ELV-00511, Criticality Analysis of Vogtle Fresh Fuel Racks
| ML20244D565 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 01/31/1989 |
| From: | Boyd W, Cobb R, Fecteau M WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20244D559 | List: |
| References | |
| ELV-00511, ELV-511, NUDOCS 8906190136 | |
| Download: ML20244D565 (20) | |
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ATTACHMENT 2 TO ENCLOSURE 2 0F LETTER ELV-00511 l
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J CRITICALITY ANALYSIS OF THE VOGTLE FRESH FUEL RACKS January 1989 W. A. Boyd R. C. Cobb 4 M. W. Facteau l W. A. Bordogna e
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4 TABLE OF CONTENTS 1
1.0 int-idu. tion ..............................................
1.1 Design Description ...........................'......1 1.2 Design Criteria ........ .......................... 1 2.0 Criticality Analytical Method ................................. 2 3.0 Criticality Analysis of Fresh Fuel Racks ......................... 3 3.1 Full Density Moderation Analysis ....................... 3 3.2 Low Density Optimum Moderation Analysis ................. 5 3.3 Postulated' accidents ................................6 4.0 Acceptance Criterion For Criticality ..............;............. 7 8
5.0 Conclusion ...............................................
Bibliography .................................................. 15 .
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Table of Contents i v
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LIST OF TABLES Table 1. Benchmark Critical Experiments (5,63 ................. 9 Table 2. Fuel Parameters Employed in Criticality Analysis ......... 10 N
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List of Tables il k
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LIST OF ILLUSTRATIONS Figure 1. Vogtie Fresh Fuel Storage Cell Nominal Dimensions ....... 11-Figure 2. Vogtle Fresh Fuel Rack Axial Layout ................. 12 Figure 3. Vogtle Fresh Fuel Rack Radial Layout ................ 13 Figure 4. Sensitivity of Ken to Water Density in the Vogtle Fresh Fuel Storage Racks- ............................... 14 i
List of Illustrations lll
1.0 INTRODUCTION
The Vogtle fresh fuel rack design described herein employs an existing array of racks, which'will be analyzed at a higher enrichment. This analysis will re-analyze the fresh fuel array for criticality to show that 5.0 w/o fuel can be stored in the rack in all storage locations. The fresh fuel rack design is an unpoisoned array, previously analyzed for storage of Westinghouse 17x17 fuel assemblies with enrichments up to 3.5 w/o U' *
- utilizing every storage lo-cation.
The fresh fuel rack remnalysis is based on maintaining Kev s 0.95 for storage of Westinghouse 17x17 OFA and STD fuel at 5.0 w/o U8 8 8 with_ an uncertainty of 0.05 w/o under full water density and optimum moderation conditions.
1.1 DESIGN DESCRIPTION The fresh fuel rack storage cell design is depicted schematically in Figure 1.
The fresh fuel rack layout is shown in Figures 2 and 3.
1.2 DESIGN CRITERIA Criticality of fuel assemblies in a fuel storage rack is prevented by the design of the rack which limits fuel assembly interaction. This is done by fixing the minimum separation between assemblies.
The design basis for preventing criticality outside the reactor is that, including uncertainties, there is a 95 percent probability at a 95 percent confidence level that the effectivo multiplication factor (K.er) of the fuel assembly array will be less than 0.95 as recommended in ANSI 57.3-1983 and in Reference 1.
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Introduction 1
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2.0 CRITICALITY ANALYTICAL METHOD The criticality calculation method and cross-section values are verified by comparison with critical experiment data for assemblies similar to those for which the racks are designid, This benchmarking data is sufficiently diverse to l
' establish that the method bias and uncertainty will apply to rack conditions which include strong neutron absorbers, large water gaps and low moderator densities.
The design method which insures the criticality safety of fuel assemblies in the 83 system of codes for cross-spent fuel storcge rack uses the AMPX' 8, I]J k section generation and KENO IV(
- 8 for reactivity determination.
p The 227 energy group cross-section library that is the common starting point for all cross-sections used for the benchmarks and the storage rack is generated k t83 program includes, in this library, the from ENDFIB-V( 8 8 data. The NITAWL l p self-shielded resonance cross-sections that are appropriate for each particular geometry. The Nordheim integral Treatment is used. Energy and spatial l LJ I weighting of cross-sections is performed by the XSDRNPM' 8 3 program which is a 'one-dimensional Sn transport theory code. These multigroup cross-section
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W sets are then used as input to KENO IV(
- 3 which is a three dimensional Monte {
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'Julo theory program designed for reactivity calculations.
