47 15.46 32.74394 0.0212 394 391 393 392 330 334 15.5 630.1 3.48 15.46 32.77


138




(°C) (ohms)* (°C) (°C) (°C) (°C) (°C) (°C) (MPa) (amps)(m3/hr)

(V) (hr)

379 0.0209 379 379 377 379 329 332 15.7 628.1 3.42 15.16 0.00380 0.0209 380 381 378 381 330 333 15.6 628.4 3.40 15.17 2.50380 0.0209 380 382 378 381 330 333 15.7 628.5 3.42 15.16 5.27380 0.0209 380 382 378 381 329 333 15.6 629.3 3.43 15.19 8.05380 0.0209 380 382 378 381 329 333 15.6 629.0 3.42 15.18 10.83380 0.0209 380 383 379 382 330 333 15.6 629.7 3.42 15.21 13.61381 0.0209 381 384 379 382 330 333 15.6 630.0 3.43 15.19 16.40381 0.0209 381 385 379 383 330 334 15.6 629.1 3.44 15.19 19.18381 0.0209 381 385 380 383 330 333 15.6 629.0 3.42 15.19 21.96382 0.0209 382 386 380 384 330 333 15.7 628.8 3.43 15.18 24.74382 0.0210 382 387 380 384 330 333 15.7 628.3 3.43 15.19 27.52382 0.0210 382 388 381 385 330 333 15.6 628.6 3.42 15.21 30.30383 0.0210 383 388 381 386 330 333 15.6 629.0 3.42 15.21 33.08383 0.0210 383 388 382 386 330 333 15.6 629.2 3.42 15.21 35.86383 0.0210 383 387 382 386 330 333 15.6 629.0 3.43 15.21 38.64383 0.0210 383 387 382 386 330 333 15.6 628.9 3.44 15.22 41.42384 0.0210 384 386 382 386 330 333 15.7 629.3 3.43 15.22 44.20383 0.0210 383 386 382 386 330 333 15.6 629.1 3.42 15.22 46.98384 0.0210 384 385 382 386 329 333 15.6 628.7 3.41 15.22 49.76384 0.0210 384 385 382 385 329 333 15.7 629.0 3.44 15.19 52.54384 0.0210 384 385 382 385 330 333 15.6 629.0 3.44 15.20 55.32384 0.0210 384 385 382 385 330 333 15.6 629.3 3.43 15.23 58.10384 0.0210 384 384 382 384 330 333 15.6 629.1 3.42 15.20 60.88384 0.0210 384 384 382 384 330 333 15.6 629.6 3.42 15.22 63.66384 0.0210 384 384 382 384 330 333 15.6 629.0 3.43 15.22 63.95384 0.0210 384 384 382 384 330 333 15.7 629.3 3.44 15.22 64.78384 0.0210 384 384 382 384 330 333 15.6 628.8 3.40 15.21 65.67


139





378 0.0209 377 378 380 378 329 333 15.6 629.4 3.44 15.65 0.00378 0.0209 378 378 380 378 329 333 15.6 629.3 3.45 15.63 1.25378 0.0209 377 377 380 378 330 333 15.6 629.2 3.44 15.64 2.50378 0.0209 378 378 380 378 330 333 15.6 629.0 3.45 15.63 3.75378 0.0209 378 377 380 378 329 333 15.6 629.2 3.45 15.65 5.00379 0.0209 378 378 381 379 330 333 15.6 629.1 3.46 15.66 6.25379 0.0209 378 378 380 378 329 333 15.6 628.8 3.45 15.65 7.50379 0.0209 378 378 381 378 329 333 15.6 628.9 3.43 15.66 8.75379 0.0209 378 378 381 379 329 333 15.6 628.6 3.46 15.66 10.00379 0.0209 378 378 381 379 329 333 15.6 629.0 3.46 15.66 11.25379 0.0209 378 378 381 378 329 333 15.6 628.9 3.45 15.64 12.50379 0.0209 378 378 381 378 329 333 15.6 629.1 3.45 15.63 13.75379 0.0209 378 378 381 379 329 333 15.6 629.1 3.45 15.62 15.00379 0.0209 378 378 381 379 330 333 15.7 629.1 3.45 15.65 16.25379 0.0209 378 378 381 379 329 333 15.6 629.4 3.46 15.65 17.50379 0.0209 378 378 381 379 329 333 15.6 628.9 3.45 15.65 18.75379 0.0209 379 379 381 379 330 333 15.6 629.2 3.44 15.65 20.00379 0.0209 379 379 381 379 329 333 15.6 629.5 3.47 15.63 21.25380 0.0209 379 379 382 379 330 333 15.6 629.3 3.46 15.66 22.50380 0.0209 379 379 382 380 329 333 15.6 629.3 3.44 15.65 23.75380 0.0209 379 379 382 380 330 333 15.6 629.3 3.44 15.67 25.00380 0.0209 379 379 382 380 329 333 15.6 629.4 3.46 15.66 26.25380 0.0209 379 379 382 380 330 333 15.6 629.3 3.46 15.68 27.50380 0.0209 379 379 382 380 330 333 15.6 629.4 3.44 15.67 28.75380 0.0209 380 379 382 380 329 333 15.6 629.3 3.43 15.68 30.00380 0.0209 379 379 382 380 329 333 15.6 629.2 3.44 15.67 31.27380 0.0209 380 379 382 380 329 333 15.7 629.3 3.44 15.68 32.46


140

5.2.2 Overall crud Thermal Conductivity Calculations and Results

Thermal conductivity is the quantity of heat transmitted through a unit thickness

in a direction normal to a surface of unit area, due to a unit temperature gradient under

steady state conditions. Thermal conductivity is an intrinsic material property for which

the values depend on the chemical composition, porosity, density, structure, and fabric of

the material.

The overall crud Thermal Conductivity can be calculated based on other direct

measurements as discussed in Chapter 3. The procedure for the overall crud Thermal

Conductivity measurement includes data selection, which has been discussed in the last

section. The measurement results are entered into an Excel spreadsheet, which includes a

steam table installed, equations coding, and thermal conductivity calculations as

discussed in previous chapters. After the selected data from Section 5.2.1 are entered into

the Excel file, the overall crud Thermal Conductivity values returned in a separate Excel

window.

