ML20128H020

Spent Fuel Rack Criticality Analysis W/Credit for Soluble Boron

ML20128H020
Person / Time
Site:	Vogtle
Issue date:	08/31/1996
From:	Fecteau M, Lesko J, Newmyer W WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML20128G993	List: ML20128G988 ML20128H020 ML20128H028 ML20128H049
References
WCAP-14720, NUDOCS 9610090208
Download: ML20128H020 (66)

Text

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WCAP-14720 1

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I Vogtle Units 1 and 2 Spent Fuel Rack l

Criticality Analysis With Credit for Soluble Boron

. 1 August 1996 i

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l N. L. Domenico i W. D. Newmyer S Srinilta j Prepared : //)

~W. D. ewm>6r l l

Criti ality Services Team Verified: '

J.d. Lesko f

. Criticality ServicesTenm Approved: [ M

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ffd. W. Fe 'cau, Manager l Core An sis A l

O Westinghouse Commerical Nuclear Fuel Division i '

9610090208 DR 961004 ADOCK 05000424 PDR

Table of Contents ,

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1.0 Introduction............................................................................................................I  !

1.1 Design Description....... . .. ......... . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . ..2 1.2 Design Criteria.. .. .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . ....3 2.0 A n a l yt i ca l M et h od s .......... ............ .... .. ...... .... ...... .......... .. ...................... .. ............... 4 3.0 Criticality Analysis of Unit i All Cell Storage................ .................................... 5 3.1 Maximum Fea sible K err Calc utation..... ... . .................... ...................... .... .... ..... . 5 3.2 Soluble Boron Credit K err Calculations .......... ... . . ... .... .. ..... ...... .. ........ .. ......... 6 3.3 Burnup Credit Reactivity Equivalencing ............... ......... .. ...... . .. .... ... .... .... .. 8 i 1

4.0 Criticality Analysis of Unit 1 3-out-of-4 Storage.................................................. 9 4.1 Maximum Feasible Kerr Calculation... .. ....... .. ....... ...................................9 4.2 Soluble Boron Credit Ke rr Calculations.. . ... ...... .... . .................................... .. ..10 4.3 B urnup Credit Reactivity Equivalencing ..... ......... ............ ..... . .. . ...... .. ..........12 l

5.0 Criticality Analysis of Unit 1 2-out-of-4 Storage..................................................13 j 5.1 Maximum Feasible K err Calcutation............... .............................. ... .. .............13 l 5.2 Soluble Boron Credit Ke rr Calculations... ..... . . ... . ...... . ...... .. . ...... ........ ... ..14 I i

6.0 Criticality Analysis of Unit 2 All Cell Storage......................................................17 6.I M aximu m Feasible Ke rr Calc u tation.. . . ....................................... .................. . .... 17 6.2 Soluble Boron Credit K err Calculations......... ........ ............. ............ . . ....... .... ... I 8 6.3 B urnup Credit Reactivity Equivalencing ............. . .. ............ ...... ... . .. ........ . .. 20 l

1 7.0 Criticality Analysis of Unit 2 3-out-of-4 Storage.................................................. 21 '

. 7.1 M aximum Feasible K rre Calculation ... ..... ... .. ...................... .............. .......... . .......... 21 7.2 S oluble B oron Credit Kerr Calculations ........................... ... ................................ ... 22 7.3 Burnup Credit Reactivity Equivalencing . ................. ........ . ................ ............... .. 24 8.0 Criticality Analysis of Unit 2 2-out-of-4 Storage.................................................. 25 8.1 Maximum Feasible K err Calculation......... .................................... ...... ... ..... . .... . 25

8.2 Soluble Boron Credit K err Calculations..... ....... ...... . 76 l

9.0 Criticality Analysis of Unit 2 3x3 Checkerboard ................................................. 29 9.I d

Maximum Feasible K err Calculation........ .... ............. .. . . .. . ......... ............. ....... . 29 9.2 S oluble Boron Credit K err Calculations .. . ........ .. .. ...................... ....... ... . ............ 30 i 9.3 Burnup Credit Reactivity Equivalencing ... .......... ..... ..................... ...... ... . .. .... 32 l 9.4 IFB A Credit Reactivity Equivalencing...... .... . . ... ............... .......... ......... . . . .. 33 10.0 Discussion of Postulat ed Acci dents........................................................................ 35 11.0 Sol u ble Boron C red it S u m ma ry ............................................................................ 37

. 12.0 S u mma ry of Criticali ty Results ............. .............................................................. 38 B i b l i og ra p h y .... .. .... .... .... .... .. .. ...... .. .. .. .. .. .. .... .. .. .. .. .. .. .. .. .. .. .... .. .. .. .. .. .. .. .. .... .... .. .. .... 6 2 Vogtle Units 1 and 2 Spent Fuel Racks .

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List of Tables Table 1. Nominal Fuel Parameters Employed in the Criticality Analysis. .... . .. .. ..... . 40 Table 2.

All Cell Storage Soluble Boron Credit Ke n for Vogtle Unit 1......... ........ . ... . 41 Table 3. Minimum Burnup Requirements for Vogtle Unit 1............... ... . . .. .. .. ... .. . 42 Table 4. 3-out-of-4 Checkerboard Soluble Boron Credit Keg for the Vogtle Unit 1.. .... 43 Table 5. 2-out-of-4 Checkerboard Soluble Boron Credit Ke g for the Vogtle Unit 1.. .. . 44 Table 6. All Cell Storage Soluble Boron Credit Keg for Vogtle Unit 2. ........ .. ... . .... .. 45 Table 7. Minimum Burnup Requirements for Vogtle Unit 2.... ........... . . ...... . .... .. ...... 46

. Table 8. 3-out of-4 Checkerboard Soluble Boron Credit Ke g for Vogtle Unit 2............. 47 Table 9, 2-out-of-4 Checkerboard Soluble Boron Credit Keg for Vogtle Unit 2............. 48 Table 10. 3x3 Checkerboard Soluble Boron Credit Ke rr for Vogtle Unit 2........................ 49 Table 11. 3x3 Checkerboard Minimum IFB A Requirement for Vogtle Unit 2....... .......... 50 Table 12. Postulated Accident Summary for Vogtle Units 1 and 2......... ..... .. .......... . . . 51 Table 13. Summary of Soluble Boron Credit Requirements for Vogtle Units 1 and 2...... 52 l

Vogtle Units I and 2 Spent Fuel Racks ii

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l List of Figures Figure 1. Vogtle Unit 1 Spent Fuel Storage Cell Nominal Dimensions. . . . . . . . 53 Figure 2. Vogtle Unit 2 Spent Fuel Storage Cell Nominal Dimensions.. . . . .54 Figure 3. Vogtle Unit 2 Rack Module A-5 Limiting Water Gaps and Equivalent Cell. . 55 i Figure 4. Vogtle Unit 1 Burnup Credit Requirements. . .. .. . . . . . . . . . . .56

! Figure 5. Vogtle Units 1 and 2 Checkerboard Storage Configurations.. .. . . . . . .57 Figure 6. Vogtle Unit 2 Bumup Credit Requirement. .. .. . . . . . . .. .. 5 8 i Figure 7. Vogtle Unit 2 3x3 Checkerboard IFB A Requirement . . . .. . .... .............59 Figure 8. Vogtle Unit i Soluble Boron Worth. . .... . . . . . . . . . . . . . . . . . . . . . 60 Figure 9. Vogtle Unit 2 Soluble Boron Worth. . .. . . . . . . . .. . . . . . .61 4

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Vogtle Units 1 and 2 Spent Fuel Racks iii

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1.0 Introduction This report presents the results of a criticality analysis of the Vogtle Units I and 2 spent fuel storage racks with credit for spent fuel pool soluble boron. The methodology employed here is I contained in the topical report, " Westinghouse Spent Fuel Rack Critical:ty Analysis Methodology"W.

The Vogtle Units 1 and 2 spent fuel racks have been reanalyzed to allow storage of Westinghouse 17x17 fuel assemblies with nominal enrichments up to 5.00 w/o 235 U in the allowable storage cell l locations using soluble baron credit. This analysis will also ignore the presence of the spent fuel '

rack Boraflex poison panels. The analysis uses the maximum feasible K e g < l.0 condition to determine the acceptable storage of 17x17 fuel assemblies with no credit for soluble boron and soluble boron credit to provide safety margin by maintaining Keg s 0.95 including uncertainties, tolerances and accident conditions in the presence of spent fuel pool soluble boron. l The following storage configurations and enrichment limits were considered in this analysis:

Unit i Enrichment Limits All Cell Storage Storage of 17x17 fuel assemblies in any cell location. Fuel assemblies must have an initial nominal enrichment no greater 235 than 2.00 w/o U or satisfy a minimum burnup requirement for higher initial enrichments. The soluble boron credit required for this storage configuration is 850 ppm.

3-out-of-4 Storage of 17xl7 fuel assemblies in a 3-out-of-4 checkerboard Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 2.70 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. A 3-out-of-4 checkerboard with empty cells means that no more than 3 fuel assemblies can occupy any 2x2 matrix of storage celle. The j soluble boron credit required for this storage configuration is 950 ppm.

2 out of-4 Storage of 17x17 fuel assemblies in a 2-out-of-4 checkerboard Checkertmard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 5.00 w/o 235 U. A 2-out-of-4 checkerboard with empty cells means that no 2 fuel assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent. The soluble boron credit required for this storage configuration is 1100 ppm.

1 Introduction i

Unit 2 Enrichment Limits All Cell Storage Storage of 17xl7 fuel assemblies in any celllocation. Fuel assemblies must have an initial nominal enrichment no greater 235 than 1.82 w/o U or satisfy a minimum burnup requirement for higher initial enrichments. The soluble boron credit required for this storage configuration is 750 ppm.

3-out-of-4 Storage of 17x17 fuel assemblies in a 3-out-of-4 checkerboard Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 2.54 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. A 3-out-of-4 checkerboard with empty cells means that no more than 3 fuel assemblies can occupy any 2x2 matrix of storage cells. The soluble boron credit required for this storage configuration is 950 ppm.

2-out-of-4 Storage of 17x17 fuel assemblies in a 2-out-of-4 checkerboard Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 5.00 w/o 235 U. A 2-out-of-4 checkerboard with empty cells means that no 2 fuel assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent. The soluble boron credit required for this I storage configuration is 1250 ppm.

3x3 Checkerboard Storage of Westinghouse 17x17 fuel assemblies with nominal Storage enrichments no greater than 4.00 w/o 235 U (equivalent enrichment 1 with IFBA credit)in the center of a 3x3 checkerboard. The j surrounding fuel assemblies must have an initial nominal enrichment no greater than 1.48 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. The soluble boron credit required for this storage configuration is 800 ppm.

The Vogtle Units I and 2 spent fuel rack analysis is based on maintaining Keff < 10 under maximum feasible conditions with no soluble baron for storage of 17x17 fuel assemblies. Soluble boron credit is used to provide safety margin by maintaining Keg s 0.95 including uncertainties, tolerances and accident conditions in the presence of spent fuel pool soluble boron.

1.1 Design Description The Vogtle Unit I spent fuel storage cell is shown in Figure 1 on page 53 and the Vogtle Unit 2 spent fuel storage cell is shown in Figure 2 on page 54 with nominal dimensions provided on each figure.