LJ A set of 33 critical experiments has been analyzed using the above method to demonstrate its applicability to criticality analysis and to establish the method bias and variability. The experiments range from water moderated, oxide fuel
[
L~ arrays separated by various materi.ds 64C, steel, water, etc) thit simulate LWR fuel shipping and storage conditions ' 3 to dry, harder spectrum uranium metal t
cylinder arrays with various interspersed materials
- 3 (Plexiglas and air) that
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demonstrate the wide range of applicability of the method. Table 1 summarizes these experiments.
The everage K.es of the benchmarks is 0.992. The standard deviation of the bias valu. is 0.0008 Ak. The 95/95 one sided tolerance limit factor for 33 values is 2.19. Thus, there is a 95 percent probability with a 95 percent confidence level that the uncertainty in reactivity, due to the ' method, is not greater than 0.0018 Ak.
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2 Criticality Analytical Method
3.0 CRITICALITY ANALYSIS OF FRESH FUEL RACKS Since the fresh fuel racks are maintained in a dry condition, the criticality analysis will show that the rack K.n is less than 0.95 for the full density and low density optimum moderation conditions. The full density and low density optimum moderation scenarios are accident situations in which no credit can be taken for soluble boron.
The following assumptions were used to develop the KENO model for the storage of fresh fuel in the fresh fuel racks smder full density and low density optimum moderation conditions:
1.
The fuel assembly contains the highest enrichment authorized, is at its most reactive point in life, and no credit is taken for any burnable poison in the -
fuel rods. All fuel pellets are modelled at 96 percent theoretical density without dishing or chamfers to bound the maximum fuel assembly uranium loading.
2.
All fuel rods contain uranium dioxide at an enrichment of 5.0 w/o U * *
- over the entire length of each rod.
3.
No credit is taken for any U* *
- or U* *
- in the fuel, nor is any credit taken for the buildup of fission product poison material.
- 4. No credit is taken for any spacer grids or spacer sleeves.
5.
The fuel rack center-to-center spacing is assumed to be 21 inches in all directions except for the low density optimum moderation case which ex-plicitly models the fuel pit geometry.
3.1 FULL DENSITY MODERATION ANALYSIS in the KENO model for the full density moderation analysis, the moderator is pure water at a temperature of 68'F. A conservative value of 1.0 gm/cm* is used for the density of water. The fuel array is infinite in all directions which precludes any neutron leakage from the fuel array. ,
Calculations for fuel racks have shown that the Westinghouse 17x17 OFA fuel assemblies yield a larger K.n (approximately 1 - 2 %Ak/k) than does the Westinghouse 17x17 STD fuel assembly when both fuel assemblies have the Criticality Analysis of Fresh Fuel Rocks 3
~ ~ ' ~ ~ ---- ~
same U' *
- enrichment. Thus only the Westinghouse 17x17 OFA fuel assembly was analyzed see Table 2 for fuel parameters).
The KENO calculation for the nominal case resulted in a Ken of 0.9119 with e 95 perc'ent probability 95 percent confidence level uncertainty of 10.0084.
The maximum K.n under normal conditions arises from consideration of me-chanical and material thickness tolerances resulting from the manufacturing process in addition to asymmetric positioning of fuel assemblies within the storage cells. Studies of asymmetric positioning of fuel assemblies within the storage cells has shown that symmetrically placed fuel assemblies yield con-servative results in rack K n . Due to the relatively large cell spacing, the small tolerances on the cell I.D. and center-to-center spacing are not considered since they will have an insignificant effect on the fuel rack reactivity. The sheet metal thickness is reduced by 0.004 inches. Thus, the most conservative, or
worst case", KENO model of the fresh fuel storage racks contains the minimum sheet metal thickness with symmetrically placed fuel assemblies.
Based on the analysis described above, the following equation is used to de-velop the maximum K.n for the Vogtle fresh fuel storage racks:
K.n= Kworst + Bm.tnos + B.=.cn + (((ks)* .or , + (ks)8m.inoe + (ks) *.nrien ]
where: .
Kworst = worst case KENO Ken with full dent;ty water Bm.inoa = method bias determined from benchmark critical comparisons B.nrica = bias for 0.05 w/o enrichment uncertainty ks.or, = 95/95 uncertainty in the worst case KENO Ken ksm.inen = 95/95 uncertainty m the method bias ks.nrien = uncertainty in the enrichment bias Substituting calculated values in the order listed above, the result is:
K.n = 0.9159 + 0.0083 + 0.0022 + (((0.0076)* + (0.0018)8 + (0.0063)* ] = 0.9364 Since K.n is less than 0.95 including uncertainties at a 95/95 probability confi-dance level, the acceptance criteria for criticality is met. ~
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l Criticality Analysis of Fresh Fuel Racks 4 7 .. . - _ - _ _ _ _ _ _ - _ _ _ _ _ _ _
3.2 LOW DENSITY OPTIMUM MODERATION ANALYSIS in the low density optimum moderation analysis, the fuel array is finite in all directions. The nominal model described above is used in KENO except that -
the concrete walls and floor are explicitly mocelled as shown in Figures 2 and
- 3. The reduced steel thickness used in the full density worst case model will have an insignificant effect on the optimum raoderator density and reactivity.
