stratified flow phenomena, graphite oxidation, and mitigation strategies of air ingress accident inl...

Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident

INL / Chang Oh, U.S. PI

Background and MotivationBackground and Motivation

• What happens following What happens following LOCA ?LOCA ? Depressurization Stratified Flow Diffusion Natural Convection

• T/H Safety IssuesT/H Safety Issues Core maximum temperature Potential core collapse

• Technical RequirementsTechnical Requirements Accurate stratified flow

modeling Accurate graphite oxidation

and collapse modeling Accurate power distribution

with neutronics model

ObjectivesObjectives

• To conduct experiments to supply information to model important phenomena in air-ingress accident, and code V&V. Effect of density-driven stratified flow on the air-ingress Oxidation and density variation of the graphite structures Internal pore area density of the graphite structures Effect of burn-off on the structural integrity of the graphite

structures

• To develop a coupled neutronics and thermal-hydraulic capability in the GAMMA code Development of core neutronics model Coupling neutronic-thermal hydraulic tools Coupled core model V&V

• To evaluate various methods for the mitigation of air-ingress

Project OrganizationProject Organization

Schematic diagram of all tasks involved

GAMMACode

Task 1 (INL)

Stratified Flow Analysis(CFD & GAMMA)

Task 2 (INL)

Stratified Flow Experiment

Validation

Stratified Flow Study

Models and Parameters

Task 4 (INL)

Full Air-ingress AnalysisAir-ingress Mitigation Study

Analysis

Task 3 (INL)

Advanced Graphite Oxidation Study

Task 5 (KAIST)

Experiment of Burn-offIn the Bottom Reflector

Task 6 (KAIST)

Structural Test of Burn-off Bottom Reflector

Task 7 (KAIST)

Coupling Neutronic Thermal Hydraulic Tool

Task 8 (KAIST)

Core Neutronic Model

Task 9 (KAIST)

Coupled Core ModelV&V

Advanced Graphite Oxidation Study


Core Neutronic Model


Stratified Flow in Air-ingress

• Previous air-ingress analyses are all based on the assumption that the main air-ingress is dominated by molecular diffusion.• •Previous analyses were performed using 1-D and a vertical geometry

• A new issue has been raised for the possibility of a convective flow driven by local density gradient.

• After depressurization, there is large density differences between inside(Helium) and outside(Air) of vessel.

• The density driven stratified flow can highly accelerate the whole air-ingress scenarios.

New Assumption on the Air-ingress AnalysisNew Assumption on the Air-ingress Analysis

Diffusion Assumption(40000 sec)

Stratified Flow Assumption(60 sec)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Air

Ve

loci

ty (

m/s

)

Pipe Diameter (m)

Density-Gradient Driven Stratified FlowDensity-Gradient Driven Stratified Flow

Air

Helium Hα

Hα)-(1 H

L

AirHelium

centerP

HgHgPHgPP HeairaircenterHecentertop

222

HgHgPHgPP HeairHecenteraircenterbottom

2

)1(

2

)1(

2

)1(

(1) Volumetric flowrates of air and helium are the same.

(2) At the interface, the shear stresses are the same between Air and Helium.

HLf

gu

air

Heair

)1(

5.0

2

Air ingress velocities by density driven flow

He Air He Air He Air

(1) Depressurization (2) Onset-of Flow (3) Density-driven Flow

Density Driven Stratified FlowDensity Driven Stratified Flow

Helium Air

Water and Salted Water Experiment

WaterSalted Water

New Scenario for Air IngressNew Scenario for Air Ingress

HeliumAir Helium Air Helium Air Helium

(a) Depressurization (b) Stratified Flow (c) Diffusion (d) Natural Convection

Several Minute

Several Minute

Several Days

CFD Analysis on the Stratified Flow in VHTRCFD Analysis on the Stratified Flow in VHTR

23.7 m

5.4 m

11.0 m

2.4 m

4.5 m

0.4 m

GT-MHR 600 MWt

6.8m

0.5 m

01.5 m

12.1 m

25.2 m

0.4 m

Mesh (GAMBIT / FLUENT)

51,566 nodes

Porous Media Approach• Core and Plenum were assumed to be porous media.• Porosity and Permeability should be determined.

