north carolina state university - calculation no. nrp-98 ... · nrp-98-01 . criticality analysis...

Attachment 4

North Carolina State University

Calculation No. NRP-98-01

Criticality Analysis for a 250 Fresh Pin Storage Rack

of pages

NCSU NUCLEAR REACTOR PROGRAM ANALYSIS/CALCULATION FOR

Title Criticality Analysis for a 250 Fresh Fuel Pin

Storage Rack

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~repared by /Date Reviewed by /Date Approved by /Date

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Keywords Criticality. Fuel Storage

NRP-98-01

Criticality Analysis for a 250 Fresh Fuel Pin Storage Rack

Page 2 Date 23 Nov 1998

Prepared by P.B. Perez Checked by K. Verghese Approved by C.W. Mayo

ABSTRACT

A criticality analysis was performed using the SCALE code CSAS sequence to analyze a new fresh fuel pin storage rack. The rack will store up-to 250 pins of 6% or less enrichment in a fixed geometry. The results of the analysis demonstrate the multiplication factor of the assembly is less than 0.9 as required by the facility technical specifications.

NRP-98-01



Prepared by P .B. Perez Checked by K. Verghese Approved by C.W. Mayo

TABLE OF CONTENTS

i. Abstract ii. List of Figures iii. List of Tables 1.0 Introduction 2.0 Discussion of Design and Analysis Methodology 3.0 Numerical Analysis Results 4.0 Safety Evaluation 5.0 Conclusions 6.0 References

Page No. 2 4 5 6 6

10 10 10 13

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Figure 1 Figure 2 Figure 3

Checked by, K. Yergbese Approved by C.W. Mayo

LIST OF FIGURES

Page

Fuel Pin Unit Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Storage Rack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Analytical Model of Storage Rack . . . . . . . . . . . . . . . . . . . . 12

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Prepared bY. P.B. Perez

Table I Table II

Checked bY. K Verghese Approved bY. C.W. Mayo

LIST OF TABLES

Page

Unit Cell Dimensions • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 CSAS Parametric Study Summary . . . . . . . . . . . . . . . . . . . 11

NRP-98-01



Prepared by P.B. Perez Checked by K. Verghese Approved by C. W. Mayo

1.0 Introduction

The PUISTAR reactor facility acquired 246 fresh fuel pins of 6% enrichment through the U.S. Department of Energy Fuel Assistance Program. The individual fuel pins were to be stored in two racks included with the fuel. These storage racks were not selected for storing fuel pins at the PULST AR in favor of a new design .

A new storage rack was designed by following established criticality safety guidelines [1 ]. The criticality of the storage rack assembly was analyzed with the SCALE code CSAS analysis sequence to demonstrate the multiplication factor is within the Technical Specification limit.

2.0 Discussion of Design and Analysis Methodology

2.1 Design

Geometry and material control are the major factors affecting criticality in an assembly. The new PULSTAR fuel storage rack addresses the two factors by mechanically fixing the geometry and administratively controlling material. The criticality analysis is based on a full rack containing 250 pins.

Criticality may occur by fast or thermal neutron multiplication. Fast criticality in heterogeneous systems is associated with dry conditions and thermal criticality involves flooded (moderated) assemblies. The U02 mass present in 250 6% U02 fuel pins is 145.675 kg and the corresponding U-235 mass is 7.757 kg [2]. The U-235 mass present in U02 is not sufficient for fast criticality since the required U-235 (metal) mass for fast critical is approximately 49 Kg in optimum geometry [3,4]. A result, the criticality calculation will focus on thermal criticality.

The fuel rack consists of 250 aluminum seamless tubes in 5 by 50 array. The tubes are constructed of 6061-T6 aluminum and measure 0.75 inches OD with a 0.065 inch wall thickness. The tubes are fixed in place with a pitch of 1.50 inch with multiple aluminum spacer plates. The assembly includes aluminum angle-bar for mounting on the PULST AR bay West wall.

2.2 Analysis Methodology

The SCALE {Standardized Computer Analyses for Licensing Evaluation) Code Package is widely used to support criticality safety, radiation shielding, spent fuel characterization, and heat transfer analyses of nuclear facilities and package design [5]. SCALE was developed

NRP-98-01



Prepared by, P.B. Perez Checked by, K. Verghese Approved by C.W. Mayo

by ORNL for the Nuclear Regulatory Commission and has been very well documented, validated, and verified with a benchmark effort which included many critical experiments.

The concept of SCALE is to provide standardized analysis sequences for the user to focus on the problem specific properties such as material and geometry. The Criticality Safety Analysis Sequence (CSAS) provides automated material and cross-section processing prior to the criticality analysis [6]. Heterogeneous and resonance (self-shielding) corrections are performed automatically during the cross-section processing.