In this dissertation, overall crud Thermal Conductivity is calculated for crud-

regimes II and partially III and IV. This is important for simulating the top grid span in

the PWR normal operating conditions, including the hot rods. The calculated overall crud

Thermal Conductivity is most important for use in Crud Induced Power Shift (CIPS) and

Crud Induced Localized Corrosion (CILC) analyses.

Using the Excel spreadsheet, the boiling duty is calculated in order to determine

the crud-regime conditions. Then, a direct solution method using existing correlations is

used to calculate the crud thermal conductivities for WALT test runs with heater rods

#22B, #30, #34, #39, #43, and #44.

141

5.2.2.1 Overall crud Thermal Conductivity for Heater Rod #22B

The selected data for heater rod #22B in Table 5-2 is plotted in Figure 5-22,

which shows the boiling curve at the end of this test. Meanwhile, boiling duty is

calculated and plotted in Figure 5-22, which indicates that the crud condition is in crud-

regime II/III for the three data points at the end of the curve. For nucleate boiling, Thom

correlation is utilized to calculate the crud outer surface temperature.

( )

satwall Te

qT P +

′′×=1260

5.0072.0 (5-3)

where, q ′′ is the heat flux at the heater rod outer surface (Btu/hr-ft2). P is the WALT

system pressure (psia). wallT and satT are cladding wall and saturation temperatures (°F),

respectively. SI units must be converted into English units for using Equation (5-3).

Wall superheats in sub-cooled boiling region are not very high and, consequently,

it is frequently deemed adequate to extend the heat transfer correlation from nucleate

boiling region down into the sub-cooled boiling region. To do this is slightly more

conservative [55] because of the slight overestimates of the wall temperatures by using

Thom correlation. Then, the heater transfer coefficient is calculated using Newton's law

of cooling.

Finally, Equation (3-7) is used for the overall overall crud Thermal Conductivity

calculations. All these equations are coded in an Excel spreadsheet with a steam table

installed. And the calculation results from this Excel spreadsheet are given in Table 5-8.

142

A Boiling Curve for Heater Rod #22B(Pr = 15.5 MPa, G = 0.195 kg/s-cm^2, Tinlet = 324 Deg-C, OD = 0.95cm)

320.0

330.0

340.0

350.0

360.0

370.0

380.0

390.0

400.0

9.1 14.8 22.9 28.6 37.7 47.5 71.2 85.6 94.3

Heat Flux, W/cm^2

Cla

d T

emp

erat

ure

, Deg

-C

0.00

0.05

0.10

0.15

0.20

Ste

amin

g R

ate,

kg

/hr-

cm^

2

Boiling Curve (TC1)Boiling Curve (TC2)Boiling Curve (TC3)Boiling Curve (TC4)Steaming Rate (Calculated)

Figure 5-22 Boiling Curve for Test with Heater Rod #22B

143

Parameters 1st end point 2nd end point 3rd end point

T1, °C 386.1 381.3 374.2

T4, °C 325.5 326.0 325.2

Δt, °C 60.6 55.3 49.0

Q, watts 7926.8 7194.0 5979.6

q''', watts/cm3 1756.4 1594.1 1325.0

r1, cm 0.417 0.417 0.417

r2, cm 0.474 0.474 0.474

Dcrud, μm 57.30 57.30 57.30

r3, cm 0.480 0.480 0.480

Lh, cm 28.19 28.19 28.19

k (watts/cm-°C) 0.19 0.19 0.18

A 2.750 2.495 2.074

B 0.537 0.488 0.405

C 93.207 84.591 70.312

kd (watts/cm-°C) 0.0232 0.0246 0.0267

h (watts/cm2-°C) 4.11 3.85 3.11

Porosity, % 44 44 44

Table 5-8: Overall crud Thermal Conductivity Calculations for Heater Rod #22B

144

5.2.2.2 Overall crud Thermal Conductivity for Heater Rod #30

Similarly, the selected data for heater rod #30 in Table 5-3 is plotted in Figure 5-

23, which shows the boiling curve at the end of this test. Meanwhile, steaming rate is

calculated and plotted in Figure 5-23, which indicates that the crud condition is in crud-

regime II/III for the four data points at the end of the curve. For nucleate boiling, Thom

correlation (Equation 5-3) is utilized to calculate the crud outer surface temperature. Then,

the heater transfer coefficient is calculated using Newton's law of cooling.

Finally, Equation (3-7) is used for the overall crud Thermal Conductivity



5.2.2.3 Overall crud Thermal Conductivity for Heater Rods #34, #39, #43, and #44

From Tables 5-4 through 5-7, we can see that the test is stabilized at the end of the

tests for heater rods #34, #39, #43, and #44. The boiling duty are calculated about ~300

lbm/hr-ft2 (or 0.407 kg/s-m2), which indicates that the crud condition is in crud-regime

II/III at the end of the test. For nucleate boiling, Thom correlation (Equation 5-3) is

utilized to calculate the crud outer surface temperature. Then, the heater transfer

coefficient is calculated using Newton's law of cooling.

Finally, Equation (3-7) is used for the overall crud Thermal Conductivity



145

A Boiling Curve Heater Rod #30(Pr = 15.6 MPa, G = 0.204 kg/s-cm^2, Tinlet = 330 Deg-C, OD = 0.95cm)

260.0

280.0

300.0

320.0

340.0

360.0

380.0

400.0

420.0

96.3 88.0 75.6 60.1 47.5 37.4 28.2 21.0 14.1 8.9 4.9 2.1

Heat Flux, W/cm^2

Cla

d T

emp

erat

ure

, Deg

-C

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Ste

amin

g R

ate,

kg

/hr-

cm^

2

Boiling Curve (TC1)

Boiling Curve (TC2)

Boiling Curve (TC3)

Boiling Curve (TC4)

Steaming Rate (Calculated)