The fuel parameters relevant to this analysis are given in Table 1 on page 40. The fuel rod and guide tube cladding are modeled with zircaloy in this analysis. This is conservative with respect to the Westinghouse ZIRLO product which is a zirconium alloy containing additional elements Introduction 2

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l including niobium._ Niobium has a small absorption cross section which causes more neutron capture in the cladding regions resulting in a lower reactivity. Therefore, this analysis is i conservative with respect to fuel assemblies containing ZlRLO cladding in fuel rods and guide

! tubes.

l l The Vogtle Unit 2 spent fuel storage racks contain as-built storage racks which are not consistent

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with the nominal dimensions provided in Figure 2. Specifically, the as-built spacing between storage cells is not consistent with the nominal spacing between storage cells. A criticality analysisW was previously performed to address the inconsistencies between the nominal storage i rack cell water gap spacing and as-built storage rack cell water gap spacings. Based on data from the previous analysis, the as-built water gap spacings of rack module A-5 were determined to

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bound all the rack modules in the Vogtle Unit 2 spent fuel pool. The limiting water gap spacings for the worst case 3x3 array of cells in rack module A-5 is shown in Figure 3 on page 55. The j criticality analysis performed in this report was based on an equivalent cell shown in Figure 3 I which yields a reactivity which is equivalent to the reactivity of the as-built 3x3 array in rack l module A-5 with the worst combination of water gap spacings. This equivalent cell was used as a basis for the calculations of reactivity in the Vogtle Unit 2 spent fuel racks.

1.2 Design Criteria 1

Criticality of fuel assemblies in a fuel storage rack is prevented by the design of the rack which l

limits fuel assembly interaction. This is done by fixing the minimum separation between fuel  ;

assemblies and controlling the placement of assemblies into selected storage cells. l In this report, the reactivity of the spent fuel racks was analyzed such that K eg remains less than 1.0 under maximum feasible conditions with no soluble boron as defined in Reference 1. To provide safety margin in the criticality analysis of the spent fuel racks, credit is taken for the soluble boron present in all PWR spent fuel pools.

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 effective neutron multiplication factor, Keg, of the fuel assembly arra will be less than or equal to 0.95. This requirement as currently stated in ANSI 57.2-1983 3 ), and the NRC paper, OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications,@) does not allow for reactivity credit due to the presence of soluble boron. This criticality analysis report takes exception to this and shows that the effective neutron multiplication factor, K eg, of the fuel assembly array is less than 1.0 under maximum feasible conditions and less than or equal to 0.95 when credit is taken for the presence of a portion of the spent fuel pool soluble boron as defined in the topical report,

" Westinghouse Spent Fuel Rack Criticality Analysis Methodology"W, l

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Introduction 3

2.0 Analytical Methods The criticality calculation method and cross-section values are verified by comparison with l critical experiment data for fuel assemblies similar to those for which the racks are designed. This I benchmarking data is sufficiently diverse to establish that the method bias and uncertainty will apply to rack conditions which include strong neutron absorbers, large water gaps, low moderator densities and spent fuel pool soluble boron. I 1

The design method which insures the criticality safety of fuel assemblies in the fuel storage rack l is described in detail in the Westinghouse Spent Fuel Rack Criticality Analysis Methodology  !

topical report, WCAP-14416W . This report describes the computer codes, benchmarking, and methodology which are used to calculate the criticality safety limits presented in this report for Vogtle Units I and 2.

l As determined in the benchmarking in the topical report, the method bias using the described methodology of NITAWL-II, XSDRNPM-S and KENO-Va is 0.00770 AK with a 95 percent probability at a 95 percent confidence level on the bias of 0.00300 AK. These values will be used j throughout this report as needed.

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Analytical Methods 4

3.0 Criticality Analysis of Unit 1 All Cell Storage This section describes the analytical techniques and models employed to perform the criticality  !

l analysis and reactivity equivalencing evaluations for the storage of fuel in all cells of the Vogtle Unit I spent fuel storage racks with credit for soluble boron. l Section 3.1 describes tb maximum feasible K ge KENO-Va calculations. Section 3.2 discusses the results of the spent fuel rack K g e soluble boron credit calculations. Finally, Section 3.3 presents the results of calculations performed to show the minimum burnup requirements for assemblies with initial enrichments above those determined in Section 3.1.

3.1 Maximum Feasible Keg Calculation l

The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in all cells of the Vogtle Unit I spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westinghouse 17x17 STD fuel design (see Table 1 on page 40 for fuel parameters).

Calculations show that for the enrichment and storage configuration considered here, the Westinghouse 17x17 STD design was more reactive than the Westinghouse 17x17 OFA fuel assembly design.

2. Fuel assemblies contain uranium dioxide at a nominal enrichment of 2.00 w/o 235 U over the entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U n the fuel, nor was any credit taken for the buildup of tission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any burnable absorber in the fuel rods.

8. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Boraflex j volume was replaced with water.

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I1. All available storage cells were loaded with fuel assemblies. 4 1

With the above assumptions, the KENO-Va calculation resulted in a Keg of 0.98111 under normal

! conditions. The reactivity bias calculated in PHOENIX-P for the normal temperature range of the

( spent fuel pool water (50*F to 185'F) was 0.01067 AK. Finally, the methodology bias associated with the benchmarking of the Westinghouse criticality methodology was 0.00770 AK.

Criticality Analysis of Unit 1 All Cell Storage 5

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Based on the results above, the following equation was used to develop the maximum feasible Keg for all cell storage in the Vogtle Unit I spent fuel storage racks:

K,y = K ,,,,,,,,ai + B,,,, + B ,,,yo, where:

K ,,,,r m a t =

normal conditions KENO-Va Ke g B ,,,,,p temperature bias for normal temperature range of spent fuel pool water (50*F to 185'F)

B method # b *' ' "' O # ' '"

comparisons Substituting the calculated values in the order listed above, the result was:

K,g = 0.98111 + 0.01067 + 0.00770 = 0.99948 Since Ke g is less than 1.0, the Vogtle Unit I spent fuel racks will remain suberitical under  ;

maximum feasible conditions when all cells are loaded with 2.00 w/o 235U 17x17 fuel assemblies and no soluble boron is present in the spent fuel pool water, in the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron required to maintain K eg s 0.95 including tolerances and uncenainties.

3.2 Soluble Boron Credit K eg Calculations To determine the amount of soluble boron required to maintain Keg s 0.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of material and construction tolerance variations. A final 95/95 K eg was developed by statistically combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for all cell storage in the Vogtle Unit I spent fuel racks were similar to those in Section 3.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 300 ppm soluble boron.

With the above assumptions, the KENO-Va calculation for the nominal case with 300 ppm soluble boron in the moderator resulted in a Keg of 0.88950.

Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Criticality Analysis of Unit 1 All Cell Storage 6

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Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodologv was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and mechanical / construction diniensions, PHOENIX-P perturbation calculations were performed. For the Vogtle Unit I spent fuel rack all cell storage configuration, UO material 2 tolerances were considered along with construction tolerances related to the cell I.D., storage cell pitch, and stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance ofi0.05 w/o :35 U about the nominal reference enrichment of 2.00 w/o 235 U was considered.

UO 2Density: A i2.07c variation about the nominal reference theoretical density (the nominal reference values are listed in Table 1 on page 40) was considered.

Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.07c to twice the nominal

, dishing (the nominal reference values are listed in Table 1 on page 40) was considered.

Storage Cell I.D.: The +0.050/-0.025 inch tolerance about the nominal 8.80 inch reference cell l.D. was considered.

Storage Cell Pitch: The 40.00/-0.320 inch tolerance about the nominal 10.60 inch reference cell pitch was considered.

Stainless Steel Wall Thickness: The i0.015 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations

. show that an increase in reactivity can occur if the comers of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

The maximum K eg was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown in Table 2 on page 41 and results in a maximum Keg of 0.94494.

Criticality Analysis of Unit 1 All Cell Storage 7

Since Keg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 l

probability / confidence level, the acceptance criteria for criticality is met for all cell storage of l

17x17 fuel assemblies in the Vogtle Unit I spent fuel racks. Storage of fuel assemblies with 1 nominal enrichments no greater than 2.00 w/o 235 U is acceptable in all cells including the presence of 300 ppm soluble boron.

1 3.3 Burnup Credit Reactivity Equivalencing Storage of fuel assemblies with initial enrichments higher than 2.00 w/o 235 U in all cells of the l Vogtle Unit I spent fuel .acks is achievable by means of burnup credit using reactivity i equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease

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associated with fuel depletion. For burnup credit, a series of reactivity calculations is performed to l generate a set of enrichment-fuel assembly discharge burnup ordered pairs which all yield an  !

equivalent K,g when stored in the spent fuel storage racks.

Figure 4 on page 56 shows the constant Ke rr contours generated for all cell storage in the Vogtle Unit I spent fuel racks. This cmve represents combinations of fuel enrichment and discharge burnup which yield the same rack multiplication factor (Keg) as the rack loaded with 2.00 w/o 235 U fuel assemblies at zero burnup in all cell locations.

Uncertainties associated with burnup credit include a reactivity uncertainty of 0.01 AK at 30,000 MWD /MTU applied linearly to the burnup credit requirement to account for calculation and depletion uncertainties and 5% on the calculated burnup to account for burnup measurement uncertainty. The amount of additional soluble boron needed to account for these uncertainties in the burnup requirement of Figure 4 was 250 ppm. This is additional boron above the 300 ppm required in Section 3.2. This results in a total soluble boron credit of 550 ppm.

It is important to recognize that the curve in Figure 4 is based on calculations of constant rack reactivity. In this way, the environment of the storage rack and its influence on assembly reactivity is implicitly considered. For convenience, the data from Figure 4 are also provided in Table 3 on l page 42. Use of linear interpolation between the tabulated values is acceptable since the curve  !

shown in Figure 4 is linear in between the tabulated points.

The effect of axial burnup distribution on assembly reactivity has been considered in the j development of the Vogtle Unit I all cell storage bumup credit limit. Previous evaluations have been performed to quantify axial burnup reactivity effects and to confirm that the reactivity equivalencing methodology described in Reference I results in calculations of conservative i burnup credit limits. The evaluations show that axial burnup effects only become important at burnup-enrichment combinations which are above those calculated for the Vogtle Unit 1 all cell storage burnup credit limit. Therefore, additional accounting of axial burnup distribution effects in the Vogtle Unit I all cell storage burnup credit limit is not necessary.

Criticality Analysis of Unit 1 All Cell Storage 8

4.0 Criticality Analysis of Unit 13-out-of-4 Storage Tais section describes-the analytical techniques and models employed to perform the criticality analysis and reactivity equivalencing evaluations for the storage of fuel in 3-out-of-4 cells of the l Vogtle Unit I spent fuel storage racks with credit for soluble boron.

Section 4.1 describes the maximum feasible K eg KENO-Va calculations. Section 4.2 discusses the results of the spent fuel rack K eg soluble boron credit calculations. Finally, Section 4.3 presents the results of calculations performed to show the minimum burnup requirements for assemblies with initial enrichments above those determined in Section 4.1.

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4.1 Maximum Feasible Keg Calculation l l

The following assumptions were used to develop the maximum feasible KENO-Va model for l storage of fuel assemblies in 3-out-of-4 cells of the Vogtle Unit I spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westinghouse 17x17 STD fuel design (see Table 1 on page 40 for fuel parameters). l Calculations show that for the enrichment and storage configuration considered here, the '

Westinghouse 17x17 STD design was more reactive than the Westinghouse 17x17 OFA fuel assembly design.

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2. Fuel assemblies contain uranium dioxide at a nominal enrichment of 2.70 w/o U over the entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup of fission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any burnable absorber in the fuel rods.

8. No credit was taken for the presence of spent fuel rack Borafiex poison panels. The Boraflex volume was replaced with water.

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I1. Fuel storage cells were loaded with fuel assemblies in a 3-out-of-4 checkerboard arrangement as shown in Figure 5 on page 57. A 3-out-of-4 checkerboard with empty cells means that no more than 3 fuel assemblies can occupy any 2x2 matrix of storage cells.

Criticality Analysis of Unit 13-out-of-4 Storage 9

' i With the above assumptions, the KENO-Va calculation resulted in a K eg of 0.97740 under normal  :

conditions. The reactivity bias calculated in PHOENIX-P for the normal temperature range of the spent fuel pool water (50*F to 185'F) was 0.00615 AK. Finally, the methodology bias associated with the benchmarking of the Westinghouse criticality methodology was 0.00770 AK.