Porous Zones

imagii vvCvS

2

12

Additional Momentum Source

permeability Inertia resistance

Porous Media ParametersPorous Media Parameters


Porosity

2

2

43

81

p

d

A

A

V

V

total

fluid

total

fluidcore

Core Hole Pattern(d = 1.58 cm, p = 3.27 cm)

Geometry of Lower Plenum(d = 0.212 m, p = 0.36 m)

2

22

43

81

43

LP

LPLP

total

fluid

total

fluidmlowerplenu

p

dp

A

A

V

V

In the Core

In the Lower Plenum

21.0core

68.0PlenumLower


Friction Factor in the Circular Channel

Flow Resistance Parameters in the Core• Empirically determined based on the friction correlations• Flow resistance in the radial was assumed to be infinitely large.

In the Core

Re

55015.0 f

D

LuP 2

2

1

Re

55015.0

D

Lu

uDP 2

2

1

)/(

55015.0

22 2

1015.0

2

55u

Du

DL

P

262 1008.955

2mD

12 949.0

015.0 mD

C

1000 10000 100000 1000000 1E70.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

from Moody Chart

Laminar

f

Re

Turbulent


1000 100000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Kays and London Chilton and Generaux Gunter-Shaw Zukauskas

fric

tion

fact

or

Re

Friction Factor in the Staggered Array

In the Lower Plenum

200769.0 m1

2 0326.0 mC

Re

20042.0 f

2002116.0 m1

2 913.0 mC

For axial direction

For radial direction

Flow Resistance Parameters in the Lower Plenum• Flow resistance in the radial direction was calculated based on the friction data in the staggered array.• Flow resistance in the axial direction was calculated based on the friction data in the circular pipe flow.

Simulation of Stratified FlowSimulation of Stratified Flow

Natural convection started about 160 sec after simulation.

Temperature

Air-Mole Fraction

Initial ConditionsStratified Flow Simulation (by FLUENT 6.3)

Turbulence ModelsTurbulence Models

Air-ingress AnalysisAir-ingress Analysis

Multi-step Approach for Air-ingress Analysis• Stratified flow phase was solved by CFD code (FLUENT).• Depressurization and Diffusion/natural convection phase were solved by GAMMA code.

520

515(9x2)

513(9x3)

604

603

512(9x1)

Block Core514

(9x10)

511

605

701

602

ReactorCavity300

(2x15)

350Vair=49,000 m3

160 110

105

115

130

200205

220

143(12)

141(12)

142(12)

165

210

225231~235

251~255

260 265

125

144(12)

145(12)

120(

7)

215~216

702

FLUENT Simulation GAMMA Simulation

1. Depressurization Analysis

2. Stratified Flow Analysis 3. Natural Convection AnalysisData Transfer

Data Transfer

CFD Code System Code

Air Ingress Analysis - ResultsAir Ingress Analysis - Results

0 100 200 300 400

800

900

1000

1100

1200

1300

1400

1500

1600

Tem

pera

ture

(C

)

Time (hrs)

DDA-1 DDA-2 SFDA-1 SFDA-2

Maximum Temperature Criteria

0 100 200 300 400550

600

650

700

750

800

850

900

950

1000

Tem

pera

ture

(C

)

Time (hrs)


0 100 200 300 4000.50

0.55

0.60

0.65

0.70

0.75

0.80

Vo

id F

ract

ion

of G

rap

hite

Str

uct

ure

Time (hrs)


Stratified Flow Assumption

Diffusion Assumption

Temperature(Core)

Temperature(Bottom Reflector)

Corrosion(Lower Plenum)

DDA-1Diffusion Dominated Air-IngressIn the Infinite Vault (1X1010 m3)

DDA-2Diffusion Dominated Air-IngressIn the Finite Vault (25,000 m3)

SFDA-1Stratified Flow Dominated Air-IntressIn the Infinite Vault (1X1010 m3)

SFDA-2Stratified Flow Dominated Air-IntressIn the finite Vault (25,000 m3)

0

(At the center of the core)

0


0


Experimental Plan - 1Experimental Plan - 1

HeliumPulsed Laser

Source

Pressurized Pressurized

CO2

PP

Vacuum Pump

CCD Camera

CO2 (10.6 atm)

N2 (6 atm)

Annular Pipe

Isothermal Experiment in the Horizontal Circular Pipe (TEST-1)

• Focused on the separate effect of stratified flow phenomena• A simple scaling method used for pipe sizing and test conditions.