Figure 5-23 Boiling Curve for Test with Heater Rod #30

146

Parameters 1st end point 2nd end point 3rd end point 4th end point

T1, °C 412.8 407.4 399.1 388.2

T4, °C 329.3 334.9 335.8 335.1

Δt, °C 83.5 72.5 63.3 53.0

Q, watts 8095.5 7395.6 6350.7 5049.9

q''', watts/cm3 1793.8 1638.7 1407.2 1119.0

r1, cm 0.417 0.417 0.417 0.417

r2, cm 0.474 0.474 0.474 0.474

Dcrud, μm 75.00 75.00 75.00 75.00

r3, cm 0.482 0.482 0.482 0.482

Lh, cm 28.19 28.19 28.19 28.19

k (watts/cm-°C) 0.19 0.19 0.19 0.19

A 2.808 2.565 2.203 1.752

B 0.717 0.655 0.562 0.447

C 94.841 86.642 74.401 59.162

kd (watts/cm-°C) 0.0146 0.0145 0.0145 0.0146

h (watts/cm2-°C) 4.86 6.29 5.87 4.57

Porosity, % 44 44 44 44

Table 5-9: Overall crud Thermal Conductivity Calculations for Heater Rod #30

147

Parameters Heater Rod #34 Heater Rod #39 Heater Rod #43 Heater Rod #44

T1, °C 381.0 392.6 383.3 380.3

T4, °C 336.7 331.7 331.5 331.0

Δt, °C 44.3 60.9 51.8 49.3

Q, watts 7927.1 8001.1 7875.2 7858.3

q''', watts/cm3 1756.5 1772.9 1745.0 1741.3

r1, cm 0.417 0.417 0.417 0.417

r2, cm 0.474 0.474 0.474 0.474

Dcrud, μm 42.70 29.85 53.65 29.70

r3, cm 0.479 0.477 0.480 0.477

Lh, cm 28.19 28.19 28.19 28.19

k (watts/cm-°C) 0.19 0.19 0.19 0.19

A 2.750 2.775 2.732 2.726

B 0.401 0.283 0.500 0.277

C 93.495 94.623 92.671 92.937

kd (watts/cm-°C) 0.0230 0.0096 0.0252 0.0168

h (watts/cm2-°C) 7.79 5.69 5.36 5.14

Porosity, % 39 25 45 35

Table 5-10 Overall crud Thermal Conductivity Calculations for Heater Rod #34, 39, 43, and 44

148

5.2.3 Crud Thermal Conductivity Result Analyses

The calculated overall crud Thermal Conductivity based on other direct

measurements have been documented in Tables 5-8 through 5-10. In this section, the

overall crud Thermal Conductivity results are analyzed by considering the effects of crud

thickness, crud porosity, and chemistry components. The crud-regimes are also identified

in the analyses.

5.2.3.1 Effect of Crud Porosity – Crud-regime II

Four cases were used to investigate the porosity effects. Measured or calculated

results of crud thermal conductivities, crud porosities, and crud thickness for heater rods

#34, #39, #43, #44 are listed in Table 5-10. The overall crud Thermal Conductivity

versus porosity curve is plotted in Figure 5-24.

As listed in Table 5-10, the crud thickness for the four test cases with heater rod

#34, #39, #43, and #44 shown in Figure 5-24 are ranging from 29.9 mμ (or 1.18 mils) to

53.7 mμ (or 2.11 mils). On the other hand, Figures 5-16 through 5-19 show that the main

chemistry component is Nickel Iron Oxide / Trevorite (NiFe2O4) for crud from heater rod

#34, #39, #43, and #44.

Based on the above information and Figure 5-24, it is suggested that overall crud

Thermal Conductivity is a function of porosity within certain range of the crud thickness

and chemistry components. This is explained as that more liquid entering into the porous

pores in crud deposit enhanced the heat transfer at higher porosity crud (for the same

boiling rate.). This is a dynamic steady-state condition for crud thermal conductivity.

149

y = 2.7503Ln(x) + 4.7385

R2 = 0.9626

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Porosity

Kd

(wa

tts/

m-°

K)

Figure 5-24 Overall Crud Thermal Conductivity (Kd) vs. Porosity

150

The fitted equation shown on Figure 5-24 describes the relationship between

independent variable of crud porosity (x) and the dependent variable of overall crud

Thermal Conductivity (y). The equation is provided below.

( ) 7385.4ln2.7503 += xy (5-4)

The applicable conditions or parameter range are that the crud thickness is in

between 30 mμ (or 1.2 mils) and 54 mμ (or 2.2 mils) and the main chemistry component

is Nickel Iron Oxide / Trevorite (NiFe2O4). The boiling rate is about 300 lbm/hr-ft2. The

crud porosity is 45.025.0 ≤≤ x , which indicates the condition in crud is in crud-regime

II.

151

5.2.3.2 Effect of Chemistry Component – Crud-regime IV

Due to zinc injection, some of the crud pores are partially filled with zinc oxide. It

is now necessary and important to understand the property of ZnO in order to investigate

the overall crud thermal conductivity change. Researchers have studied ZnO for many

years [56, 57, 58]. With a direct bandgap only slightly lower than that of GaN, ZnO has a

Wurtzite structure identical to that of GaN. With a crystal structure of Wurtzite; some

ZnO phases are stable under high pressure or metastable growth conditions. The ZnO

density is measured as 355.85 lb/ft3 (or 5.7 g/cm3) [59]. In addition, the thermal

conductivity of ZnO is measured from 0.075 Btu/hr-ft-°F (or 0.13 W/cm-K) to 0.5779

Btu/hr-ft-°F (or 1.0 W/cm-K) [59, 60].

In the present work, a number of WALT tests have been studied with respect to

the oxidation compound behavior. The experimentally determined oxidation layer

impacts are comparable with those available in the literature [56, 57, 48, 49, 60]. A major

finding is that the overall crud Thermal Conductivity is significantly affected by the

partially filled impurities, such as ZnO or Silica, etc. The exact amounts, type and

distribution of the impurity materials must be carefully measured depending on the

particular crud layer application.

As shown in Figure 5-24, the crud layer from heater rod #39 has a relatively lower

thermal conductivity due to lower porosity, which is because of the ZnO filled partially in

the crud pores. As discussed earlier in this section, ZnO has a low thermal conductivity.

Thus, the overall overall crud Thermal Conductivity for heater rod #39 is measured lower.

This case is identified as crud-regime IV.