Based on the results above, the following equation was used to develop the maximum feasible Keg for the storage of fuel in 3-out-of-4 cells in the Vogtle Unit I spent fuel storage racks:

K,y =- K,,o,,,,,g + B emp + B,,,,,g,,

where:

K,,y,m ui =

normal conditions KENO-Va Ke g B ,,,,,y temperature bias for normal temperature range of spent fuel pool water (50*F to 185'F)

B ma,nos "

meM Nas kened hm &&& &al comparisons Substituting the calculated values in the order listed above, the result was:

1 K,g = 0.97740 + 0.00615 + 0.00770 = 0.99125 l l

Since Keg is less than 1.0, the Vogtle Unit 1 spent fuel racks will remain suberitical under l maximum feasible conditions when 3-out-of-4 cells are loaded with 2.70 w/o 235 U 17x17 fuel I

assemblies and no soluble boron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron required to maintain K eg s 0.95 including tolerances and uncertainties.

4.2 Soluble Boron Credit K en Calculations i To determine the amount of soluble boron required to maintain Keg 50.95, KENO-Va was used to  :

establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of l material and construction tolerance variations. A final 95/95 K en was developed by statistically l combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nommal KENO-Va reference reactivity.

The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for 3-out-of-4 cell storage in the Vogtle Unit I spent fuel racks were similar to those in Section 4.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 300 ppm soluble boron.

With the above assumptions, the KENO-Va calculation for the nominal case with 300 ppm soluble boron in the moderator resulted in a Keg of 0.90121.

Criticality Analysis of Unit 13-out-of-4 Storage 10

Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and mechanical / construction dimensions, PHOENIX-P perturbation calculations were performed. For the Vogtle Unit I spent fuel rack 3-out-of-4 checkerboard configuration,'iO 2material tolerances were considered along with construction tolerances related to the cell I.D., storage cell pitch, and stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance ofi0.05 w/o 235 U about the nominal reference enrichment of 2.70 w/o 235 U was considered.

UO 2Density: A 12.0% variation about the nominal reference theoretical density (the nominal reference values are listed in Table 1 on page 40) was considered.

Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.07c to twice the nominal dishing (the nominal reference values are listed in Table 1 on page 40) was considered.

Storage Cell I.D.: The +0.050/-0.025 inch tolerance about the nominal 8.80 inch reference cell I.D. was considered.

Storage Cell Pitch: The +0.00/-0.320 inch tolerance about the nominal 10.60 inch reference cell pitch was considered.

Stainless Steel Wall Thickness: The iO.015 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations show that an increase in reactivity can occur if the comers of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

The maximum K g e was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown in Table 4 on page 43 and results in a maximum Keg of 0.94233.

Criticality Analysis of Unit 13-out-of-4 Storage i1

Since K eg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met for 3-out-of-4 storage of 17x17 fuel assemblies.in the Vogtle Unit I spent S fuel racks. Storage of fuel assemblies with nominal enrichments no greater than 2.70 w/o U is acceptable in 3-out-of-4 cells including the presence of 300 ppm soluble boron.

l 4.3 Burnup Credit Reactivity Equivalencing '

Storage of fuel assemblies with initial enrichments higher than 2.70 w/o 235U in 3-out-of-4 cells of the Vogtle Unit I spent fuel racks is achievable by means of burnup credit using reactivity equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease l associated with fuel depletion. For burnup credit, a series of reactivity calculations is performed to )

generate a set of enrichment-fuel assembly discharge burnup ordered pairs which all yield an equivalent K eg when stored in the spent fuel storage racks.

Figure 4 on page 56 shows the constant Ke g contours generated for 3-out-of-4 cell storage in the Vogtle Unit I spent fuel racks. This curve represents combinations of fuel enrichment and discharge burnup which yield the same rack multiplication factor (K eg) as the rack loaded with 2.70 w/o 235 U fuel assemblies at zero bumup in all cell locations.

Uncertainties associated with burnup credit include a reactivity uncertainty of 0.01 AK at 30,000 MWD /MTU applied linearly to the burnup credit requirement to account for calculation j and depletion uncertainties and 5% on the calculated burnup to account for bumup measurement uncertainty. The amount of additional soluble boron needed to account for these uncertainties in the burnup requirement of Figure 4 was 200 ppm. This is additional boron above the 300 ppm required in Section 4.2. This results in a total soluble boron credit of 500 ppm.

It is important to recognize that the curve in Figure 4 is based on calculations of constant rack reactivity. In this way, the environment of the storage rack and its influence on assembly reactivity is implicitly considered. For convenience, the data from Figure 4 are also provided in Table 3 on page 42. Use of linear interpolation between the tabulated values is acceptable since the curve shown in Figure 4 is linear in between the tabulated points.

The effect of axial burnup distribution on assembly reactivity has been considered in the development of the Vogtle Unit 13-out-of-4 cell storage burnup credit limit. Previous evaluations have been performed to quantify axial burnup reactivity effects and to confirm that the reactivity equivalencing methodology described in Reference I results in calculations of conservative burnup credit limits. The evaluations show that axial burnup effects only become important at burnup-enrichment combinations which are above those calculated for the Vogtle Unit 1 3-out-of-4 cell storage burnup credit limit. Therefore, additional accounting of axial burnup distribution effects in the Vogtle Unit 13-out-of-4 cell storage burnup credit limit is not necessary.

Criticality Analysis of Unit 13-out-of-4 Storage 12

l 5.0 Criticality Analysis of Unit 12-out-of-4 Storage This section describes-the analytical techniques and models employed to perform the criticality analysis for the storage of fuel in 2-out-of-4 cells of the Vogtle Unit I spent fuel storage racks with credit for soluble boron.

Section 5.1 describes the maximum feasible Keg KENO-Va calculations and section 5.2 discusses the results of the spent fuel rack Ke g soluble boron credit calculations.

5.1 Maximum Feasible K err Calculation The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in 2-out-of-4 cells of the Vogtle Unit I spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westinghouse 17x17 STD and 17x17 OFA fuel designs (see Table 1 on page 40 for fuel parameters).

2. Westinghouse 17x17 STD and 17x17 OFA fuel assemblies contain uranium dioxide at a nominal enrichment of 5.00 w/o 235 U over the entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup of fission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any bumable absorber in the fuel rods.

8. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Boraflex volume was replaced with water.

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I 1. Fuel storage cells were loaded with fuel assemblies in a 2-out-of-4 checkerboard arrangement as shown in Figure 5 on page 57. A 2-out-of-4 checkerboard with empty cells means that no 2 fuel assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent.

With the above assumptions, the KENO-Va calculations of Keg under normal conditions resulted in a K eg of 0.93263 and 0.93670 for Westinghouse STD and OFA fuel assemblies, respectively.

The reactivity bias calculated in PHOENIX-P for the normal temperature range of the spent fuel pool water (50*F to 185'F) was 0.00028 AK and 0.00024 AK for Westinghouse STD and OFA fuel assemblies, recoectively. Finally, the methodology bias associated with the benchmarking of the Westinghouse cnicality methodology was 0.00770 AK.

Criticality Analysis of Unit 12-out-of-4 Storage 13

1 Based on the results above, the following equation was used to develop the maximum feasible Keg for the storage of fuel in 2-out-of-4 cells in the Vogtle Unit I spent fuel storage racks:

! l Kag = K,v,,,_ ,,g + B,,,, + B,,,n,, \

where:

l l K no, mat =

normalconditions KENO-Va Keg

=

l B,,,p temperature bias for norma! temperature range of spent fuel j pool water (50*F to 185'F) l l

B ,,, nog

=

meM Nas demined ham &namd uid comparisons Substituting the calculated values in the order listed above for Westinghouse STD fuel, the result )

was:  !

I K,g = 0.93263 + 0.00028 + 0.00770 = 0.94061 Substituting the calculated values in the order listed above for Westinghouse OFA fuel, the result was:

K,g = 0.93670 + 0.00024 + 0.00770 = 0.94464 Since Keg is less than 1.0, the Vogtle Unit i spent fuel racks will remain suberitical under 235 maximum feasible conditions when 2-out-of-4 cells are inaded with 5.00 w/o U 17x17 STD or 17x17 OFA fuel assemblies and no soluble baron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron required to maintain K eg s 0.95 including tolerances and uncertainties.

5.2 Soluble Boron Credit K eg Calculations To determine tt:e amount of soluble boron required :o maintain Ke g 50.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of material and construction tolerance variations. A final 95/95 K eg was developed by statistically combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for 2-out-of-4 cell storage in the Vogtle Unit I spent fuel racks were similar to those in Section 5.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 100 ppm soluble boron for both the Westinghouse 17x17 STD and l 17x17 OFA fuel assembly types.

Criticality Analysis of Unit 12-out-of-4 Storage 14

I l

With the above assumptions, the KENO-Va calculation for the nominal case results in a Ke g of 0.91126 and 0.92077 for Westinghouse STD and OFA fuel assembly types respectively.

Temperature and methodology biases must be considered in the final Keg summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for  !

the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and mechanical / construction dimensions, PHOENIX-P perturbation calculations were performed. For the Vogtle Unit I spent fuel rack 2-out-of-4 checkerboard configuration, UO2material tolerances were c ansidered along with construction toleraces related to the cell I.D., storage cell pitch, and stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance of10.05 w/o 235 U about the 235 nominal reference enrichment of 5.00 w/o U was considered.

UO 2Density: A i2.0% variation about the nominal reference theoretical density (the nominal  :

reference values are listed in Table 1 on page 40) was considered.

Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.0% to twice the nominal dishing (the nominal reference values are listed in 'lable 1 on page 40) was considered.

Storage Cd. I.D.: The +0.050/-0.025 inch tolerance about the nominal 8.80 inch reference cell 1.D. was considered.

Storage Cell Pitch: The +0.00/-0.320 inch tolerance about the nominal 10.60 inch reference cell pitch was considered.

Stainless Steel Wall Thickness: The 10.015 inch tolerance about the nomina 10.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations show that an increase in reactivity can occur if the comers of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in.

the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Criticality Analysis of Unit 12-out-of-4 Storage 15

l 1

The maximum K g e was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown inTable 5 on page 44 and results in a maximum K eg of 0.92947 and 0.93754 for Westinghouse STD and OFA fuel assembly types, respectively.

Since K eg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met for 2-out-of-4 cell storage of 17x17 fuel assemblies in the Vogtle Unit I spent fuel racks. Storage of fuel assemblies 235 with nominal enrichments no greater than 5.00 w/o U is acceptable in 2-out-of-4 cells i including the presence of 100 ppm soluble baron.

1 l

Criticality Analysis of Unit 12-out-of-4 Storage 16

I 6.0 Criticality Analysis of Unit 2 All Cell Storage This section describe >the analytical techniques and models employed to perform the criticality analysis and reactivity equivalencing evaluations for the storage of fuel in all cells of the Vogtle Unit 2 spent fuel storage racks with credit for soluble boron.

Section 6.1 describes the maximum feasible K err KENO-Va calculations. Section 6.2 discusses the results of the spent fuel rack K err soluble boron credit calculations. Finally, Section 6.3 l presents the results of calculations performed to show the minimum burnup requirements for assemblies with initial enrichments above those determined in Section 6.1.

l 6.1 Maximum Feasible Keg Calculation i

The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in all cells of the Vngtle Unit 2 spent fuel storage rack:

1. The fuel assembly parameters relevant ta i criticality analysis were based on the Westinghouse 17x17 STD fuel design (see Table 1 on page 40 for fuel parameters).  ;

Calculations show that for the enrichment and storage configuration considered here, the Westinghouse 17x17 STD design was more reactive than the Westinghouse 17x17 OFA fuel I assembly design.