1~1.5 m

0.5~1.0 m

1m

Tube diameter = 20 cm


Scaling Analysis of Stratified Flow in a Simple Channel

Countercurrent stratified flow behavior in the VHTR hot duct

2/1,

2/1,

2/3 )()()()()( stdRstdRcRRR dP

C

HCLP

dgu

)(

5.0

RC

RRd

2/1

2/3

)(

)()(

2/1

)(RC

RLP

du

(Turner(1973))

By gas law By Reynolds number similitude

Air-Helium CO2-Helium Ar Helium Diameter ratio ( Rd ) 0.1 0.133 0.1 0.133 0.1 0.133

Viscosity ratio ( R ) 1 1 0.833 0.833 1.26 1.26

Density ratio at std ( R ) 1 1 1.571 1.571 1.43 1.43

Density different ratio at std ( R ) 1 1 1.667 1.667 1.5 1.5

Pressure ratio ( RP ) 31.6 20.5 16.2 10.6 26.6 17.2

Summary of Scaling Results (ratio = scaled down/full-scale)

2/1,

2/1,

2/3 )()()()()( stdRstdRcRRR dP


Water In

Water Out

Pressure

Temperature

Flow Rate

Heater

Insulation(can be detached)

Water Tank

Valve-1

Valve-2Valve-3

Glass Chamber

Buffer Layer

Non-isothermal Test (TEST-2)• Focused on the coupling effect of stratified flow and natural convection.• On-set of natural convection is the main measuring parameter.

0.8~1.0 mPipe diameter 5 cm


10 sec 20 sec 40 sec 80 sec

300 sec 400 sec 410 sec 420 sec

Fluent Simulation for Density Driven Air-ingress Experiment

Onset of natural convection occurred at around 400 sec.


Time (sec)

0 200 400 600 800 1000

Mas

s F

low

(kg

/s)

-0.002

-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Time (Sec)

0 200 400 600 800 1000

Tem

pera

ture

(K

)

290

300

310

320

330

340

350

Time vs. TemperatureTime vs. Flow-rate

onset of natural convection

onset of natural convection

Flow rate and Temperature can be used as signals for onset-natural-circulation.

FLUENT Results for Density Driven Air-ingress Experiment (at Valve-3)

Experimental Plan – 3 and 4Experimental Plan – 3 and 4

Water In

Water Out

Pressure

Temperature

Flow Rate

Heater


Water Tank

Valve-1

Valve-2Valve-3

Glass Chamber

Buffer Layer

Metal or Ceramic Pebbles

Water In

Water Out

Pressure

Temperature

Flow Rate

Heater


Water Tank

Valve-1

Valve-2Valve-3

Glass Chamber

Buffer Layer

Metal or Ceramic Structure

Non-isothermal Test (TEST-3, TEST-4)• Focused on the coupling effects

Stratified Flow + Natural Convection + Porous Media + Chemical Reaction

• Basic Experimental Procedures are the same as TEST 2

Non-isothermal test with Pebbles Non-isothermal test with Structures

Metal (TEST-3) or Graphite (TEST-4)

Experiment on the Oxidized Graphite Experiment on the Oxidized Graphite FractureFracture

Experimental Set-up• The experiment was performed at 650 oC for uniform oxidation.• The test procedure and set-up is based on ASTM standard test method.• IG-110 and H451 graphite was used for testing.

Cage

Load

GraphiteSample

1 in

1 in

1 in

0.5 in

Sample load and holder

Experiment on the Oxidized Graphite Experiment on the Oxidized Graphite FractureFracture

0 20 40 60 80

0.0

0.2

0.4

0.6

0.8

1.0 Ishihara et al. (2004) This work Correlation (Eto and Growcock (1981)) Correlation (This Work)

Nor

mal

ized

Com

pres

sive

Str

engt

h

Burnoff [%]

IG-110

400

0.400 /17.0/83.0/ SS

5.600 // SS

0 20 40 60 80

0.0

0.2

0.4

0.6

0.8

1.0 Ishihara et al. (2004) This Work Correlation (Eto and Growcock (1981)) Correlation (This Work)

No

rma

lize

d C

om

pre

ssiv

e S

tre

ng

th

Burnoff [%]

H-451

400

5.30 )/(21.0)/(79.0/ oSS

25.600 )/(/ SS

Normal Compressive Stress vs. Burn-off

IG-110 H-451

old data

New dataNew data

old data

Graphite Surface Area DensityGraphite Surface Area Density

Graphite Surface Area Density (Unoxidized Initial Value)• The graphite surface area density was calculated from the BET surface area measured by previous investigations.