152

5.2.3.3 Effect of Crud Thickness – Crud-regime III

The effect of crud thickness is investigated by selected WALT tests with the same

or very close crud porosity. Three cases of heater rods #22B, #30, and #43 were used to

investigate this effect. Measured or calculated results of crud thermal conductivities, crud

porosities, and crud thickness for heater rods #22B, #30, and #43 are listed in Tables 5-8

through 5-10, respectively . The overall crud Thermal Conductivity versus crud thickness

curve is plotted in Figure 5-25.

The crud thickness for the three test cases with heater rods #22B, #30, and #43

shown in Figure 5-25 are ranging from 53.7 mμ (or 2.11 mils) to 75.0 mμ (or 2.95 mils).

On the other hand, Figures 5-14, 5-15, and 5-18 shows that the main chemistry

component is Nickel Iron Oxide / Trevorite (NiFe2O4) for crud from heater rod #22B,

#30, and #43.

Based on the above information and Figure 5-25, it is suggested that overall crud

Thermal Conductivity is a function of crud thickness with the same porosity and

chemistry components in crud. Crud on heater rod #30 with the highest crud thickness

has the lowest overall crud Thermal Conductivity. This can be understood when the crud

is thick enough at high heat flux level, it may cause heater rod surface dryout. The study

in Figure 5-25 examines the effects of crud formation on heater rod clad dryout. If we

grow the crud thicker and/or with higher heat flux, dryout effect is more significant due

to much lower overall crud Thermal Conductivity. In Figure 5-25, the conditions in crud

for heater rod #30 is at the beginning of crud-regime III.

153

y = -3.1918Ln(x) + 15.242

R2 = 0.99

1.000

1.500

2.000

2.500

3.000

50.0 55.0 60.0 65.0 70.0 75.0 80.0

Crud Thickness (Micron)

Kd

(wat

ts/m

-°K

)

Figure 5-25 Overall Crud Thermal Conductivity ( dk ) vs. Crud Thickness

154

The fitted equation shown on Figure 5-25 describes relationship between

independent variable of crud thickness (x) and dependent variable of overall crud

Thermal Conductivity (y). The equation is also provided below.

( ) 242.15ln-3.1918 += xy (5-5)

The applicable conditions or parameter range are that the crud thickness is in

between 54 mμ (or 2.2 mils) and 75 mμ (or 3.0 mils) and the main chemistry component

is Nickel Iron Oxide / Trevorite (NiFe2O4). The boiling rate is about 300~400 lbm/hr-ft2.

The crud porosity is 40%.

5.2.3.4 Error Analysis

R-squared and relative error analyses are performed in the section for the selected

cases.

5.2.3.4.1 R-squared Calculation and Analysis

In this section, error analysis is performed to investigate the sum of squares of

residuals to the fitted the line or equation. R-squared, often called the coefficient of

determination, is defined as the ratio of the sum of squares explained by a regression

model or fitted equation and the "total" sum of squares around the mean.

total

error

total

regression

SS

SS

SS

SSR −== 12 (5-6)

155

The R-Sq values are calculated to be 0.9626 and 0.99 in Figures 5-24 and 5-25

respectively. R-Sq is a measure describing the quality of regression or fitted equations. In

Figures 5-24 and 5-25, 96.26% and 99% of variation in overall crud Thermal

Conductivity can be explained by variation in crud porosity and crud thickness,

respectively.

5.2.3.4.2 Relative Error Analysis

Sensitivity study has been performed to investigate the temperature measurement

error propagation. All selected cases have been investigated by varying the delta T in

Equation (3-7), the calculated overall crud Thermal Conductivity changes from 1% to

3.4% per degree Fahrenheit variation. The typical thermocouple measurement error is

about ±1.5 °F. Thus, the worse temperature measurement error could cause a calculated

overall crud Thermal Conductivity error of 10.2%.

Figures 5-24 and 5-25 are repotted as Figures 5-26 and 5-27 showing data for the

overall crud Thermal Conductivity of all selected cases. The uncertainty bars shown are

for a 10.2 % relative error.

In general, the overall crud Thermal Conductivity error due to temperature

measurement error is dominating. Other parameter error is relatively small. Thus, other

factors will not significantly increase the uncertainty of the measurement or calculation of

thermal conductivity of the crud.

156

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.1 0.2 0.3 0.4 0.5

Porosity

Kd

(wat

ts/m

-°K

)

Figure 5-26 Overall Crud Thermal Conductivity (with error) vs. Porosity

157

1.0

1.5

2.0

2.5

3.0

50.0 55.0 60.0 65.0 70.0 75.0 80.0

Crud Thickness (Micron)

Kd

(wat

ts/m

-°K

)

Figure 5-27 Overall Crud Thermal Conductivity (with error) vs. Crud Thickness

158

5.2.3.5 Crud Thermal Conductivity Results

In Sections 5.2.3.1 through 5.2.3.4, the overall crud thermal conductivity is

calculated. In order to calculate and utilize the crud thermal conductivity ( crudk ) in the

BOA computer code, Equation (3-1) is re-written as the following.

( )

φφααφ

−−−−

=1

1 vldcrud

kkkk (5-7)

The crud thermal conductivity ( crudk ) can be easily calculated in the BOA

computer code, if Equation (5-7) is coded in BOA. This is because that the thermal

conductivities for water and vapor ( lk and vk ), the porosity (φ ) as well as the void

fraction (α ) in Equation (5-7) are calculated based on the local fluid conditions inside

BOA code. The overall crud thermal conductivity is obtained from Equations (5-4) and

(5-5) at specified conditions. Thus, the crud thermal conductivity ( crudk ) is calculated

using Equation (5-7). A sample calculation is provided in Table 5-11.

159

Table 5-11 A Sample Calculation for Crud Thermal Conductivity ( crudk )

Parameters Measured/Calculated Values

System Pressure (MPa) 15.5

Water Temperature (°C) 344.7

lk (W/m-K) 0.459

vk (W/m-K) 0.121

α 0.5

φ 0.5

dk (W/m-K) 1.0

crudk (W/m-K) 1.7

160

5.3 Crud Structure Comparisons between Plant and WALT data

Researchers have performed many studies on the crud structure analyses for the

nuclear power plants. A set of sample data from different plants presented by Dr. W. A.

Byers [45] is provided in Table 5-12 in the following. A sample set of data from the

WALT loop test are shown in Table 5-13 and Figures 5-28 and 5-29 below.