2. Fuel assemblies contain uranium dioxide at a nominal enrichment of 1.82 w/o 235 U over the entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing l 4

fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets. i 234 236

5. No credit was taken for any U or U in the fuel, nor was any credit taken for the buildup of fission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any burnable absorber in the fuel rods. l

8. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Boraflex volume was replaced with water.

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I 1. All available storage cells were loaded with fuel assemblies.

With the above assumptions, the KENO-Va calculation resulted in a Ke rrof 0.97863 under normal conditions. The reactivity bias calculated in PHOENIX-P for the normal ten.perature range of the spent fuel pool water (50*F to 185'F) was 0.00931 AK. Finally, the methedology bias associated with the benchmarking of the Westinghouse criticality methodology was'J.00770 AK.

1 Criticality Analysis of Unit 2 All Cell Storage 17

1 l

l Based on the results above, the following equation was used to develop the maximum feasible Keg for all cell storage in the Vogtle Unit 2 spent fuel storage racks:

Kag = Kno,,,g + B,,,,+ B ,,,gog where:

K,y, mat =

normal conditions KENO-Va Ke g

=

B ,,,g temperature bias for normal temperature range of spent fuel pool water (50*F to 185'F)

B ,,,nos meM Nas dewrmined ham &&maA cMd comparisons Substituting the calculated values in the order listed above, the result was:

I K,g = 0.97863 + 0.00931 + 0.00770 = 0.99564 i Since Keg is less than 1.0, the Vogtle Unit 2 spent fuel racks will remain suberitical under maximum feasible conditions when all cells are loaded with 1.82 w/o 235 U 17x17 fuel assemblies and no soluble boron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron required to maintain K eg s 0.95 including tolerances and uncertainties.

6.2 Soluble Boron Credit K err Calculations To determine the amount of soluble boron required to maintain Ke g 50.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of material and construction tolerance variations. A final 95/95 K eg was developed by statistically combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for all cell storage in the Vogtle Unit 2 spent fuel racks were similar to those in Section 6.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 200 ppm soluble boron.

With the above assumptions, the KENO-Va calculation for the nominal case with 200 ppm soluble baron in the moderator resulted in a K eg of 0.91531.

Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Criticality Analysis of Unit 2 All Cell Storage 18

_ - _ _ - . = . _ _-- - . . _ - - - _

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and mechanical / construction dimensions, PHOENIX-P perturbation calculations were performed. For the Vogtle Unit 2 spent fuel rack all cell storage configuration, UO2 material tolerances were considered along with construction tolerances related to the cell I.D., storage cell pitch, and stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance of10.05 w/o 235 U about the I nominal reference enrichment of 1.82 w/o 235 U was considered.

UO 2Density: A i2.07c variation about the nominal reference theoretical density (the nominal reference values are listed in Table 1 on page 40) was considered.

Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.07c to twice the nominal dishing (the nominal reference values are listed in Table 1 on page 40) was considered.

Storage Cell I.D.: The i0.030 inch tolerance about the nominal 8.75 inch reference cell I.D.

was considered.

Storage Cell Pitch: The i0.040 inch tolerance about the nominal 10.40 inch (E-W) and 10.58 inch (N-S) reference cell pitch was considered.

Stainless Steel Wall Thickness: The 0.005 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations show that an increase in reactivity can occur if the corners of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered. l The maximum K g e was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown in Table 6 on page 45 and results in a maximum Ke g of 0.94409.

l Criticality Analysi; of Unit 2 All Cell Storage 19

1 Since Keg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 i

probability / confidence level, the acceptance criteria for criticality is met for all cell storage of l 17x17 fuel assemblies in the Vogtle Unit 2 spent fuel racks. Storage of fuel assemblies with l nominal enrichments no greater than 1.82 w/o 235 U is acceptable in all cells including the presence of 200 ppm soluble boron.

6.3 Burnup Credit Reactivity Equivalencing 235 Storage of fuel assemblies with initial enrichments higher than 1.82 w/o U in all cells of the Vogtle Unit 2 spent fuel racks is achievable by means of burnup credit using reactivity equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease associated with fuel depletion. For burnup credit, a series of reactivity calculations is performed to generate a set of enrichment-fuel assembly discharge burnup ordered pairs which all yield an equivalent K eg when stored in the spent fuel storage racks.

Figure 6 on page 58 shows the constant Ke g contours generated for all cell storage in the Vogtle Unit 2 spent fuel racks. This curve represents combinations of fuel enrichment and discharge burnup which yield the same rack multiplication factor (Keg) as the rack loaded with 1.82 w/o 235 U fuel assemblies at zero burnup in all cell locations.

! Uncertainties associated with burnup credit include a reactivity uncertainty of 0.01 AK at 30,000 MWD /MTU applied linearly to the burnup credit requirement to account for calculation and depletion uncertainties and 5% on the calculated burnup to account for burnup measurement uncertainty. The amount of additional soluble boron needed to account for these uncertainties in the burnup requirement of Figure 6 was 250 ppm. This is additional boron above the 200 ppm j required in Section 6.2. This results in a total soluble boron credit of 450 ppm.

1 It is important to recognize that the curve in Figure 6 is based on calculations of constant rack reactivity. In this way, the environment of the storage rack and its influence on assembly reactivity

is implicitly considered. For convenience, the data from Figure 6 are also provided in Table 7 on i page 46. Use of linear interpolation between the tabulated values is acceptable since the curve shown in Figure 6 is linear in between the tabulated points.

, The effect of axial bumup distribution on assembly reactivity has been considered in the development of the Vogtle Unit 2 all cell storage burnup credit limit. Previous evaluations have been performed to quantify axial burnup reactivity effects and to confirm that the reactivity equivalencing methodology described in Reference 1 results in calculations of conservative burnup credit limits. The evaluations show that axial burnup effects only become important at burnup-enrichment combinations which are above those calculated for the Vogtle Unit 2 all cell storage bumup credit limit. Therefore, additional accounting of axial burnup distribution effects in the Vogtle Unit 2 all cell storage burnup credit limit is not necessary.

1 a

Criticality Analysis of Unit 2 All Cell Storage 20

7.0 Criticality Analysis of Unit 2 3-out-of-4 Storage l

This section describes the analytical techniques and models employed to perform the criticality j analysis and reactivity equivalencing evaluations for the storage of fuelin 3-out-of-4 cells of the 1 Vogtle Unit 2 spent fuel storage racks with credit for soluble boron.

Section 7.1 describes the maximum feasible K err KENO-Va calculations. Section 7.2 discusses the results of the spent fuel rack K g e soluble boron credit calculations. Finally, Section 7.3 presents th e results of calculations performed to show the minimum burnup requirements for assemblies with initial enrichments above those determined in Section 7.1. j 7.1 Maximum Feasible K err Calculation The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in 3-out-of-4 cells of the Vogtle Unit 2 spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westinghouse 17xl7 STD fuel design (see Table 1 on page 40 for fuel parameters).

Calculations show that for the enrichment and storage configuration considered here, the l Westinghouse 17x17 STD design was more reactive than the Westinghouse 17x17 OFA fuel assembly design.

2. Fuel assemblies contain uranium dioxide at a nominal enrichment of 2.54 w/o 235U over the i entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup of fission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any burnable absorber in the fuel rods.

8. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Boraflex volume was replaced with water.

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm 3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I 1. Fuel storage cells were loaded with fuel assemblies in a 3-out-of-4 checkerboard arrangement as shown in Figure 5 on page 57. A 3-out-of-4 checkerboard with empty cells means that no more than 3 fuel assemblies can occupy any 2x2 matrix of storage cells.

Criticality Analysis of Unit 2 3-out-of-4 Storage 21

l l

With the above assumptions, the KENO-Va calculation resulted in a Keg of 0.98443 under normal conditions. The reactivity bias calculated in PHOENIX-P for the normal temperature range of the '

! spent fuel pool water (50*F to 185'F) was 0.00522 AK. Finally, the methodology bias associated I with the benchmarking of the Westinghouse criticality methodology was 0.00770 AK.

{

Based on the results above, the following equation was used to develop the maximum feasible Keg for the storage of fuel in 3-out-of-4 cells in the Vogtle Unit 2 spent fuel storage racks- l l

K,g = Kruorinal + B,,,,,, + B,,,,,og l 1

where:

K,wr,nai =

normal conditions KENO-Va Ke g

= l B ,,,,y temperature bias for normal temperature range of spent fuel <

, pool water (50*F to 185'F)

B,nethod

=

mew Nas demined ham &nchmd cual comparisons 1

Substituting the calculated values in the order listed above, the result was:

i; K,g = 0.98443 + 0.00522 + 0.00770 = 0.99735 4

Since Ke g is less than 1.0, the Vogtle Unit 2 spent fuel racks will remain suber;tical under maximum feasible conditions when 3-out-of-4 cells are loaded with 2.54 w/o 235 U 17x17 fuel assemblies and no soluble boron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron

required to maintain K eg 5 0.95 including tolerances and uncertainties.

l

7.2 Soluble Boron Credit K err Calculations To determine the amount of soluble boron required to maintain K eg 5 0.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of material and construction tolerance variations. A final 95/95 Keg was developed by statistically

combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

l The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for 3-out-of-4 storage in the Vogtle Unit 2 spent fuel racks were similar to those in Section 7.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 250 ppm soluble boron.

With the above assumptions, the KENO-Va calculation for the nominal case with 250 ppm soluble boron in the moderator resulted in a Ke g of 0.91778.

l Criticality Analysis of Unit 2 3-out-of-4 Storage 22

1 Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and I i mechanical / construction dimensions, PHOENIX-P perturbation calculations were performed. For

)

the Vogtle Unit 2 spent fuel rack 3-out-of-4 checkerboard configuration, UO2 material tolerances l were considered along with construction tolerances related to the cell I.D., storage cell pitch, and I stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncerta'.ty I

, statistical summation: >

e 235 U Enrichment: The standard DOE enrichment tolerance of10.05 w/o 235 U about the 235 nominal reference enrichment of 2.54 w/o U was considered.

UO 2Density: A i2.0% variation about the nominal reference theoretical density (the nominal

reference values are listed in Table 1 on page 40) was considered.

! Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.0% to twice the nominal dishing (the nominal reference values are listed in Table 1 on page 40) was considered.

Storage Cell I.D.: The 10.030 inch tolerance about the nominal 8.75 inch reference cell 1.D.

was considered.

2 Storage Cell Pitch: The i0.040 inch tolerance about the nominal 10.40 inch (E-W) and 10.58 inch (N-S) reference cell pitch was considered.

Stainless Steel Wall Thickness: The t0.005 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The r "NO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations show that an increase in reactivity can occur if the corners of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

The. maximum Keg was developed by adding the calculational and methodology biases and the

, statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown in Table 8 on page 47 and results in a maximum Keg of 0.93983.

Criticality Analysis of Unit 2 3-out-of-4 Storage 23

Since K eg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met for 3-out-of-4 storage of 17x17 fuel assemblies in the Vogtle Unit 2 spent fuel racks. Storage of fuel assemblies with nominal enrichments no' greater than 2.54 w/o 235 U is acceptable in 3-out-of-4 cells including the presence of 250 ppm soluble boron.

7.3 Burnup Credit Reactivity Equivalencing Storage of fuel assemblies with initial enrichments higher than 2.54 w/o 235U in 3-out-of-4 cells of the Vogtle Unit 2 spent fuel racks is achievable by rneans of burnup credit using reactivity equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease associated with fuel depletion. For burnup credit, a series of reactivity calculations is performed to ,

generate a set of enrichment-fuel assembly discharge burnup ordered pairs which all yield an )

equivalent K eg when stored in the spent fuel storage racks.