The CSAS analysis starts by selecting materials for the problem from the Material Information Processor Library (MIPLIB) which contains common materials that are associated with elements, mixtures, and nuclides. The problem specific cross-section processing for a CSAS25 sequence is performed by BONAM! and NITAWL-11.

BONAM! performs resonance self-shielding calculations for nuclides containing Bondarenko correction data. NITAWL-11 applies the Nordheim integral method for resonance self-shielding corrections to nuclides having resonance parameters and produce a problem specific working library. KENO-V.a performs the Monte Carlo Transport calculation for determining the multiplication factor for a 3-D assembly.

The SCALE manual provides recommendations for selecting a cross-section library. The library selected for this analysis is the 44 group (22-fast, 22-thermal) collapsed from the 238 group ENDF /B-V data using fuel cell spectrum from a L WR fuel. This cross-section library has been validated by 92 critical experiments and one sub-critical.

The criticality analysis assumes the rack is flooded in water to find the maximum multiplication factor (kc11 ) for the array. The CSAS25 calculation for the PULST AR fuel storage rack defines two unit cells for the calculation. The first unit cell contains the fuel, aluminum tube and water (Figure 1 ). This unit cell is then duplicated 250 times to construct the full rack model.

The fuel unit cell consists of the fuel pellet, gap, clad, a water gap, and the aluminum tube. The tube is surrounded by a liquid volume and the geometry includes the pitch of the assembly. The material were selected from the list supported by MIPLIB. The pitch was selected from a parametric study which also included a sensitivity study of available crosssection libraries.

A second unit cell is defined to model a 19 cm thick water reflector surrounding the fuel storage array. The water unit cell has the same dimensions as the fuel unit cell and is multiplied to surround the 250 fuel array (Figure 2).

NRP-98-01



Prepared by P.B. Perez Checked by K. Verghese Approved by C.W. Mayo

The fuel storage rack model does not include the mounting and supporting structures which consist of aluminum and would displace water. As a result, the model will yield slightly conservative results since the calculation will over-predict the multiplication factor.

~ - -------------------------T---------r--r rr r--C, R, R, R, R. Rs

Figure 1

PULSTAR Fuel Pin Unit Cell

2.3 CSAS25 Output

The CSAS sequence results is the KENO code output which provides tabulated values for the calculated k:rr per generation, the average kcm and other parameters. The results are also presented graphically by plotting the average kcrr as a function of generations run and skipped. The KENO code provides kcrr uncertainties which are the standard deviation of the mean.

The output of the code is carefully reviewed to ensure convergence has been accomplished. The plots are very useful for verifying convergence.

NRP-98-01



Prepared by P.B. Perez Checked by, K. Verghese Approved by, C.W. Mayo

Table I

Unit Cell Dimensions

CSAS Geometry Description Dimension (cm, [in])

Cell - Cuboid Water cuboid cell 3.81 [1.50]

Cylinder R1 CL - R 1 Fuel Pellet 0.5372 [0.2115]

Cylinder R2 R1 - R2 Helium Gap 0.5588 [0.2200]

Cylinder R3 R2 - R3 Zirc Clad 0.6109 [0.2405]

Cylinder Rs R3 - Rs Water Annulus 0.7874 [0.3100]

Cylinder Rs R4 - Rs Aluminum Tube 0.9525 [0.3750]

222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222

. 222221111111111111111111111111111111111111111111111111122222 222221111111111111111111111111111111111111111111111111122222 222221111111111111111111111111111111111111111111111111122222 222221111111111111111111111111111111111111111111111111122222 222221111111111111111111111111111111111111111111111111122222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222 222222222222222222222222222222222222222222222222222222222222

1 - Fuel Unit Cell 2 - Water Unit Cell

Figure 2

CSAS Fuel Storage Rack Model

NRP-98-01



Prepared by: P.B. Perez Checked by: K. Verghese Approved by, C. W. Mayo

3.0 Numerical Analysis Results

A parametric study was performed applying the CSAS25 analysis sequence to determine the fuel pitch necessary to arrive at a calculated ~ of 0.9 or less. The desired pitch was later utilized to evaluate calculation sensitivity to other analysis parameters such as cross-section libraries, clad material, and aluminum tube thickness. A summary of the parametric study is presented in Table Il.

The results of the parametric study were also used to confirm the cross-section library selected was suitable for our problem.

Convergence was achieved in approximately 100 generations. The results for the thermal criticality analysis is kcir = 0.803901 ± 0.0007173 corresponding to the largest eigenvalue of the fission production for 1000 generations.