Comparing plant crud data (Table 5-12) with the WALT loop crud data (Table 5-

13 and Figures 5-28 and 5-29), it can be seen that the WALT loop crud chimney diameter

and chimney density or concentrations are very similar or very close to those from the

plant fuel assemblies. In particular, Figure 5-29 shows boiling chimney diameter

distribution with most chimneys having a diameter of about 5µm, which is similar or

close to plant data shown in Table 5-12. Meanwhile, Figure 5-28 shows that the WALT

crud boiling chimneys occurred in patches with variances in density and number of

boiling chimneys across the given areas is somewhat consistent with plant data in Table

5-12.

In the last five years, a number of successful cases have been done on the WALT

loop. Some crud images have been taken using a Scanning Electron Microscope (SEM)

instrument. These figures are provided in Figures 5-30 through 5-33 below. From these

pictures, it can be seen that the WALT loop generated crud has a unique needle or rod-

like structure. This is very similar to the crud found in the nuclear power plant fuel

assemblies as discussed previously and shown in Figures 1-7 and 2-2. In detail, Figure 5-

34 shows the structural similarities of crud from the plants (Part a) and the WALT loop

(Part b). This means that even the crud detail structures are very close to each other.

161

Table 5-12 Plant Boiling Chimney3 Analysis Data [45]

Pore Chimney Diameter (μm)

Plants Cycles Average Minimum Maximum

Average Pore

Chimney

Concentration

(#/m2)

Beaver Valley 2 10 3.5 1.7 7.8 9.6E+09

Braidwood 2 11 2.5 1.1 5.6 1.4E+10

Plant F 9 7.3 1.7 30.0 7.8E+09

Plant F 10 30.0 30.0 30.0 6.6E+09

Plant F 13 3.5 1.5 6.3 7.1E+09

Plant E 11 3.6 2.0 5.0 1.2E+10

Plant A 5 4.9 3.8 8.8 2.5E+09

Plant B 10 3.9 1.9 6.3 1.0E+10

Plant B 11 3.5 1.9 7.2 8.7E+09

Plant C 8 4.3 1.9 9.4 1.1E+10

Plant C 9 3.3 1.6 6.3 1.8E+10

3 The crud deposit is a porous body and the pores in the crud are chimney like.

162

Table 5-13 WALT Heater Rod #40 Chimney Analysis Data

Pore Chimney Diameter (μm)

Name Rod # Average Minimum Maximum

Average Pore

Chimney

Concentration

(#/m2)

WALT Heater

Rod 40 4.0 2.0 1.0 3.7E+09

163

Figure 5-28 Chimney Density in Rod # 10 Crud Flakes (WALT Data) [45]

164

Figure 5-29 Boiling Chimney Diameter Distribution (WALT Data) [45]

(μm)

(# of Chimney/unit area)

165

Figure 5-30 A Scanning Electron Microscope (SEM) Picture for WALT Rod# 40 (100X)

166

Figure 5-31 A SEM Picture for WALT Rod# 40 (500X)

167


25 μm

168


10 μm

169

a, Plant crud b, WALT Crud (Braidwood 1) (Top View)

Figure 5-34 Comparison of Crud Structures from Plant (left) and WALT Loop (right)

10 μm

170

In other words, WALT loop can produce similar or the same crud structures those

found from the surface of plant fuel assemblies. In addition, more test cases can be runs

in the WALT loop to cover a wider ranges for different crud structures, which can cover

all possible plant crud structure types.

In conclusion, the WALT loop crud is similar to the plant fuel assembly crud. The

WALT loop crud properties, such as thermal conductivity and porosity, should be

representative to the plant crud properties. More importantly, WALT loop is simulating

the real PWR operating conditions.

171

5.4 Crud Thermal Conductivity Sensitivity Study with BOA Computer Code

In this section, the WALT loop test section is modeled with VIPRE-01/VIPRE-W and

BOA, computer codes developed for Light Water Reactors (LWR). VIPRE-W (or VIPRE-01) is

the Westinghouse version of the VIPRE-01 code developed for the Electric Power Research

Institute (EPRI). The VIPRE-01 and BOA codes are widely used in the nuclear industry for PWR

reload safety analyses and/or Crud Induced Power Shift (CIPS) risk assessments.

5.4.1 VIPRE Modeling of the WALT Loop

The VIPRE-01 subchannel model is built based on the WALT loop configurations,

dimensions, and boundary conditions. Geometric data and key operating parameters used for

subchannel analysis are given in Figure 4-2 and Table 5-14. The whole test region with a single

heater rod is simulated in the VIPRE-01 model. The test cross section simulated in the VIPRE-01

subchannel model includes the entire heater rod and one subchannel surrounding the centered

heated rod. The axial power distribution is uniform with 7 nodes along the centered heater rod in

the chimney channel. In addition, the entrance effect has been simulated in the WALT loop test

region in the VIPRE-01 subchannel model.

The code default options were generally used when they reflect the original

suggestions given by the user's guide [61]. Boundary conditions are defined as the

following:

a. specified as uniform inlet temperature;

b. specified as uniform inlet velocity across the first node (ft/s or m/s);

c. specified as uniform power in kW/ft per rod;

d. system pressure is specified.

172

Table 5-14 Operating Conditions for Selected WALT Data

Parameters Normal Values

System Pressure, MPa 15.5

Heat Flux, W/cm2 95.0

Chimney Inlet Flow Velocity, m/s 3.05

Chimney Inlet Temperature, °C 330.0

Axial Power Shape Non-Uniform

Heater Rod Cladding Material ZIRLO

Heater Rod Heated Length, cm 29.7

Heater Rod OD, cm 0.95

Heater Rod ID, cm 0.84

173

5.4.2 BOA Modeling of the WALT Loop

The same as in the VIPRE-01 model, the BOA model is developed for the WALT

loop core using code options commonly used for PWR design or risk assessment

applications. In order to correctly model the WALT loop test section geometry and

operating conditions, the following conditions are defined the same as in the VIPRE-01

WALT loop model.

a. number of axial nodes;

b. length of the axial levels;

c. outer diameter of the heater rod;

d. rod pitch or hydraulic diameter;

e. channel flow area;

f. heated perimeter;

g. test section inlet temperature;

h. test section outlet temperature;

i. other operating parameters (e.g. heat flux and pressure, etc.) are from the

VIPRE-01 “.aoa” files.