Figure 6 on page 58 shows the constant Keg contours generated for 3-out-of-4 storage in the Vogtle Unit 2 spent fuel racks. This curve represents combinations of fuel enrichment and discharge burnup which yield the same ract: multiplication factor (K eg) as the rack loaded with 2.54 w/o 235 U fuel assemblies at zero burnup in 3-out-of-4 cell locations.

i Uncertainties associed with burnup credit include a reactivity uncertainty of 0.01 AK at i 30,000 MWD /MTU applied linearly to the burnup credit requirement to account for calculation l and depletion uncertainties and 5% on the calculated burnup to account for burnup measurement uncertainty. The amount of additional soluble boron needed to account for these uncertainties in the burnup requirement of Figure 6 was 200 ppm. This is additional boron above the 250 ppm required in Section 7.2. This results in a total soluble boron credit of 450 ppm.

It is important to recognize that the curve in Figure 6 is based on calculations of constant rack reactivity. In this way, the environment of the storage rack and its influence on assembly reactivity is implicitly considered. For convenience, the data from Figure 6 are also provided in Table 7 on page 46. Use of linear interpolation between the tabulated values is acceptable since the curve shown in Figure 6 is linear in between the tabulated points.

The effect of axial bumup distribution on assembly reactivity has been considered in tie development of the Vogtle Unit 2 3-out-of 4 cell storage bumup credit limit. Previous evaluati ans have been performed to quantify axial burnup reactivity effects and to confirm that the reactivity equivalencing methodology described in Reference i results in calculations of conservative burnup credit limits. The evaluations show that axial burnup effects only become important at burnup-enrichment combinations which are above those calculated for the Vogtle Unit 2 3-out-of-4 cell storage burnup credit limit. Theref^re, additional accounting of axial burnup distribution effects in the Vogtle Unit 2 3-out-of-4 cell storage burnup credit limit is not necessary.

Criticality Analysis of Unit 2 3-out-of-4 Storage 24

8.0 Criticality Analysis of Unit 2 2-out-of-4 Storage This section describes.the analytical techniques and models employed to perform the criticality analysis for the storage of fuel in 2-out-of-4 cells of the Vogtle Unit 2 spent fuel storage racks  ;

with credit for soluble boron. I Section 8.1 describes the maximum feasible K eg KENO-Va calculations and section 8.2 discusses the results of the spent fuel rack Keg soluble boron credit calculations.

8.1 Maximum Feasible K err Calculation The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in 2-out-of-4 cells of the Vogtle Unit 2 spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the

Westinghouse 17x17 STD and 17x17 OFA fuel designs (see Table 1 on page 40 for fuel parameters).

, 2. Westinghouse 17x17 STD and 17x17 OFA fuel assemblies contain uranium dioxide at a nominal enrichment of 5.00 w/o 235 U over the entire length of each rod.

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

4. No credit was taken for any natural or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup 4

of fission product poison material.

6. No credit was taken for any spacer grids or spacer sleeves.

7. No credit was taken for any bumable absorber in the fuel rods.

8. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Borallex volume was replaced with water. l

9. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm3was used.

10. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

I

11. Fuel storage cells were loaded with fuel assemblies in a 2-out-of-4 checkerboard arrangement as shown in Figure 5 on page 57. A 2-out-of-4 checkerboard with empty cells means that no 2 fuel assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent.

With the above assumptions, the KENO-Va calculations of K eg under normal conditions resulted in a K ge of 0.93575 and 0.94622 for both Westinghouse STD and OFA fuel assemblies, respectively. The reactivity bias calculated in PHOENIX-P for the normal temperature range of the spent fuel pool water (50*F to 185'F) was 0.00059 AK and 0.00027 AK for Westinghouse STD and OFA fuel assemblies, respectively. Finally, the methodology bias associated with the

! benchmarking of the Westinghouse criticality methodology was 0.00770 AK.

Criticality Analysis of Unit 2 2-out-of-4 Storage 25

l l

l Based on the results above, the following equation was used to develop the maximum feasible Keg for the storage of fuel in 2-out-of-4 cells in the Vogtle Unit 2 spent fuel storage racks:

Kag = K,,o,,a; + B,,,,,, + B,,,,, n,g where:

K,,y,ma t =

normal conditions KENO-Va Ke g

=

B ,,,,,p temperature bias for ncimal temperature range of spent fuel i pool water (50*F to 185'F) l B,,,nos meM Nas Memind imm &ndmark crhical comparisons Substituting the calculated values in the order listed above for Westinghouse STD fuel, the result was ,

l i

K,g = 0.93575 + 0.00059 + 0.00770 = 0.94404 Substituting the calculated values in the order listed above for Westinghouse OFA fuel, the result was:

K,g = 0.94622 + 0.00027 + 0.00770 = 0.95419 Since Ke g is less than 1.0, the Vogtle Unit 2 spent fuel racks will remain suberitical under maximum feasible conditions when 2-out-of-4 cells are loaded with 5.00 w/o 235 U 17x17 STD or 17x17 OFA fuel assemblies and no soluble boron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the I amount of soluble boron required to maintain K eg s 0.95 including tolerances and uncertainties.

I 8.2 Soluble Boron Credit K eg Calculations l To determine the amount of soluble boron required to maintain Ke g s 0.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of j material and construction tolerance variations. A final 95/95 Keg was developed by statistically l combining the individual tolerance impacts with the calculational and methodology uncertainties i and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

i The assumptions used to develop the nominal case KENO-Va model for soluble baron credit for 2-out-of-4 storage in the Vogtle Unit 2 spent fuel racks were similar to those in Section 8.1 except for assumption 9 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 50 ppm soluble boron for both the Westinghouse 17x17 STD and 17x17 OFA fuel assembly types.

Criticality Analysis of Unit 2 2-out-of-4 Storage 26

I 1

With the above assumptions, the KENO-Va calculation for the nominal case results in a Keg of I

' 0.92588 and 0.93412 for Westinghouse STD and OFA fuel assembly types, respectively. l Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and mechanical / construction dimensions, PHOENIX P perturbation calculations were performed. For the Vogtle Unit 2 spent fuel rack 2-out-of-4 checkerboard configuration, UO2 material tolerances were considered along with construction tolerances related to the cell 1.D., storage cell pitch, and i stainless steel wall thickness. Uncertainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance of10.05 w/o 235 U about the 235 nominal reference enrichment of 5.00 w/o U was considered.

UO 2Density: A i2.0% variation about the nominal reference theoretical density (the nominal reference values are listed in Table 1 on page 40) was considered.

Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.0% to twice the nominal dishing (the nominal reference values are listed in Table 1 on page 40) was considered.

Storage Cell I.D.: The iO.030 inch tolerance about the nominal 8.75 inch reference cell I.D.

was considered.

Storage Cell Pitch: The 10.040 inch tolerance about the nominal 10.40 inch (E-W) and 10.58 inch (N-S) reference cell pitch was considered.

Stainless Steel Wall Thickness: The i0.005 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations show that an increase in reactivity can occur if the corners of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered.

Criticality Analysis of Unit 2 2-out-of-4 Storage 27

4 j

The maximum K rt e was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The  ;

summation is shown in Table 9 on page 48 and results in a maximum Ke rrof 0.93934 and 0.94794 I for Westinghouse 17x17 STD and 17x17 OFA fuel assembly types, respectively.

Since Kerr is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met for 2-out-of-4 cell storage of 17x17 fuel assemblies in the Vogtle Unit 2 spent fuel racks. Storage of fuel assemblies with nominal enrichments no greater than 5.00 w/o 235 U is acceptable in 2-out-of-4 cells l including the presence of 50 ppm soluble boron.

1 l

j Criticality Analysis of Unit 2 2-out-of-4 Storage 28

9.0 Criticality Analysis of Unit 2 3x3 Checkerboard This section describes the analytical techniques and models employed to perform the criticality analysis and reactivity equivalencing evaluations for the storage of fuel in a 3x3 checkerboard in the Vogtle Unit 2 spent fuel storage racks with credit for soluble boron.

Section 9.1 describes the maximum feasible K eg KENO-Va calculations. Section 9.2 discusses the results of the spent fuel rack K eg soluble boron credit calculations. Finally, Section 9.3 presents the results of calculations performed to show the minimum burnup requirements for assemblies with initial enrichments above those determined in Section 9.1.

9.1 Maximum Feasible Keg Calculation The following assumptions were used to develop the maximum feasible KENO-Va model for storage of fuel assemblies in a 3x3 checkerboard in the Vogtle Unit 2 spent fuel storage rack:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westinghouse 17xl7 STD fuel design (see Table 1 on page 40 for fuel parameters).

Calculations show that the Westinghouse 17x17 STD design was the most reactive fuel assembly type.

2. Westinghouse 17x17 STD fuel assemblies stored in the middle of the 3x3 checkerboard contain uranium dioxide at a nominal enrichment of 4.00 w/o 235 U over the entire length of each rod.

3. Westinghouse 17x17 STD fuel assemblies surrounding the center of the 3x3 checkerboard contain uranium dioxide at a nominal enrichment of 1.48 w/o 235U over the entire length of each rod.

4. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction.

5. No credit was taken for any natural or reduced enrichment axial blankets.

No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup 6.

of fission product poison material.

7. No credit was taken for any spacer grids or spacer sleeves.

8. No credit was taken for any burnable absorber in the fuel rods.

9. No credit was taken for the presence of spent fuel rack Boraflex poison panels. The Boraflex volume was replaced with water.

10. The moderator was water with 0 ppm soluble boron at a temperature of 68'F. A water density of 1.0 gm/cm 3was used.

I 1. The array was infinite in lateral (x and y) extent and finite in axial (vertical) extent.

12. Fuel storage cells were loaded with fuel assemblies in a 3x3 checkerboard arrangement as shown in Figure 5 on page 57.

f l Criticality Analysis of Unit 2 3x3 Checkerboard 29

~ - . _ _ - _ _ - - - .. _- ~_ - .. -. . .

With the above assumptions, the KENO-Va calculations of Keg under normal conditions resulted  :

i in a K eg of 0.98373. The reactivity bias calculated in PHOENIX-P for the normal temperature

)

range of the spent fuelpool water (50*F to 185'F) was 0.00772 AK. Finally, the methodology bias 1 associated with the benchmarking of the Westinghouse criticality methodology was 0.00770 AK.

i Based on the results above, the following equation was used to develop the maximum feasible Keg for the storage of fuel in a 3x3 checkerboard in the Vogtle Unit 2 spent fuel storage racks:

i l K,y = Kno,,ag+ B,,,,+ B ,,, nog

l where

K ngemat =

normalconditions KENO-Va Ke g

=

l B ,,,p temperature bias for normal u mperature range of spent fuel pool water (50*F to 185'F)

B ,,,nos meM Nas dete&cd ham &hd &al comparisons Substituting the calculated values in the order listed above, the result was:

J K,g = 0.98373 + 0.00772 + 0.00770 = 0.99915 Since Keg is less than 1.0, the Vogtle Unit 2 spent fuel racks will remain suberitical under maximum feasible conditions when cells are loaded in a 3x3 checkerboard with a 4.00 w/o 235U 17x17 fuel assembly surrounded by 1.48 w/o 235 U 17x17 fuel assemblies and no soluble boron is present in the spent fuel pool water. In the next section, soluble boron credit will be used to provide safety margin by determining the amount of soluble boron required to maintain Ke g 5 0.95 including tolerances and uncertainties.

9.2 Soluble Boron Credit K err Calculations To determine the amount of soluble boron required to maintain Keg 50.95, KENO-Va was used to establish a nominal reference reactivity and PHOENIX-P was used to assess the effects of material and construction tolerance variatiens. A final 95/95 K eg was developed by statistically combining the individual tolerance impacts with the calculational and methodology uncertainties and summing this term with the temperature and method biases and the nominal KENO-Va reference reactivity.

The assumptions used to develop the nominal case KENO-Va model for soluble boron credit for 3x3 checkerboard storage in the Vogtle Unit 2 spent fuel racks were similar to those in Section 9.1 except for assumption 10 regarding the moderator soluble boron concentration. The moderator was replaced with water containing 250 ppm soluble boron. .