Density[g/m3]

Specific Surface Area[m2/g]

Surface Area Density[m2/m3]

NBG-18(Contescu (2008)) 1790 0.21 375.9

NGB-10(Contescu (2008)) 1790 0.29 519.1

PCEA(Contescu (2008)) 1790 0.21 375.9

20-20(Contescu (2008)) 1790 0.46 823.4

IG-11(Eto and Growcock (1981)) 1750 2.8 4900

IG-110(Nakano et al. (1997)) 1780 0.5 890

H451(Pawelko et al. (2001)) 1760 0.75 1320

PGX(Eto and Growcock (1981)) 1730 0.7 1211

Effect of Graphite Burn-off on the Oxidation Effect of Graphite Burn-off on the Oxidation RateRate

Graphite Oxidation Rate and Burn-off• The reaction rate increases with the increasing burn-off in the beginning because of the increase of pore size and porosity open.• The reaction rate decreases at high burn-off because the pores join together, decreasing the reaction surface area.

0 10 20 30 40 50 60 70 80

1000

2000

3000

4000

5000

6000

650oC

Su

rfa

ce A

rea

De

nsi

ty (

m2 /m

3 )

Burn-off (%)

IG-110 H-451

Modeling of Graphite Oxidation and Modeling of Graphite Oxidation and Fracture Fracture in Air-ingressin Air-ingress

23.7 m

5.4 m

11.0 m

2.4 m

4.5 m

0.4 m

GT-MHR 600 MWt

6.8m

0.5 m

01.5 m

12.1 m

25.2 m

0.4 m

Core Graphite Structure

Reference Reactors (GT-MHR 600 MWt)

Estimation of Corrosion Depth by GAMMA Estimation of Corrosion Depth by GAMMA codecode

Most Seriously Damaged!Most Seriously Damaged!

• Burn-offBurn-off refers to the oxidation of the graphite’s internal body, causing reduction of density, leading to reduction of stiffness and mechanical strength.

• CorrosionCorrosion refers to oxidation taking place on the outer surface exposed to airflow. The corrosion decreases the cross-sectional area available to support the weight leading to stress concentration.

Detailed Geometry of the Supporting BlockDetailed Geometry of the Supporting Block

20c

c

DL

4j

s

DL

10jD

s

4.1jr

r

7pD

H jV

pV

Coolant channels from core

Lower reflector

blocks

Oxidized State ResultsOxidized State Results

Compressive stress distribution on plenum head, 6.5 days after ONC

Corrosion ProgressionCorrosion Progression

1/6 cyclic symmetry unit of the modified plenum head for each day

* The specified time is the elapsed time after natural convection.

Oxidized State ResultsOxidized State Results

Failure occurs 5.5 to 6 days after the start of natural convection

Maximum Compressive Stress

0

10

20

30

40

50

60

70

80

90

5 6 7 8 9 10 11 12 13Time [days]

Str

ess

[MP

a]

EdgeInsideMinimum Compressive Strength

Maximum Tensile Stress

0

5

10

15

20

25

30

35

40

45

5 6 7 8 9 10 11 12 13Time [days]

Str

ess

[MP

a]

EdgeInsideMinimum Tensile Strength

Task 5 - Experimental facility Task 5 - Experimental facility

F

F

T

COOLER

GasAnalyzer

COCO2O2

2nd Regulator

MFCGas

Injection

Check Valve

Test Section

InfraredThermometer

He

O2

Vent Reservior

InductionHeater

P

TP

TT

Schematics of Experimental Facility

Kinetics

1. Activation energy2. Order of reaction

Experimental facility

1. Design2. Installation

Mass transfer

1. Heat/mass analogy

Other effects

1. Geometry2. Burn-off3. Moisture

Graphite selection

1. IG-1102. IG-4303. NBG-184. NBG-25

Bottom reflector Burn-off model

Task ProgressCompleted activityPlanned activity

Task 5 - Test ConditionTask 5 - Test Condition

Infrared Thermometer

Induction Heater

graphite

7.6 cm2.1 cm

3 cm

Graphite Temperature (Temperature (ooC)C) 540 ~ 800540 ~ 800

Flow rate (SLPM)Flow rate (SLPM) ~ 10 SLPM (0.04 m/s)~ 10 SLPM (0.04 m/s)

Oxygen fraction (%)Oxygen fraction (%) ~ 34 %~ 34 %

Picture of the Test Section

Task 5 – Kinetics (I)Task 5 – Kinetics (I)

IG-110 IG-430Activation energy (kJ/mol) 218 ± 4 158.5 ± 1.5Order of reaction, n 0.75 ± 0.15 0.37 ± 0.04

1.00 1.05 1.10 1.15 1.20 1.25-26.0

-25.5

-25.0

-24.5

-24.0

-23.5

-23.0

-22.5

-22.0

Ea = 158.5kJ/mol

ln(

Rg(k

g/s

) )