A VIPRE-01/BOA Calculation Overview Diagram is shown in Figure 5-35. A number of

cases have been selected from the WALT loop experimental data. Calculations have been

performed and comparisons between the VIPRE-01/BOA calculated results and the WALT loop

experimental data have been made in the following section.

Figure 5-35 VIPRE-01/BOA Calculation Overview Diagram

BOA Sensitivity Study

Start

BOA Code To Adjust Crud Thermal Conductivity

WALT Loop Tests and Measurements

WALT Loop Dimensions and

Operating Conditions

BOA Calculations

Revise Crud Thermal

Conductivity

No

Stop

Reasonably Agree with

WALT Data ?

WALT Loop Dimensions and

Operating Conditions

BOA Input Model

VIPRE-01 Modeling and Calculating of

“.aoa” File for BOA Input

Yes

WALT Crud Thermal

Conductivity Calculation and

Correlation

(With coding changes)

175

5.4.3 VIPRE/BOA Calculations and Comparisons with Experimental Results

Both VIPRE-01 (VIPRE-W) and BOA calculations have been performed for nine

selected cases from the WALT loop experimental data. Calculated results of cladding

temperature differences (with and without crud deposition) are for crud thickness of 8, 16,

17, 24, 32, 40, 48, 55, 110 microns, which were typical crud thickness range on the fuel

pin surface of PWRs. This section provides two types of results/plots: (1) cladding

temperature difference comparisons (with and without crud deposition) between VIPRE-

01/BOA calculations and the WALT loop measured results with conditions provided in

the following, and (2) crud dryout fractions vs crud thickness.

Nine cases of WALT loop data have been selected and listed in Table 5-15. The

steady state nominal operating conditions for the selected nine cases are provided in

Table 5-14.

In Table 5-15, the cladding temperature difference is defined as the crudded

cladding surface temperature minus the clean cladding surface temperature. In addition,

the lower thickness values for WALT loop Rod# 22 were obtained by assuming that the

same thickness of crud is deposited with each crud solution injection. This assumption is

reasonable for steady state operations. The final crud thickness of 48 microns shown in

Table 5-15 is measured after the experiment is completed.

The experimental results given in Table 5-15 and the corresponding VIPRE/BOA

calculated results of cladding temperature difference are compared and shown in Figure 5-36.

176

Table 5-15 Data from Selected WALT Tests

Rod#

Crud Thickness

(microns)

Cladding Temperature

Difference (°C)

22 7.9 0.0

22 16.0 0.6

17 17.0 1.1

22 23.9 0.7

22 32.0 2.5

22 39.9 5.7

22 48.0 17.4

11 55.1 16.7

18 110.0 51.1

177

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0 20 40 60 80 100 120

Crud Thickness (Microns)

Cla

ddin

g D

elta

_T (

°C)

Calculated Delta_T

Measured Delta_T

( T(F) = 1.8T(K)-459.67, and 1.0 mil = 25.4 Microns)

Figure 5-36 Cladding Delta_T Comparison

178

Figure 5-36 shows that the cladding temperature does not increase much until the

crud thickness reaches about 25 microns, after which the cladding temperature is

significantly increased and the temperature change is quite impressive. A steep slope is

shown in Figure 5-36. The calculated cladding temperature difference in Figure 5-36 has

the same trend and very similar steep slope. Figure 5-37 shows that the calculated

cladding temperature will increase when crud dryout portion increases, as predicted by

BOA. It can be seen that the cladding temperature increases as the crud becomes thicker

and thicker and the dry crud percentage is also increased with a very steep slope. The

trend of the crud dryout and the cladding temperature increase agree with each other.

However, an improved crud dryout model with new crud thermal conductivity model is

suggested to be developed and coded in the BOA code in order to better predict the

cladding temperature and crud dryout percentage for difference crud thickness and other

conditions.

5.4.4 Sensitivity Study on Cladding Temperature Differences and Crud Thermal

Conductivity

Sensitivity studies have been performed to identify what key parameter changes

could influence the calculated cladding temperature results. This section provides the

sensitivity study on crud thermal conductivity. The crud thermal conductivity factor

values used in the study are 0.25, 0.50, 0.75, and 1.00.

This is an input variable in the BOA model. Even though a default value is

recommended in the BOA model, users can choose different thermal conductivity factor

values to calculate the crudded cladding temperatures. In the BOA code, a thermal

conductivity factor of 0.5 is recommended [34].

179

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0 20 40 60 80 100 120

Crud Thickness (Microns)

Cla

ddin

g D

elta

_T (

°C)

0.0

10.0

20.0

30.0

40.0

50.0Clad Delta_T

Crud Dry%

Dry

Cru

d (%

)

( T(F) = 1.8T(K)-459.67, and 1.0 mil = 25.4 Microns)

Figure 5-37 Calculated Cladding Delta_T and Dry Crud (%)

180

Different values of the thermal conductivity factor of 0.25, 0.50, 0.75, and 1.00

(Btu/hr-ft-°F, 1 Btu/hr-ft-°F = 1.73 W/m-°K) were used in this sensitivity study and

showed a significant effect on the crudded cladding temperature predictions (Figure 5-38).

The sensitivity study results show that the default value of the thermal conductivity factor

of 0.5 yields good agreement for thick crud temperature increase predictions as shown in

Figure 5-36. The crud thermal conductivities are calculated based on other direct

measurements as discussed previously. Given the different cladding temperature

differences for different crud thickness and other crud structure conditions, an improved

crud dryout model with new crud thermal conductivity model is suggested to be

developed and coded in the BOA code in order to better predict the cladding temperature

and crud dryout percentage for difference crud thickness and other conditions.

181

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0 0.2 0.4 0.6 0.8 1

Crud Thermal Conduct ivity Factor

Cla

ddin

g D

elta

_T (

°C)

(T(F) = 1.8T(K)-459.67)

Figure 5-38 Cladding Delta_T vs Crud Thermal Conductivity Factor

Chapter 6

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

1. A four-regime crud theoretical model is proposed based on crud deposition

observations. This theoretical model is used to guide the WALT loop experiments

for crud thermal conductivity measurement at different PWR operating conditions.