Criticality Analysis of Unit 2 3x3 Checkerboard 30

With the above assumptions, the KENO-Va calculation for the nominal case results in a Ke g of 0.91902.

Temperature and methodology biases must be considered in the final Ke g summation prior to comparing against the 0.95 Ke g limit. The following biases were included:

Methodology: The benchmarking bias as determined for the Westinghouse KENO-Va i methodology was considered.

l Water Temperature: A reactivity bias determined in PHOENIX-P was applied to account for the effect of the normal range of spent fuel pool water temperatures (50*F to 185'F).

To evaluate the reactivity effects of possible variations in material characteristics and

mechanical / construction dimensions, PHOENIX-P perturbation calculations were performed. For the Vogtle Unit 2 spent fuel rack 3x3 checkerboard configuration, UO2 material tolerances were considered along with construction tolerances related to the cell I.D., storage cell pitch, and stainless steel wall thickness. Uncenainties associated with calculation and methodology accuracy were also considered in the statistical summation of uncertainty components.

, The following tolerance and uncertainty components were considered in the total uncertainty statistical summation:

235 U Enrichment: The standard DOE enrichment tolerance of 0.05 w/o 235U about the I

nominal reference enrichment of 4.00 w/o 235 U for the center assembly and 1.48 w/o 235 U for the surrounding assemblies was considered.

UO 2Density: A i2.09c variation about the nominal reference theoretical density (the nominal q reference values are listed in Table 1 on page 40) was considered. l

[ Fuel Pellet Dishing: A variation in fuel pellet dishing fraction from 0.07c to twice the nominal l dishing (the nominal reference values are listed in Table 1 on page 40) was considered, i

i Storage Cell I.D.: The 0.030 inch tolerance about the nominal 8.75 inch reference cell I.D.

was considered.

Storage Cell Pitch

The 10.040 inch tolerance about the nominal 10.40 inch (E-W) and 10.58 inch (N-S) reference cell pitch was considered.

1 Stainless Steel Wall Thickness: The 0.005 inch tolerance about the nominal 0.075 inch reference stainless steel wall thickness was considered.

Assembly Position: The KENO-Va reference reactivity calculation assumed fuel assemblies were symmetrically positioned (centered) within the storage cells. Conservative calculations i show that an increase in reactivity can occur if the corners of the four fuel assemblies were positioned together. This reactivity increase was considered in the statistical summation of spent fuel rack tolerances.

Calculation Uncertainty: The 95 percent probability /95 percent confidence level uncertainty on the KENO-Va nominal reference K eg was considered.

Criticality Analysis of Unit 2 3x3 Checkerboard 31

Methodology Uncertainty: The 95 percent probability /95 percent confidence uncertainty in the benchmarking bias as determined for the Westinghouse KENO-Va methodology was considered. -

The maximum K eg was developed by adding the calculational and methodology biases and the statistical sum of independent uncertainties to the nominal KENO-Va reference reactivity. The summation is shown in Table 10 on page 49 and results in a maximum Ke g of 0.94931.

Since Keg is less than or equal to 0.95 including soluble boron credit and uncertainties at a 95/95 probability / confidence level, the acceptance criteria for criticality is met for the 3x3 checkerboard storage configuration of 17x17 fuel assemblies in the Vogtle Unit 2 spent fuel racks when cells are loaded in a 3x3 checkerboard with a 4.00 w/o 17x17 fuel assembly surrounded by 1.48 w/o 235U 17x17 fuel assemblies including the presence of 250 ppm soluble boron.

9.3 Burnup Credit Reactivity Equivalencing Storage of fuel assemblies with initial enrichments higher than 1.48 w/o 235U in the surrounding cells of the 3x3 checkerboard in the Vogtle Unit 2 spent fuel racks is achievable by means of burnup credit using reactivity equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease associated with fuel depletion. For burnup credit, a series of reactivity calculations is performed to generate a set of enrichment-fuel assembly discharge burnup ordered pairs which all yield an equivalent K eg when stored in the spent fuel storage racks.

Figure 6 on page 58 shows the constant Ke g contours generated for surrounding cells of the 3x3 checkerboard in the Vogtle Unit 2 spent fuel racks. This curve represents combinations of fuel enrichment and discharge burnup which yield the same rack multiplication factor (K err) as the rack loaded with 1.48 w/o 235 U fuel assemblies at zero burnup in surrounding cell locations of a 3x3 checkerboard.

Uncertainties associated with burnup credit include a reactivity uncertainty of 0.01 AK at 30,000 MWD /MTU applied linearly to the burnup credit requirement to account for calculation and depletion uncertainties and 5% on the calculated burnup to account for burnup measurement uncertainty. The amount of additional soluble boron needed to account for these uncertainties in the burnup requirement of Figure 6 was 200 ppm. This is additional boron above the 250 ppm required in Section 9.2. This results in a total soluble baron credit of 450 ppm.

It is important to recognize that the curve in Figure 6 is based on calculations of constant rack reactivity. In this way, the environment of the storage rack and its influence on assembly reactivity is implicitly considered. For convenience, the data from Figure 6 are also provided in Table 7 on page 46. Use of linear interpolation between the tabulated values is acceptable since the curve shown in Figure 6 is linear in between the tabulated points.

The effect of axial burnup distribution on assembly reactivity has been considered in the development of the Vogtle Unit 2 3x3 checkerboard burnup credit limit. Previous evaluations have been performed to quantify axial burnup reactivity effects and to confirm that the reactivity equivalencing methodology described in Reference 1 results in calculations of conservative Criticality Analysis of Unit 2 3x3 Checkerboard 32

i burnup credit limits. The evaluations show that axial burnup effects only become imponant at burnup-enrichment combinations which are above those calculated for the Vogtle Unit 2 3x3 )

checkerboard burnup credit limit. Therefore, additional accounting of axial bumup distribution ,

effects in the Vogtle Unit 2 3x3 checkerboard burnup credit limit is not necessary.

)

- 9.4 IFBA Credit Reactivity Equivalencing Storage of fuel assemblies with nominal enrichments greater than 4.00 w/o U235 n the middle cell of the 3x3 checkerboard in the Vogtle Unit 2 spent fuel storage racks is achievable by means of

, IFBA credit using reactivity equivalencing. The concept of reactivity equivalencing is predicated upon the reactivity decrease associated with the addition of Integral Fuel Burnable Absorbers (IFBA)W. IFBAs consist of neutron absorbing material applied as a thin ZrB2 coating on the

, outside of the UO 2fuel pellet. As a result, the neutron absorbing materialis a non-removable or integral part of the fuel assembly once it is manufactured.

l A series of reactivity calculations were performed to generate a set of IFBA rod number versus

enrichment ordered pairs which all yield the equivalent Keg when the fuel is stored in the middle i 1 of the 3x3 checkerboard in the Vogtle Unit 2 spent fuel racks. The following assumptions were i

used ist the IFBA rod assemblies in the PHOENIX-P models:

1. The fuel assembly parameters relevant to the criticality analysis were based on the Westing-

house 17xl7 STD design (see Table 1 on page 40 for fuel parameters).

2. The fuel assembly was modeled at its most reactive point in life.

i

3. The fuel pellets were modeled assuming nominal values for theoretical density and dishing fraction, l

j 4. No credit was taken for any natural enrichment or reduced enrichment axial blankets.

5. No credit was taken for any 234 U or 236 U in the fuel, nor was any credit taken for the buildup i of fission product poison material.

j 6. No credit was taken for any spacer grids or spacer sleeves.

4

7. The IFBA absorber material was a zirconium diboride (ZrB 2) coating on the fuel pellet. Nom-inal IFBA rod 10B loadings of 1.50 milligrams 10 B per inch (1.0X) and 2.25 milligrams10B per inch (1.5X) were used in determining the IFBA requirement.

! 8. The IFBA B10loading was reduced by 16.67% to conservatively model a minimum poison length of 120 inches.

9. The moderator was pure water (no boron) at a temperature of 68'F with a density of l.0 gm/cm3.

10. The array was infinite in lateral (x and y) and axial (vertical) extent. This precludes any neu-tron leakage from the array.

I1. Standard Westinghouse IFBA patterns for 17x17 fuel assemblies were considered.

Criticality Analysis of Unit 2 3x3 Checkerboard 33

- . _ . . . . - _ - - - . . - . _ . . - . . . _ _ . - . - _ . _ - . . = . . - . - _ . - - . _ . . - _ _ _ .

1 i

1 Figure 7 on page 59 shows the constant Keff contour generated for the Vogtle Unit 2 spent fuel racks. The data in Figure 7 is also provided on Table 11 on page 50 for both 1.0X and 1.5X IFBA rods. -

It is important to recognize that the curve in Figure 7 is based on reactivity equivalence calculations for the specific enrichment and IFBA combinations in actual rack geometry (and not just on simple comparisons ofindividual fuel assembly infinite multiplication factors). In this '

way, the environment of the storage rack and its influence on assembly reactivity is implicitly considered. ,

i l Uncertainties associated with IFBA credit include a 5% manufacturing tolerance and a 10%

calculational uncertainty on the10B loading of the IFBA rods. The amount of additional soluble i boron needed to account for these uncertainties in the IFBA credit requirement of Figure 7 was l 50 ppm. This is additional boron above the 250 ppm required in Section 9.2. The 50 ppm needed l for IFB A credit is bounded by the 200 ppm required for burnup credit in the 3x3 checkerboard in j the Vogtle Unit 2 spent fuel racks. Therefore, the total soluble boron credit required for the 3x3 l checkerboard in the Vogtle Unit 2 spent fuel racks remains at 450 ppm.

l r

i I

l l

Criticality Analysis of Unit 2 3x3 Checkerboard 34

10.0 Discussion of Postulated Accidents Most accident conditiens will not result in an increase in IQ of the rack. Examples are:

4 Fuel assembly drop The rack structure pertinent for criticality is not excessively deformed on top of rack and the dropped assembly which comes to rest horizontally on top of the rack has sufficient water separating it from the active fuel height of stored assemblies to preclude neutronic interaction.

Fuel assembly drop Typically, the design of the spent fuel racks and fuel handling between rack equipment is such that it precludes the insertion of a fuel assembly in modules or between other than prescribed locations. However, in cases where this is not rack modules and true, the reactivity increase caused by this accident is bounded by the spent fuel pool wall mis-placement of a fuel assembly inside the spent fuel racks.

However, two accidents can be postulated for each storage configuration which can increase reactivity beyond the analyzed condition. The first postulated accident would be a change in the spent fuel pool water temperature and the second would be a misload of an assembly into a cell for which the restrictions on location, enrichment, or burnup are not satisfied. All accident conditions are analyzed without the presence of Boraflex neutron absorbing panels.

For the change in spent fuel pod water temperature accident, a temperature range of 32*F to 240*F is considered. Calculations were performed for all Vogtle Unit I and 2 storage configurations to determine the reactivity change caused by a change in the Vogtle Units 1 and 2 ,

spent fuel pool water temperature outside the normal range (50*F to 185'F). The results of these calculations are tabulated in Table 12 on page 51.

For the mistoaded assembly accident, calculations were performed to show the largest reactivity increase caused by a 5.00 w/o 17x17 STD or OFA unirradiated fuel assembly misplaced into a storage cell for which the restrictions on location, enrichment, or burnup are not satisfied. The results of these calculations are also tabulated in Table 12.

For an occurrence of the above postulated accident conditions, the double contingency principle of ANSI /ANS 8.1-1983 can be applied. This states that one is not required to assume two unlikely, independent, concurrent events to ensure protection against a criticality accident. Thus, for these postulated accident conditions, the presence of additional soluble boron in the storage pool water (above the concentration required for normal conditions and reactivity equivalencing) can be assumed as a realistic initial condition since not assuming its presence would be a second unlikely event.