4.0 Validation and Verification

The SCALE code validation and verification was verified for this application. The CSAS25 sequence was executed for the benchmark case "Example 311 found in the manual on page C4.5.6. This case is a light-water reactor 4% (weight} enrichment fuel assemblies in wet storage. The manual provides a solution kcrr of 0.7940 ± 0.0035, however, there is no reference to the computational resources. The sample case was executed on a Spare 5 workstation at NCSU and yielded results of 0.7931 + 0.0032. These results are considered acceptable and differences may simply be due to hardware differences.

5.0 Conclusions

The numerical analysis results predict a kcir of 0.80 which is with in Technical Specification limits of 0.9.

NRP-98-01



Prepared by, P .B. Perez Checked by K. Verghese Approved by, C.W. Mayo

Table II Summary of SCALE/CSAS Parametric Study Results

CASE PITCH SEPARATION k..a Cross-Section Comments ID (cm) (cm, [in]) (AYCrago) Library

Pitch

A 1.9050 0.0 [0.0] 0.7384 44-group (22-fast 22-thermal). Broad group

B 2.2225 03175 [0.125] 0.8329 version of the 238 group ENDF /B-V Data. Collapsed using fuel cell spectrum of L WR

c 2.5400 0.6350 [0.25] 0.8580 fuel. Validation based on 92 critical

44groupndf5 experiments and 1 sub-critical. Run with 100 D 3.1750 1.270 [0.5] 0.8596 generations.

E 3.8100 1.905 [0.75] 0.7982

F 4.4450 2.540 [1.00] 0.7162

Cross-Section Library

EO 0.7982 44groupndf5 See above

El 0.8235 Hansen-Roach 16-group (12-fast 4 thermal). Primarily for 3.8100 1.905 [0.75] fast systems (100 generation).

E2 0.7909 27groupndf4 27-group (14-fast 13-thermal). (100 generations).

Clad Material

EO 0.7982 Elemental Zirc (100 generations) 3.8100 1.905 [0.75] 44groupndf5

EZ2 0.7989 Zirc-2 (100 generations)

Enrichment

ET2 0.8038 6.0% (weight) Run with 200 generations 3.8100 1.905 [0.75] 44groupndf5

ET4 0.8127 6.25% (weight) Run with 200 generations

Monte Carlo Neutron Generation

ET2 0.8016 6.0% (weight) Run with 200 generations 3.8100 1.905 [0.75] 44groupndf5

ET6 0.8039 6.0% (weight) Run with 1000 generations

Aluminum Tube Wall Thickness

EO 0.7982 0.0625 inch wall thickness 3.8100 1.905 [0.75] 44groupndf5

ETl 0.7927 0.0650 inch wall thickness

NRP-98-01



Prepared by: P.B. Perez Checked by, K. Verghese Approved by, C. W. Mayo

=csas25 PULSTAR fuel storage rack n E a criticality for 6% fuel Buffalo Rack 44groupndf5 latticecell uo2 1 den=l0.71 1 293. 92235 6.0 92238 94.0 end zr 2 1 end h2o 3 1 end al 4 1 end he 5 1 end end comp squarepitch 3.8100 1.07442 1 3 1.22174 2 1.11760 5 end keno geometry description for fuel and array read param tme=350 gen=lOOO npg=lOOO mka=YES end param read geom unit 1 com='fuel pin' cylinder 1 1 0.53721 2P30.48 cylinder 5 1 0.55880 2P30.48 cylinder 2 1 0.61087 2P30.48 cylinder 3 1 0.78740 2P30.48 cylinder 4 1 0.95250 2P30.48 cuboid 3 1 2Pl.905 2Pl.905 2P30.48 unit 2 com='water reflector' cuboid 3 1 2Pl.905 2Pl.905 2P30.48 end geom read array ara=l nux=60 nuy=15 nuz=l fill 300r2 5r2 50rl 5r2 4q60 300r2 end fill end array end data end =clec_out end

Figure 3

CSAS Analytical Model of Storage Rack

NRP-98-01



Prepared by, P .B. Perez Checked by, K. Verghese Approved by, C. W. Mayo

6.0 References

1. TID-7028, Critical Dimensions of Systems Containing U(235), Pu(239), and U(233), 1964

2. DOE/NRC Form 741, Nuclear Materials Transaction Report, 17 July 1998

3. TID-26286, Nuclear Criticality Safety, 1974, Table I, pp. 6

4. ANL-5800, Reactor Physics Constants, 2ad Edition, Table 7-12, pp. 585

5. NUREG/CR-0200, Revision 5, The SCALE Code Package, March 1997

6. NUREG/CR-0200, Revision 5, Volume 1, Section C-4, The SCALE Code PackageCSAS: Control Module for Enhanced Criticality Safety Analysis Sequences, March 1997

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