These four-regimes included in this theoretical model are Flooding Model,

Mixture Model, Dryout Model, and Particle Model, which are described or

defined as:

(1) Flooding Model simulates conditions of liquid flooding in crud chimneys or

pores. This condition typically applies to relatively lower power operating

conditions in PWR.

(2) Mixture Model applies to conditions of vapor and liquid mixed in crud

chimneys or pores. This situation occurs during normal high power operating

conditions in PWR.

(3) Dryout Model is for simulating crud dryout, occurring gradually from the

bottom to the top of crud layer at higher power operating conditions in PWR.

(4) Particle Model is for crud pores filled or partially filled with solid particles.

This situation may occur when a plant has zinc injection or significant impurities

183

in the reactor coolant system (RCS).

2. Two types of methods have been proposed to calculate crud thermal conductivity

based on measured data from the WALT loop. The first method is the mathematic

solution method, which use two different WALT test cases with similar but

slightly different conditions to solve for the crud thermal conductivity. The

second method is to use a WALT test case and one existing and applicable

correlation to solve the unknowns.

3. In order to understand crud formation on the fuel rod cladding surfaces of

pressurized water reactors (PWRs), a crud Thermal-Hydraulic test facility referred

to as the Westinghouse Advanced Loop Tester (WALT) is built at the

Westinghouse Science and Technology Department Laboratories in October 2005.

Since then, a number of updates have been made. After these updates were made,

the WALT system operated with higher stability and fewer failures.

4. In the WALT loop, crud can be deposited on the heater rod surface and the

character of the crud is similar to what has been observed in the PWRs. In

addition, chemistry in the WALT loop can be varied to study its impact on crud

morphology and associated parameters. The WALT loop has been successful in

generating crud and measuring its thermal impact as a function of crud thickness.

Currently, this test facility is supporting an Electric Power Research Institute

(EPRI) program to assess the impact of zinc additions to PWR reactor coolant.

184

Meanwhile, the WALT system is also being utilized by Westinghouse to perform

dry-out and hot spot tests, etc. These tests support the industry goal of zero fuel

failures by 2010 set by commercial nuclear industry executives at an executive

meeting of Institute of Nuclear Power Operations (INPO).

5. In this dissertation, six WALT test cases have been selected and data reductions

have been performed for crud thermal conductivity calculations. Crud assessment

has been performed using SEM and XRD analyses. Preliminary crud thermal

conductivity results have been calculated and documented based on the WALT

test data, SEM and XRD results. The SEM and XRD results were used to identify

the fourth crud regime and chemical components in the crud.

6. Benchmarking or sensitivity studies have been performed using the VIPRE and

BOA computer codes. Differences have been found between VIPRE/BOA

calculated and WALT measured cladding temperature differences for different

crud thickness and crud thermal conductivities. The newly measured crud thermal

conductivity data is one of the base cases for the BOA coding improvement in the

future work in this area.

7. The thermal conductivity measurement results will provide the nuclear industry

several benefits, in the form of data that will allow a better assessment of the

effect of crud on fuel failure. In detail, the measured crud thermal conductivities

are utilized in the following areas.

185

• Data for validation of key BOA code heat transfer models.

• Crud and heat flux limit curves against which to assess fuel failure risk

• Other areas, which may have a future potential need for crud thermal effects (it

should be noted that NRC is currently evaluating the potential effect of crud on

LOCA safety analysis).

• In the future, the method for crud thermal conductivity measurement can be

utilized to measure other thermal conductivity for other materials, if needed.

In addition, relevant information is published in three papers [50, 52, 71].

186

6.2 Recommendations for Future Work

1. Recently, an EPRI proposal, titled “Crud Thermal Conductivity Measurements at

Crud Dryout and High-Boiling-Duty PWR Conditions” [62] has been sent to

EPRI for near future work to systematically and more accurately measure crud

thermal conductivities at crud dryout conditions.

2. The crud thermal conductivity results provided in this dissertation are

preliminary. Especially, the crud thickness is averaged value. And the thermal

couple measured cladding temperatures are also averaged values. It is

recommended to use local crud thickness and local temperature measurement

results for the near future EPRI work.

3. Further BOA code benchmarking is suggested to confirm the different cladding

temperature differences for different crud thickness and other crud structure

conditions. An improved crud dryout model with new crud thermal conductivity

model is suggested to be developed and coded in the BOA code in order to better

predict the cladding temperature and crud dryout percentage for differenct

conditions.

4. The Updated Westinghouse Advanced Loop Tester (WALT) has the advantage of

being relatively easy to assemble and disassemble for tests simulating PWR hot

region fuel rod at normal operating conditions. This crud test facility has been and

187

will be utilized to perform a wide range of experiments to study effects of coolant

chemistry and cladding surface condition on crud deposition. Experiments

performed in this test facility can also provide guidance for more difficult and

expensive experiments and for evaluations of plant chemistry effects on crud

deposition. Some other future work may include the following items.

• Generate more experimental data to further investigate crud heat transfer

effect under PWR fuel rod geometry and operating conditions;

• Dry-out study with thick crud depositions;

• Local crud thermal conductivity measurement and more

analytical/sensitivity study.

5. In future work or studies, it is suggested to perform uncertainty or error analyses

including all relevant parameters.

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Appendix A

Formulations of Heat Flux and Temperatures in a Hollow Cylinder

The formulations of heat flux and temperature distributions in a hollow cylinder,

e.g. the WALT loop heater rod, are derived in the appendix based on Fourier’s Law, and

Newton's law of cooling, etc.

A.1 Formulation Derivation of Cladding Outer Surface Temperature

As discussed in Chapter 4, the WALT loop heater rod is a long hollow cylinder design

from the heat transfer point of view. The cylinder has inner radius r1 and outer radius r2. Under

steady state operation conditions, it is assumed that the inner temperature remains constant at T1

and outer temperature at T2. It is important to point out that the heat is being generated within the

cylinder wall and can only be transferred to the surrounding coolant radially. This is a reasonable

assumption. Fourier’s Law is written as,

dr

dtkAQ 0−= (A-1)

For the hollow cylinder geometry shown in Figure A-1, Fourier’s Law is re-written as the

following,

( ) ( )dr

dtLrkdrLrq H

r

r H ⋅⋅×−=⋅⋅′′′∫ ππ 221

(A-2)

197

Figure A-1 A Hollow Cylinder Heater Rod Design

198

In Equations (A-1) and (A-2), Q is the average heater rod power. HL is the heater rod or

hollow cylinder heated length. k is the thermal conductivity of the heater rod cladding or hollow

cylinder wall. 0A is the radial heat transfer area. The wall temperature of t is for location at r

within the cylinder. '''q is the volume heat flux generated in the cladding by electric current.