The reactivity change due to the presence of soluble boron in the Vogtle Units 1 and 2 spent fuel pool has been calculated with PHOENIX-P and is shown in Figure 8 on page 60 for Vogtle Unit I and Figure 9 on page 61 for Vogtle Unit 2.

Discussion of Postulated Accidents 35

The amount of soluble boron required to offset each of the postulated accidents was determined from Figure 8 for Vogtle Unit I and from Figure 9 for Vogtle Unit 2. The additional amount of soluble boron for accident conditions needed beyond the required boron for uncertainties and  !

burnup is shown in Table 12.

1 Based on the above discussion, should a loss of spent fuel pool cooling accident or a fuel  !

assembly mistoad occur in the Vogtle Units I and 2 spent fuel racks, Keg will be maintained less !

than or equal to 0.95 due to the presence of at least 1100 ppm of soluble boron in the Vogtle Unit I spent fuel pool water and 1250 ppm in the Vogtle Unit 2 spent fuel pool. '

4 e

4 i

j Discussion of Postulated Accidents 36

l l

l l

1 11.0 Soluble Boron Credit Summary Spent fuel pool soluble boron has been used in this criticality analysis to offset storage rack and j fuel assembly tolerances, calculational uncertainties, uncertainty associated with reactivity l equivalencing (burnup credit and IFBA credit) and the reactivity increase caused by postulated accident conditions. The total soluble boron concentration required to be maintained in the spent  ;

fuel pool is a summation of each of these components. Table 13 on page 52 summarizes the  !

storage configurations and corresponding soluble boron credit requirements. l l

l 1

)

Soluble Boron Credit Summary 37

12.0 Summary of Criticality Results  ;

l For the storage of Westinghouse 17x17 fuel assemblies in the Vogtle Units 1 and 2 spent fuel storage racks, the acceptance criteria for criticality requires the effective neutron multiplication factor, Keg, to be less than 1.0 under maximum feasible conditions with no soluble boron, and less than or equal to 0.95 including uncertainties, tolerances and accident conditions in the presence of spent fuel pool soluble boron. This report shows that the acceptance criteria for criticality is met for the Vogtle Units 1 and 2 spent fuel racks for the storage of Westinghouse 17x17 fuel assemblies under both normal and accident conditions with soluble boron credit and the following storage configurations and enrichment limits:

Unit i Enrichment Limits All Cell Storage Storage of 17x17 fuel assemblies in any cell location. Fuel assemblies must have an initial nominal enrichment no greater

than 2.00 w/o 235 U or satisfy a minimum burnup requirement for i

higher initial enrichments. The soluble boron credit required for this storage configuration is 850 ppm.

3-out of-4 Storage of 17x17 fuel assemblies in a 3-out-of-4 checkerboard Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 2.70 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. A l 3-out-of-4 checkerboard with empty cells means that no more than  ;

3 fuel assemblies can occupy any 2x2 matrix of storage cells. The

soluble boron credit required for this storage configuration is i j 950 ppm.

2 out-of-4 Storage of 17x17 fuel assemblies in a 2-out-of-4 checkerboard 1 i Checkerboard arrangement with empty cells. Fuel assemblies must ! a an Storage initial nominal enrichment no greater than 5.00 w/o 24. A ,

2-out-of-4 checkerboard with empty cells means that no 2 fuel I assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent. The soluble boron credit required for this

, storage configuration is 1100 ppm.

! i l

I 1

Vogtle Uaits 1 and 2 Spent Fuel Racks 38

\

Unit 2 Enrichment Limits l

l All Cell Storage , Storage of 17x17 fuel assemblies in any celllocation. Fuel I assemblies must have an initial nominal enrichment no greater l than 1.82 w/o 235 U or satisfy r 'mimum burnup requirement for l

higher initial enrichments. The soluble boron credit required for i this storage configuration is 750 ppm.

3-out of 4 Storage of 17x17 fuel assemblies in a 3-out-of-4 checkerboard Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 2.54 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. A

3-out-of-4 checkerboard with empty cells means that no more than 3 fuel assemblies can occupy any 2x2 matrix of storage cells. The soluble boron credit required for this storage configuration is 950 pm.

i 2-out-of-4 Storage of 17x17 fuel assemblies in a 2-out-of-4 checkerboard

, Checkerboard arrangement with empty cells. Fuel assemblies must have an Storage initial nominal enrichment no greater than 5.00 w/o 235 U. A 2-out-of-4 checkerboard with empty cells means that no 2 fuel assemblies may be stored face adjacent. Fuel assemblies may be stored corner adjacent. The soluble boron credit required for this l storage configuration is 1250 ppm.

3x3 Checkerboard Storage of Westinghouse 17x17 fuel assemblies with nominal Storage enrichments no greater than 4.00 w/o 235 U (equivalent enrichment with IFBA credit)in the center of a 3x3 checkerboard. The surrounding fuel assemblies must have an initial nominal enrichment no greater than 1.48 w/o 235 U or satisfy a minimum burnup requirement for higher initial enrichments. The soluble boron credit required for this storage configuration is 800 ppm.

The analytical methods employed herein conform with ANSI N18.2-1973, " Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants," Section 5.7 Fuel Handling System; ANSI 57.2-1983, " Design Objectives for LWR Spent Fuel Storage Facilities at Nuclear Power Stations,"

Section 6.4.2; ANSI N16.9-1975, " Validation of Calculational Methods for Nuclear Criticality Safety";

and the NRC Standard Review Plan, Section 9.1.2, " Spent Fuel Storage". This criticality analysis report credit may be taken for the presence of soluble also takes exception to the boron in the spent fuel pool as stated in ANSI 57.2-1983requirement that no reactivity (3) and the W and NRC shows thatposition the effective neutron multiplication factor, Ke g, of the fuel assembly array is less than 1.0 under maximum feasible conditions (no soluble boron) and less than or equal to 0.95 when credit is taken for the presence of spent fuel pool soluble boron.

Vogtle Units 1 and 2 Spent Fuel Racks 39

1 l

i 1

Table 1. Nominal Fuel Parameters Employed in the Criticality Analysis

)

Westinghouse Westinghouse arameter 17x17 STD 17x17 OFA l Number of Fuel Rods per Assembly 264 264 i

Rod Zire Clad O.D. (inch) 0.3740 0.3600 Clad Thickness (inch) 0.0225 0.0225 Fuel Pellet 0.D. (inch) 0.3225 0.3088 Fuel Pellet Density (% of Theoretical) 95 95 Fuel Pellet Dishing Factor (%) 1.2074 1.211 Rod Pitch (inch) 0.496 0.496 i

Number of Zire Guide Tubes 24 24 I Guide Tube O.D. (inch) 0.482 0.474 ,

Guide Tube Thickness (inch) 0.016 0.016 Number of Instrument Tubes 1 1 l

Instrument Tube O.D. (inch) 0.482 0.474 i Instrument Tube Thickness (inch) 0.016 0.016 Vogtle Units I and 2 Spent Fuel Racks 40

Table 2. All Cell Storage Soluble Boron Credit K,g for Vogtle Unit i Nominal KENO.Va Reference Reactivity: 0.88950 Calculational & Methodology Biases:

Methodology (Benchmark) Bias 0.00770 Pool Temperature Bias (50*F - 185'F) 0.00978 TOTAL Bias 0.01748 Tolerances & Uncertainties:

UO2EnrichmentTolerance 0.00754 1

UO2DensityTolerance 0.00375 Fuel Pellet Dishing Variation 0.00195 Cell Inner Diameter 0.00007 i

Cell Pitch 0.03607 Cell Wall Thickness 0.00613 Asymmetric Assembly Position 0.00393 Calculational Uncertainty (95/95) 0.00182 Methodology Bias Uncertainty (95/95) 0.00300 TOTAL Uncertainty (statistical) 0.03796 l

Final K,g Including Uncertainties & Tolerances: 0.94494 Vogtle Units I and 2 Spent Fuel Racks 41

l Table 3. Minimum Burnup Requirements for Vogtle Unit i I

i

'""~"~

Nominal All Cell ec er oard Enrichment Burnup

235 "

(w/o U) (MWD /MTU)

(M D ITU) 2.00 0 0 l 2.20 2647 0 2.40 5185 0 2.60 7622 0 2.70 8806 0 2.80 9967 846 3.00 12229 2524 3.20 14416 4183 3.40 16537 5824 3.60 18600 7445 i

3.80 20614 9048 4.00 22589 10632 4.20 24532 12197 4.40 26453 13744 4.60 28359 15271 4.80 30260 16780 5.00 32165 18270 Vogtle Units I and 2 Spent Fuel Racks 42

Table 4. 3-out-of 4 Checkerboard Soluble Boron Credit K d rfor the Vogtle Unit i Nomina'i KENO-Va Reference Reactivity: 0.90121 Calculational & Methodology Biases:

l Methodology (Benchmark) Bias 0.00770 Pool Temperature Bias (50*F - 185'F) 0.00531 TOTAL Bias 0.01301 Tolerances & Uncertainties:

UO2Enrichment Tolerance 0.00458 UO 2Density Tolerance 0.00329 Fuel Pellet Dishing Variation 0.00192 Cell Inner Diameter 0.00006 Cell Pitch 0.02634 Cell Wall Thickness 0.00518 Asymmetric Assembly Position 0.00453 Calculational Uncertainty (95/95) 0.00200 Methodology Bias Uncertainty (95/95) 0.00300 ,

TOTAL Uncertainty (statistical) 0.02811 Final Ker Including Uncertaintier, & Tolerances: 0.94233 l

1 Vogtle Units 1 and 2 Spent Fuel Racks 43

l 4

Table 5. 2 out-of-4 Checkerboard Soluble Horon Credit K,g for the Vogtle Unit 1 17x17 STD 17x17 OFA Nominal KENO-Va Reference Reactivity: 0.91126 0.92077 Calculational & Methodology Hiases:

Methodology (Benchmark) Bias 0.00770 0.00770 Pool Temperature Bias (50*F - 185'F) 0.00017 0.00008 l TOTAL Bias 0.00787 0.00778 l Tolerances & Uncertainties:

UO 2Enrichment Tolerance 0.00149 0.00156 UO 2Density Tolerance 0.00221 0.00257 Fuel Pellet Dishing Variation 0.00124 0.00148 Cell Inner Diameter 0.00001 0.00005 j Cell Pitch 0.00610 0.00611 Cell Wall Thickness 0.00429 0.00413 Asymmetric Assembly Position 0.00526 0.00084 Calculational Uncertainty (95/95) 0.00246 0.00233 ,

1 Methodology Bias Uncertainty (95/95) 0.00300 0.00300 TOTAL Uncertainty (statistical) 0.01034 0.00899 j i

Final K en Including Uncertainties & Tolerances: 0.92947 0.93754 l

Vogtle Units 1 and 2 Spent Fuel Racks 44

]

1 Table 6. All Cell Storage Soluble Boron Credit K,g for Vogtle Unit 2 i

Nominal' KENO-Va Reference Reactivity

0.91531 Calculational & Methodology Hiases:

Methodology (Benchmark) Bias 0.00770 i Pool Temperature Bias (50*F - 185'F) 0.00920

TOTAL Bias 0.01690 i

Tolerances & Uncertainties:

UO2Enrichment Tolerance 0.00870 UO2Density Tolerance 0.00371 Fuel Pellet Dishing Variation 0.00194 Cell Inner Diameter 0.00100 Cell Pitch 0.00454 Cell Wall Thickness 0.00236 Asymmetric Assembly Position 0.00295 l Calculational Ducertainty (95/95) 0.00175

i j Methodology Bias Uncertainty (95/95) 0.00300 TOTAL Uncertainty (statistical) 0.01188 4

Final K,n Including Uncertainties & Tolerances: 0.94409 s

Vogtle Units 1 and 2 Spent Fuel Racks 45

t Table 7. Minimum Burnup Requirements for Vogtle Unit 2 l

3-out of-4 3x3 Nonu.nal All Cell Checkerboard Checkerboard Enrichjnent Burnup l 2S Burnup Burnup (*)

(w/0 U) (MWD /MTU)

(MWD /MTU) (MWD /MTU) 1.48 0 0 0 ,

1.82 0 0 6912 2.00 2713 0 10201 2.20 5580 0 13603 2.40 8309 0 16774 2.54 10144 0 18877 2.60 10913 619 19752 2.70 12162 1598 21159 j 2.80 13410 2576 22566 3.00 15811 4401 25246 3.20 18130 6135 27815 3.40 20378 7812 30296 3.60 22' 32706 3.80 247L uo~ 35061 4.00 26795 12723 37371 4.20 28852 14361 39645 4.40 30878 16003 41886 4.60 32880 17640 44098 4.80 34859 19256 46276 5.69 36820 20828 48417

(*) Burnup required on surrounding fuel assemblies.