1000/T (K-1)

flow rate = 8 SLPMoxygen mole fraction = 5.26%

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0-23.4

-23.3

-23.2

-23.1

-23.0

-22.9

-22.8

-22.7

-22.6 923K, 10 SLPM Linear fit

ln(R

g(k

g/s

))

ln(PO2

(atm))

n = 0.371error = 0.03819

Effect of Temperature on Oxidation Rate Effect of Oxygen Concentration on Oxidation Rate

Task 5 – Kinetics (II)Task 5 – Kinetics (II)

Material Author T ( )℃Oxygen Mole

FractionFlow rate(SLPM)

Ea (kJ/mol) n Method

IG-110

Fuller 450~750 0.2 0.496 201 - TGA

Kawakami 550~650 0.2 - 210 - Gas Analysis

Ogawa 700~1500 0.05~0.19 0.2~4.5 200 - Gas Analysis

KAIST 540~630 0.03~0.32 7~18 218 0.75Gas

Analysis

IG-430

KAERI 608~808 0.2 10 161.5 - TGA

KAIST 540~800 0~0.34 8~10 158.5 0.37Gas

Analysis

Task 6 - Task ProgressTask 6 - Task Progress

Fresh graphite

Structure

1. Support column2. Support block3. Other components

Oxidized graphite

1. Uniform oxidation2. Non-uniform

oxidation

Failure test

Failure ModelDevelopment

Task ProgressCompleted activityPlanned activity

GAMMA

1. Estimation of burn-off Data collection

1. Mechanical test2. Structural analysis

D

Task 6 - Bottom structuresTask 6 - Bottom structures

Components Condition Relative temperature

Support column

A graphite column encounters oxygen first when an air-ingress event occurs

Low (cooling effect)

Support block A support block has a lot of channels. External surface is relatively large.

High (close to core)

Bottom reflector components and condition

Graphite support columns, GT-MHR 600MWth

Schematics of oxidation trend in graphite support columns

Task 6 - Test facility Task 6 - Test facility

Air outletVent

Graphite specimen

Support plate

Thermocouple

Insulating material

Air inletDistributor

Hardened steel plate

Test specimen

Spherical block

Machine cross head

Clear Plastic safety shield

Compression block

Picture of the failure test facility

Picture of the electric furnace

IG-110Φ15mm ×30mm 80.16 ± 1.97MPaΦ25mm ×50mm 78.75 ± 2.48 MPa

Average 79.46 MPa

0 10 20 30 40 500

20

40

60

80

Experimental data Linear fit of data

(M

Pa)

Slenderness ratio (L/r)

,cr buckling straight line

LC

r

91.31 MPastraight line

1.01C

79.46 91.34 1.01

11.76

L

rL

r

Buckling point:

Task 6 – Task 6 – Compressive and buckling strength of Compressive and buckling strength of fresh IG-110 columnfresh IG-110 column

Compressive strength of IG-110

Buckling strength

Task 6 - Task 6 - Compressive and buckling strength of Compressive and buckling strength of oxidized IG-110 columnoxidized IG-110 column

-2 0 2 4 6 8 10 12 14 16 18 20 22 24-4

-2

0

Strength data Linear fit

Ln

(

)

Decrease in bulk density(weight loss), d(%)

0 2 4 6 8 10 12 14 16 18 20 22 24

-2

0

x x x x x

Ln(

0)

Decrese in bulk density(weight loss), d(%)

Normalized compressive strength of oxidized graphite

Normalized compressive and buckling strength of oxidized graphite columns.

0

exp( )

0.111

kd

k

The strength of oxidized graphite column under axial load:

Sample 1 Sample 2 Sample 3

Geometry (Unit: mm)Top: Φ10 × 10Bottom: Φ20 × 10

Top: Φ10 × 20 Bottom: Φ20 × 20

Cylindrical column: Φ15 × 60 Cap: 15.1 × 10 (7 in depth)

Average fracture load (Unit: Kgf)590.1 531.3 1297.3

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

-4

-2

0

Sample 1 Sample 2 Sample 3

Ln(F

/F0)

Decreas in bulk density(weight loss), d(%)

Fracture load changes of oxidized complicated-shape samples

Task 6 - Task 6 - Fracture load changes of Fracture load changes of oxidized complicated-shape samplesoxidized complicated-shape samples

Table of complicated-shape sample test

stratified flow phenomena, graphite oxidation, and mitigation strategies of air ingress accident inl...

Documents