The Westinghouse 17x17 STD fuel as'sembly was analyzed in the model (See Table 2 for fuel parameters). Calculations have shown that the STD fuel as-sembly is more reactive by approximately 0.5 to 1.5 %Ak/k under low moderator density conditions.
Analysis of the Vogtle racks has shown that the maximum rack K.c under low density moderation conditions occurs at 0.043 gm/cm
- water density. The calculation of the Vogtle fresh rack reactivity at 0.043 gm/cm* water density resulted in a peak K.n of 0.9230 with a 95 percent probability and 95 percent confidence level uncertainty of 10.0074. Figure 4 shows the fresh fuel rack re-activity as a function of the water density.
Based. on the analysis described above, the following equation is used to de-velop the maximum K.n for the Vogtle fresh fuel storage racks under low den-sity optimum moderation conditions: .
K.n= Km . + Bm.moe + B.wien + /[(ks)* e... + (ks)* m moe + (ks)*.=6en 3 where:
Km... = maximum KENO K.n with optimum moderation Bm.mos = method bias determined from benchmark critical comparisons B .cn = bias for 0.05 w/o enrichment uncertainty k s e... = 95/95 uncertainty in the maximum KENO K.n ks mos = 95/95 uncertainty in the method bias ks.wien = uncertainty in the f.nrichment bias Substituting calculated values in the order listed above, the result is:
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l K.n = 0.9230 + 0.0083 +0.0022 + /[(0.0074)* + (0.0018)* + (0.0063)* 3 = 0.9434 Since K.n is less than 0.95 including uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met.
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Criticality Analysis of Fresh Fuel Racks 5
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3.3 POSTULATED ACCIDENTS Under normal conditions the fresh ft.el racks are maintained in a dry mode.
The full density and low density optimum moderation scenarios analyzed above are worst case accident situations in which a moderator is introduced into the fuel rack area. These accident conditions will result in the highest fuel rack K.et.
However, other accidents can be postulated which would cause some reactivity increase (i.e., dropping a fuel assembly between the rack and pool wall or on
' top of the rack). For these accident conditions, the double contingency principle of ANSI N16.1-1975 is applied. This states that one is not required to assume
- two unlikely, independent, concurrent events to ensure protection against a
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criticality accident. Thus, for accident conditions, the absence of a moderator in th'e fresh fuel storage racks can be assumed as a realistic initial condition since assuming its prssence would be a second unlikely event.
The absence of a moderator in the fresh fuel racks will cause the reactivity to
- be less than 0.70. However, for' other postulated accidents, such as those mentioned above, the maximum reactivity increase will be less than 10 %Ak/k.
As a result, for postulated accidents, the maximum rack K.ev, as shown above, E will be less than 0.95.
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Criticality Analysis of Fresh Fuel Racks 6
4.0 ACCEPTANCE CRITERlON FOR CRITICALITY The neutron multiplication factor in the fresh fuel racks shall be less than or equal to 0.95, including all uncertainties, under all conditions.
The analytical methods employed herein conform with ANSI N18.2-1973, "Nu-clear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants," Section 5.7, Fuel Handling System; ANSI N16.9-1975, " Validation of Calculational Methods for Nuc' ear Criticality Safety," NRC Standard Review Plan, Section 9.1.2, " Spent Fuel Storage"; and the NRC guidance, "NRC Position for Review and Acceptance of Spent Fuel Storage and Handling Applications," ANSI 57.3-1983, " Design Requirements for New Fuel Storage Facilities at Light Water Reactor Plants."
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5.0 CONCLUSION
The acceptance criterion for criticality is met for the storage of Westinghouse 1/x17 OFA and STD fuel with a maximum enrichment of 5.05 w/o U* *
- in the Vogtle fresh fuel storage racks.