Vogtle Units 1 and 2 Spent Fuel Racks 46

k l

Table 8. 3-out-of-4 Checkerboard Soluble Boron Credit K,g for Vogtle Unit 2 Nominal KENO Va Reference Reactivity: 0.91778 Calculational & Methodology Biases:

l Methodology (Benchmark) Bias 0.00770 Pool Temperature Bias (50*F - 185'F) 0.00514 TOTAL Bias 0.01284 Tolerances & Uncertainties:

UO 2Enrichment Tolerance 0.00508 UO2Density Tolerance 0.00330 Fuel Pellet Dishing Variation 0.00192 l

Cell Inner Diameter 0.00100 1 Cell Pitch 0.00338 Cell Wall Thickness 0.00187

)

Asymmetric Assembly Position 0.00395 Calculational Uncertainty (95/95) 0.00200 I Methodology Bias Uncertainty (95/95) 0.00300 TOTAL Uncertainty (statistical) 0.00921 Final K,g Including Uncertainties & Tolerances: 0.93983 I

i Vogtle Units I and 2 Spent Fuel Racks 47

l Table 9. 2-out-of-4 Checkerboard Soluble Boron Credit K,g for Vogtle Unit 2 17x17 STD 17x17 OFA Nominal KENO-Va Reference Reactivity: 0.92588 0.93412 Calculational & Methodology Biases:

! Methodology (Benchmark) Bias 0.00770 0.00770 Pool Temperature Bias (50*F - 185'F) 0.00046 0.00019 TOTAL Bias 0.00816 0.00789 Tolerances & Uncertainties:

UO2EnrichmentTolerance 0.00148 0.00146 (J0 2Density Tolerance 0.00232 0.00242 Fuel Pellet Dishing Variation 0.00134 0.00135 Cell Inner Diameter 0.00004 0.00100 Cell Pitch 0.00077 0.00078 Cell Wall Thickness 0.00154 0.00150 l Asymmetric Assembly Position 0.00091 0.00261 Calculational Uncertainty (95/95) 0.00244 0.00238 Methodology Bias Uncertainty (95/95) 0.00300 0.00300 TOTAL Uncertainty (statistical) 0.00530 0.00593 l

Final K en Including Uncertainties & Tolerances: 0.93934 0.94794 Vogtle Units 1 and 2 Spent Fuel Racks 48

l 1

Table 10. 3x3 Checkerboard Soluble Boron Credit K,g for Vogtle Unit 2 Nominil KENO-Va Reference Reactivity: 0.91902 Calculational & Methodology Hiases:

Methodology (Benchmark) Bias 0.00770 Pool Temperature Bias (50*F - 185*F) 0.00781 TOTAL Bias 0.01551 Tolerances & Uncertainties:

UO2EnrichmentTolerance 0.01113 UO2Density Tolerance 0.00406 >

Fuel Pellet Dishing Variation 0.00213 Cell Inner Diameter 0.00100 Cell Pitch 0.00420 Cell Wall Thickness 0.00207 Asymmetric Assembly Position 0.00618 Calculational Uncertainty (95/95) 0.00183 Methodology Bias Uncertainty (95/95) 0.00300 TOTAL Uncertainty (statistical) 0.01478 Final K,g Including Uncertainties & Tolerances: 0.94931 n

I i

l Vogtle Units I and 2 Spent Fuel Racks 49 1

4 Table 11. 3x3 Checkerboard Minimum IFBA Requirement for Vogtle Unit 2 Nominal IFBA IFBA Enrichment Requirement Requirement I

(w/o 235 0) 1.0X 1.5X 4.00 0 0 4.20 10 7 4.40 19 13 4.60 27 18 4.80 36 24 5.00 44 30 l

T l

l Vogtle Units 1 and 2 Spent Fuel Racks 50

Table 12. Postulated Accident Summary for Vogtle Units 1 and 2 Applicable ReactMty Reactidly Soluble Boron Storage Fuel Increase Caused Increase Caused

'I Required for Configuration Assembly

' a ed

'[, ,

Accidents Accident (AK) Accident (AK)

Unit 1

• STD or All Cells p 0.00520 0.05566 300 3-out-of-4 17x17 STD or 0.00249 0.07096 450 Checkerboard OFA 2-out-of-4 17x17 STD 0.0 0.12519 1000 Checkerboard 17x17 OFA 0.0 0.11487 850 Unit 2

'* I All Cells 0.00490 0.05552 300 3-out-of-4 17x17 STD or 0.00119 0.08260 500 Checkerboard OFA 2-out-of-4 17x17 STD 0.0 0.14380 1200 Checkerboard 17x17 OFA 0.0 0.13726 1050 O.00445 0.05861 350 Checkerboard OFA l

l l

l Vogtle Units 1 and 2 Spent Fuel Racks 51

l l

i i

Table 13. Summary of Soluble Boron Credit Requirements for Vogtle Units I and 2 l Applicable Soluble Horon Soluble Boron Required for Soluble Boron Total Soluble !

Storage Fuel Required for Tolerances /

Required for Horon Credit Conligurat,oni Assembly Reactivity Accidents Required

,IJPe Uncertainties Equivalencing (ppm) (ppm) (ppm) i (ppm)

Unit 1 17x17 STD or All Cells 300 250 300 850 OFA l 4 3-out-of-4 17x17 STD or 300 200 450 950 Checkerboard OFA 1

2-out-of-4 17x17 STD 100 n/a 1000 1100 i Checkerboard 17x17 OFA 100 n/a 850 950 Unit 2 l

• S or All Cells 200 250 300 750 F

3-out-of-4 17x17 STD or 350

~ 700

~ 500 950 i Checkerboard OFA 2-out-of-4 17x17 STD 50 n/a 1200 1250 Checkerboard 17x17 OFA 50 n/a 1050 1100 i l

3x3 17x17 STD or 250 200 350 800 l Checkerboard OFA j Vogtle Units 1 and 2 Spent Fuel Racks 52

• j 4

_ 1 o i 7.l* Borafles l 4 i I ,

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- i / / / / /  %.

(

/2 (0.020 GM-B 10 cm )

i s st i t 1 1 0.020 68 TAPPER d

DETAIL "A" Figure 1. Vogtle Unit 1 Spent Fuel Storage Cell Nominal Dimensions Vogtle Units 1 and 2 Spent Fuel Racks 53

i i

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Figure 2. Vogtle Unit 2 Spent Fuel Storage Cell Nominal Dimensions 1

j Vogtle Units 1 and 2 Spent Fuel Racks 54 4

- . . . - . . - - - _ . . . - - -.- - . = . _ - . _ . - .

)

i

! 1.214" 1.182" 1.151" i

1 i 1.173" 1.162" 1.207" i

i i

i 1.184" 1.171" 1.233" i

d

, Rack Module A-5 3x3 Array with Worst Case Average Water Gaps

1

) o 10.34" _t

! I I I I i i 1.22", I j i < *i i

i I j 8.75" i i I I i I I

! l l l l l l 1 I I I u______________a Reactivity Equivalent Worst Case Cell for Vogtle Unit 2 Figure 3. Vogtle Unit 2 Rack Module A-5 Limiting Water Gaps and Equivalent Cell Vogtle Units I and 2 Spent Fuel Racks 55

35000 ,

4

All Cell Storage /

30000 """" 3-out-of-4 Checkerboard /

i /

i /

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r

/

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! 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Initia 235 U Enrichment (nominal w/o)

Figure 4. Vogtle Unit 1 Hurnup Credit Requirements Vogtle Units 1 and 2 Spent Fuel Racks 56

1 1

1 1

Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z 3-out-of-4 Checkerboard Storage 2-out-of-4 Checkerboard Storage Empty Storage Cell Fuel Assembly in Storage Cell n@g ps -

.knw sa%

l.i:.'.:. $iC/...

4 3x3 Checkerboard Storage Surrounding Fuel Middle Fuel Assembly in Storage Cell d Assembly in Storage Cell l Figure 5. Vogtle Units 1 and 2 Checkerboard Storage Configurations f

Vogtle Units 1 and 2 Spent Fuel Racks 57 l

i e

50000 ,

/

45000 All Cell Storage <

3-out-of-4 Checkerboard ,

3x3 Checkerboard -

40000 i /

i

/ j

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$ 35000 / j

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1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0 Initial 235 U Enrichment (nominal w/o)

Figure 6. Vogtle Unit 2 Burnup Credit Requirement Vogtle Units I and 2 Spent Fuel Racks 58

l 48 4

/

40 /

ACCEPTABLE /

/

[

x 32

/

h 8 / /

M 1.0X IFBA Loadina / /

k24 1.5X IFBA Loading / /

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Figure 7. Vogtle Unit 2 3x3 Checkerboard IFBA Requirement i

Vogtle Units 1 and 2 Spent Fuel Racks 59

0.00 x ,

t W

-3(s, v

,, All Cell Storage -

-0'05 'i. s 3-out-of-4 Checkerboard -

k N 2-out-of-4 Checkerboard

\ , s i's. s I g T

N

-0.10 ,, '

s g ,

( , s

( ,, x

{ '(

g

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'n 'g c -0.15 ,

x e ,,

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x g -0.20

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N X

X '

3 ,

-0.25 N s ,,

N

X N -

0.30 hs '

N

'N ,

N ,

N

-0.35 0 250 500 750 1000 1250 1500 1750 2000 Soluble Boron Concentration (ppm)

Figure 8. Vogtle Unit 1 Soluble Boron Worth Vogtle Units 1 and 2 Spent Fuel Racks 60

[_ .

i 1

0.00 r ,

3s

I

!

• All Cell Storage

_P((

-0.05 , s 3-out-of-4 Checkerboard -

Q, -x 2-out-of-4 Checkerboard i O> -

3x3 Checkerboard x ..

, x s

,) '

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^

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w g -

-0.30 N

\ 'Y' ,cyl N

N '

Nm

-0.35 ,

-0.40 0 250 500 750 1000 1250 1500 1750 2000 Soluble Boron Concentration (ppm)

Figure 9. Vogtle Unit 2 Soluble Boron Worth Vogtle Units 1 and 2 Spent Fuel Racks 61

4 Bibliography

1. Newmyer, W.D., Westinghouse Spent Fuel Rack Criticality Analysis Methodology, WCAP-14416, June' 1995.

2. Turner, Stanley E., Criticality Safety Evaluation of the Vogtle Plant Spent Fuel Storage Racks With As-Built Water Gaps, HI-88250, December 1988.

3. American Nuclear Society, American National StandardDesign Requirementsfor Light Water j Reactor Spent Fuel Storage Facilities at Nuclear Power Plants, ANSIIANS-57.2-1983,0cto- i ber 7,1983.

4. Nuclear Regulatory Commission, Letter to All Power Reactor Licensees from B. K. Grimes,  !

OT Positionfor Review and Acceptance of Spent Fuel Storage and Handling Applications, '

April 14,1978.

5. Davidson, S.L., et al, VANTAGE 5 Fuel Assembly Reference Core Report, Addendum I, WCAP-10444-P-A, March 1986.

l Bibliography 62