ML20055F908

Safety Evaluation Accepting Criticality Analysis for Cycle 5 Fuel in Spent Fuel Storage Racks

ML20055F908
Person / Time
Site:	Grand Gulf
Issue date:	07/16/1990
From:	Office of Nuclear Reactor Regulation
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ML20055F906	List: ML20055F905 ML20055F908
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NUDOCS 9007190353
Download: ML20055F908 (3)

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c UNITED STATES a,

' r. , NUCLEAR REGULATORY COMMISSION .l WASHINGTON, D. C,20655

\.....lj-s SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO CRITICALITY ANALYSIS FOR CYCLE 5 FUEL l

ENTERGY'0PERATIONS. INC.- '

GRAND GULF NUCLEAR STATION. UNIT 1 DOCKET h0. 50-416 1

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1.0 INTRODUCTION

By'1etters dated April 26,1990and.' June'7,1990,thelicensee(SystemEnergy '

Resources, Inc. prior.to' June.6, 1990 and Entergy Operations, Inc. on or after that date) submitted a criticality analysis for the. storage of Grand Gulf Nuclear.

Station, Unit 1 (GGNS-1) Cycle 5 fuel in the Unit's spent-fuel storage' racks.-

These racks have been designed for storage of 8x8 fuel assemblies with an: average enrichment up.to 3.5 weight percent (w/o) U-235. Previous criticality safety

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analyses-performed for Cycle 4 reload fuel also have demonstrated that 8x8, assemblies with enrichments of 3.61 w/o and 9x9 Lead Test Assemblies.(LTAs) with enrichments storage racks.

of 3.47 w/o are acceptable for storage in-the GGNS-1 spent fuel ..

The new analysis addresses the storage of two additional 9x9 fuel assembly types-(ANF .L4L low gadolinia fuel and ANF-1.4H high gadolinia fuel) in the

-spent fuel storage racks. These assemblies are scheduled to-be-loaded into the reactor for GGNS-1 Cycle 5. Plant operation using the new fuel will'be demonstrated to.be acceptable by a cycle specific; reload safety. evaluation performed prior to fuel loading. The following'is the staff's evaluation of-the fuel storage criticality analysis; 2.0 EVALUATION L The spent fuel racks at GGNSiare a high density array of- storage cells -

I containing the fixed neutron absorber Boraflex.- The wall between adjacent cells is 0.070 t-0.007 inch thick Boraflex with 0.063 1 0.006 inch thick stainless steel cladding. on both sides. - The minimum boron-10 loading in the Boraflex is 0.0190 gm/sq cm.

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The Cycle 5 reload will:be the first full 9x9 reload for GGNS-1. This fuel is:~

l similar to the LTA assemblies loaded in Cycle 4. The assemblies contain-r~

- naturally enriched reflector regions at the top and bottom and a 3.80 w/o enriched central. region. These assemblies also contain axially distributed gadolinia.  !

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2 The analysis uses KENO-Va, a three-dimensional Monte Carlo theory computer code for reactivity calculations. However, since KENO-Va does not have the i

capability to deplete fuel assemblies, the CASMO-3G integral transport theory I code was used for burnup-dependent reactivity calculations as well as for the reactivity effects of various dimensional and material composition tolerances.

l Both codes have been extensively benchmarked, including experimental data with l boron poison sheets in arrays of bundles, and were found to adequately reproduce experimental measurements.

are acceptable. The staff concludes that these methods and models-The design basis for preventing criticality outside the reactor is that, including uncertainties, there is a 95 percent  !

confidence level (95/35 probability / confidence) that probability at a 95 multiplication the effective percent l factor (k-eff) of the fuel assembly array will be no greater than 0.95. The '

licensee has analyzed the reactivity of the spent fuel storage racks taking into account the effects of both gadolinia and burnup. The fuel was depleted using CASMO-3G to determine the maximum reactivity of the assembly over its lifetime.

The storage array k-eff was then determined assuming an infinite array of cells with all fuel at its: peak reactivity during its lifetime.

The effect of dimensional and material tolerances cn the storage rack k-eff was determined.

These tolerances include enrichment fuel pellet density, fuel pellet dish, fuel pellet diameter, Boraflex thickness, s,tainless steel thickness, storage cell pitch, channel bulge, and gadolinia enrichment.

Uncertainties due to the method were combined with these uncertainties to calculate the 95/95 upper limit on the calculated k-eff. The treatment of these uncertainties is consistent with staff guidance and is acceptable.

In addition, the analysis included Boraflex gap effects. All gaps were assumed to be 6 inches, based on an assumed maximum shrinkage of. 4% -of the panel length, in the most recent measurements of gap sizes made on the GGNS Boraflex panels by neutron attenuation (blackness testing), the maximum gap size was found to be l

2.0 inches. Therefore, the staff considers the assumption of 6-inch gaps to be adeouately conservative.

A probability distribution of the number of gaps per cell'was established and i

the gaps were assumed to be randomly distributed within the central.50% of the panel length if more than one gap per cell occurred.

cell was assumed to have the gap at the vertical centerline.At least one panel Although the in each staff has not rigorously evaluated the assumptions concerning the probability of

(. occurrence of gaps, they appear to be ri.asonably conservative. Also, the GGNS Boraflex testing program for Cycle 5 and future cycles includes Boraflex gap measurements using blackness testing. 'Through these measurements, the licensee will the confirm safety analysis. the validity of the probability of occurrence of gaps assumed in Based on these measurement confirmations and on the conservative assumption of gap size, the staff finds the effects of Boraflex

gaps on the reactivity of the spent fuel storage racks has been adequately considered at this time.

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, 3 The criticality safety analysis resulted in a rack k-eff of 0.9452, including -

biases and uncertainties at the 95/95- probability / confidence level. This meets the staff criterion of no greater than 0.95 and is acceptable.

3.0 CONCLUSION

The staff has reviewed the criticality analysis of the GGNS spent fuel storage racks to account for the changes in the fuel design to be used for Cycle 5.

The analysis included the effects of gadolinia distribution and fuel burnup as well as Boraflex gap formation. Based on this review. and on the Boraflex gap measurements included in the GGNS Boraflex testing program, the staff concludes that GGNS spent fuel storage racks can safely accommodate the ANF-1.4L and ANF-1.4H fuel.

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