After integration and simplification on Equation (A-2), the following equation is

obtained,

( )dr

dtrkrrq ⋅⋅−=−′′′ 2

12

2

1 (A-3)

Or,

dtdrr

rr

k

q −=⎟⎟⎠

⎞⎜⎜⎝

⎛−

′′′ 21

2 (A-4)

Integration on both sides of the above equation from 21 rr → , and 21 TT → respectively,

∫∫ −=⎟⎟⎠

⎞⎜⎜⎝

⎛−

′′′ 2

1

2

1

21

2

T

T

r

rdtdr

r

rr

k

q (A-5)

From Equation (A-5), we get the following,

( )122

12

21

221 2

1

2

1ln

2TTrr

r

rr

k

q −=⎥⎦

⎤⎢⎣

⎡+−⎟⎟

⎠

⎞⎜⎜⎝

⎛′′′ (A-6)

199

The following equation is obtained from Equation (A-6) and can be used to calculate the

heater rod cladding outer surface temperature. As mentioned in Chapter 3, the inner cladding

surface temperature is measured using thermocouples embedded inside the heater rod or hollow

cylinder.

⎥⎦

⎤⎢⎣

⎡+−⎟⎟

⎠

⎞⎜⎜⎝

⎛×−= 2

22

12

12112 ln2

4'''

rrr

rr

k

qTT (A-7)

Equation (A-7) is equivalent to Equation (3-4) in Chapter 3.

A.2 Formulation Derivation of Crud Outer Surface Temperature

As discussed in Chapter 4, the heat is being transferred from the WALT loop heater rod

cladding through the crud deposition, if applicable, to the water coolant. The hollow cylinder

heater rod has inner radius r1 and outer radius r2. And the crud has inner radius r2and outer radius

r3. Under steady state operation conditions, it is assumed that the crud inner temperature remains

constant at T2 and outer temperature at T3. At this time, the heat is only passing through the crud

layer and there is no heat generated inside the crud.

Apply Fourier’s Law of Equation (A-1) to the crud layer shown in Figure A-2, Equation

(A-1) is re-written as the following,

( ) ( )dr

dtLrkdrLrq Hd

r

r H ⋅⋅×−=⋅⋅′′′∫ ππ 222

1

(A-8)

After integration and simplification on Equation (A-8), the following equation is

obtained,

( )dr

dtrkrrq d ⋅⋅−=−′′′ 2

12

22

1 (A-9)

Or,

( ) dtdrr

rrqkd

−=××−′′′ 1

2

1 21

22 (A-10)

Integration on both sides of the above equation from 32 rr → , and 32 TT → respectively,

201

Figure A-2 A Hollow Cylinder Heater Rod with Crud Layer

202

( ) ∫∫ −=××−′′′ 3

2

3

2

1

2

1 21

22

T

T

r

rd

dtdrr

rrqk

(A-11)

From Equation (A-11), we get the following,

( ) ( )232

321

22 ln

2

1TT

r

rrrq

kd

−−=×−′′′ (A-12)

The following equation is obtained from Equation (A-12) and can be used to calculate the

crud layer outer surface temperature. As mentioned in Chapter 3, the crud inner surface

temperature is the same as the cladding outer surface temperature.

( )

2

32

12

223 ln

2 r

r

k

rrqTT

d

×−′′′

−= (3-13)

Equation (A-13) is equivalent to Equation (3-5) in Chapter 3. where, dk is the

overall thermal conductivity of the crud.

Appendix B

EDTA

EDTA is an abbreviation for Ethylenediaminetetraacetic Acid. A 3D chemistry

model of the EDTA structure is shown in Figure B-1. The EDTA molecule can bind to

metal ions by forming six bonds to its structure, i.e. two from nitrogen atoms in amino

groups and four from oxygen atoms in carboxyl groups. This is important for injecting

crud solution to the WALT test loop.

Figure B-1 A 3D chemistry model of the EDTA structure

VITA

Guoqiang Wang

Guoqiang Wang, born on May 3rd, 1964 in Renqiu, Hebei, People's Republic of China,

has attended High School in Renqiu, where he graduated with honors in 1983. He obtained a

Bachelor’s degree in Reactor Engineering from the Tsinghua University in Beijing in July 1988.

After graduation, he joined the China Institute of Atomic Energy (CIAE) where he specialized in

nuclear reactor safety and gained practical experience in the control theory of Heavy Water

Research Reactors (HWRR). He also participated in the feasibility studies of the Chinese

Experimental Fast Reactor (CEFR).

His background in nuclear reactor safety, nuclear technology and heat transfer, and his

competence in reactor control problems have gained him wide appreciation with the CIAE. In

consideration of his capabilities, he was delegated between 1991 and 1992 for 15 months to

Algeria/France to support, on behalf of the CIAE, the Algerian nuclear reactor development

program. He worked there as an operator/acting supervisor and this activity was concluded with

the initial criticality of an Algerian nuclear reactor.

After four years graduate study leading to concurrent MS degrees and passing a Ph.D.

comprehensive exam in the Department of Mechanical and Nuclear Engineering at The

Pennsylvania State University, he has been working at Westinghouse Electric Company since

May 8, 2000. He is now a Principle Engineer and Functional Lead in the Fuel Rod and Thermal

Hydraulic Design group of Core Engineering Department at Westinghouse. He completed a

doctoral degree in Nuclear Engineering from The Pennsylvania State University in March 2009.

His hobbies are reading books, utilizing computers, listening to music, long distance runs,

and playing Table Tennis games (Ping-Pang). His goal in his life is to contribute to the scientific

community to safely provide clean energy and a better future for the next generations.

improved crud heat transfer model for dryout on …

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