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I Table 1. Benchmark Critical Experiments (5,6)
General Enrichment Separating Soluble Description t/c U235 9eflector Material Boron ppm Keff
...........---............---....----..................----............------.---~~--------
- 1. UO2 rod 1sttice 2.46 water water O O.9857 +/- .0028
- 2. U02 rod lattice
- 3. U02 rod 1sttice 2.46 water water
- 1037 0.9906 +/- .0018 4 UO2 rod lattice 2.46 water water 764 0.9896 +/- .0015 S. 002 red lattice 2.46 water 84C pins O O.9914 +/- .0025
- 6. UO2 rod lattice 2.46 water B4C pins O O.9891 +/- .0026
- 7. U02 rod lattice 2.46 water 54C pine O O.9955 +/- .0020
- 8. U02 rod 1sttles 2.46 water 84C pins O O.9889 +/ . 0027
- 9. U02 rod lattice 2.46 2.46 water 8dC pins O O.9983 +/- .0025
- 10. U02 rod lattice water water O O.9931 +/- .0028
- 11. 002 rod lattice 2.46 2.46 water water 143 0.9928 +/- .0025
- 12. U02 roe lattice 2.4G water statniess steel- 514 0.9967 +/ .0020 water stainless steel 217 0.b943 +/ .0019
- 13. UO2 rod latt ico 2.46 water borated aluminum 15 0.9892 +/ .0023
- 14. 002 rod lattice 2.46 water borated aluminum 92 0.9884 +/- .0023
- 15. U02 rod lattice 2.46 water borated aluminum 395 0.9832 +/ .0021
- 16. U02 rod latttee 2.46 water borated aluminum 121 0.9848 +/- .0024
- 17. U02 rod lattice 2,46
- 18. UO2 rod lattice water borated aluminem 487 0.9895 +/- .0020 2.46 water borated aluminum 197 0.9885 +/ .0022
- 13. U02 red lattice 2.46 water borated aluminum 634 0.9921 +/- .0019
- 20. U02 cod la t t ice 2.46 watet- borated aluminum 320 0.9920 +/ .0020
- 28. U02 rod lattice 2.46 water borated aluminum 72 0.9933 +/ .0020
- 22. U metal cyttnders 93.2 bare
- 23. U metal cylinders 93.2 bare air O O.9905 +/- .0020
- 24. U metal cylinders 93.2 bare air O O.9976 +/- .0020
- 25. U metal cylinders 93.2 bare air O O.9947 +/ .0025
- 26. U metal cylinders 93.2 bare
. air O O.9928 +/ .0019
- 27. U metal cylindera 93.2 air O O.9922 +/ .0026
- 28. U metal cylinders bare air 0 0.9950 +/ .0027 93.2 bare plext91 ass O O.9941 +/- .0030
- 29. U metal cyttnders 93.2 paraffin plexiglass
- 30. U metal cyttnders 93.2 bare O O.9928 +/- .0041
- 31. U metal cylinders 93.2 paraffin plextglass O O.9968 +/- .0018
- 32. U metal cyttr.sers 93.2 paraffin plexiglass 0 1.0042 +/- .0019
- 33. U metal cylinders 93.2 paraffin plexiglass O O.9963 +/- .0030 plexiglass O O.9919 +/- .0032 i
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I Table 2. Fuel Parameters Employed in Criticailty Analysis Parameter W 17x17 OFA W 17x17 STANDARD Number of Fuel Rods per Assembly 264 264 Rod Zirc-4 Clad 0.D. (Inch) 0 360 0 374 Clad Thickness (Inch) 0.0225 0.0225 Fuel Pellet 0.D. (inch) 0 3088 0 3225 Fuel Pellet Density
(% of Theoretical) 96 96 Fuel Pellet Dishing Facter 0.0 0.0 Rod Pitch (inch) 0.496 0.496 Number of Zirc-4 Guide Tubes 24 24 Guide Tube 0.D, (Inch) 0.474 0.482 8 Guide Tube, Thickness (inch) 0.016 0.016 Numoer of instrument Tubes 1 1 Instrument Tube 0.D. (i nch) 0.474 0.482 instrument Tube Thickness (Inch) 0.016 0.016 E
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BIBLIOGRAPHY
- 1. Nuclear Regulatory Commission, Letter to All Power Reactor Licensees, from B. K. Grimes OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications., April 14, 1978.
- 2. W. E. Ford llt, CSRL-V: Processed ENDFIB-V 227-Neutron-Group and Pointwise Cross-Section ubraries for Criticality Safety, Reactor and Shielding Studies, ORNLICSDITM-160, June 1G82.
- 3. N. M. Greene AMPX: A Modular Code ' System for Generating Coupled Multigroup Neutron-Gamme Ubraries from ENDFIB, ORNLITM-3106, March 1976.
- 4. L M. Petrie and N. F. Cross, KENO IV--An improved Monte Carlo Celticality Program, ORNL-4938, November 1975.
S. M. N. Baldwin, Critical Experiments Supporting Close Proximity Water Storage of Power Reactor fuel, BAW-1484-7, July 1979.
- 6. J. T. Thomes Critical Three-Dimensions / Arrays of U(93.2) Metal Cylinders, ,
Nuclear Science and Engineering Volume 52, pages 350-359,1973.
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