ML20151Y466

Safety Evaluation Supporting Removal of Seismic Restraints & Qualification of Existing Spent Fuel Racks as free-standing Racks,Per 880624 Application for Amend to License

ML20151Y466
Person / Time
Site:	Millstone
Issue date:	08/22/1988
From:	Office of Nuclear Reactor Regulation
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ML20151Y462	List: ML20151Y459 ML20151Y466
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NUDOCS 8808260398
Download: ML20151Y466 (4)

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. n, g UNITED STATES l

o NUCLEAR REGULATORY COMMISSION l , h

f W ASHINGTON, D. C. 20S55

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1 SAFETY EVALUATION BY THE OFFICE _0F NUCLEAR RE_A_CTOR RE,GULATION

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1 MILLSTONE NUCLEAR POWER STAT _ ION _, UNIT NO. 1 l

DOCKET N0. 50-245 1

SPENT FUEL _P0OL EXPANSION l l

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1.0 INTRODUCTION

l By letter dated June 24, 1988, NortheastNuclearEnergyCompany(NMECO) reotested an amendment to the Millstone Nuclear Power Station Unit No.1, Technical Specifications which would specify the maximum capacity of the spent fuel storage pool. The capacity of the spent fuel pool is 2184 l fuel assemblies, with 1732 assemblies currently being stored. NNEC0 has l proposed to expand the capacity of the spent fuel pool to 3229 fuel l assemblies, plus 20 defective fuel containers. In order to accomplish '

this, NNECC has proposed removing the seismic restraints on the existing 4 spent fuel storage racks to make room for 10 new storage racks. This  !

expanded capacity will allow the licensee to have a full core offload l capability until approximately 1999. This safety evaluation addresses the first phase of spent fuel pool expansion, the removal of the seismic i restraints as detailed in the June 24, 1988 submittal. Additional l information provided by letters dated July 29 and August 12, 1988 and during meetings on June 16, July 7 and August 1,1988 was also consifiered l in the conclusions reached in this SER. The remaining areas of reviilw (structural analysis, adequacy of new racks, themal-hydraulic considerations and critcality) will be addressed in separate safety evaluations.

2.0 EVALUATION The licensee had qualified the existing spent fuel racks as part of six supenrodules braced against the walls of the pool and against each other, so that deformation of the racks needed not be considered. The evaluation considered the seismic forces resulting from the postulated earthquake at the site utilizing the response spectrum method of seismic analysis. Due to the proposed removal of the seismic restraints, each of n the subject spent fuel rack supemodules has been reanalyzed as single g rigid rack supported by a maximum of twenty-four supports.

The rack modules, identified in NNECO Report "Millstone Unit 1, Spent 8n

• O Fuel Pool Modifications", of July 1976, were assumed to be free standing and subject to a three-dfinensional time history. This evaluation utilized Q

co the DYNARACK computer code,

$o The analysis for the supermodule rack having a cross section of 143"x146" Q@

o with a height of 164" and containing 440 fuel assemblies, each having a weight of 643 pounds, incorporated the hydrodynamic forces. The analysis

%o- accounted for the appropriate rack to rack and rack to wall spacings.

Simulation for rack to rack and rack to wall impacts, were performed by the licensee for two controlling coefficients of friction of 0.8 and 0.2.

1 The 440 cell supenrodule is the largest of the rack supermodules.

It produces the maximum rattling mass inertia and the maximum stress levels. The licensee has documented the resulting stresses and displacenents for the racks designated as A and D2, and a contingency rack. These evaluations have considered several partial and full fuel loads for the racks. The licensee has identified the maximum load, stress and displacement values, even though they may not occur at the same instant of time.

The results of the evaluation of the fully loaded supermodule indicate that the raximum movement occurs at the top of the supermodule and is 0.45". Consideration of impact between the racks was dismissed based on '

the fact that the 0.45" displacement is less than the 2.0" minimum spacing between the top of the racks. The spacing between the racks and the spent fuel pool walls is much larger. The licensee has also evaluated the impact of the fuel assemblies to cells. The licensee has verified, at staff's request, that the resulting 1600 pound load between the fuel assembly and its cell produce stresses below the allowable for the material. NNECO has indicated that the grid straps have been evaluated for the 1600 pound load and that the allowable material properties are not exceeded.

The inter-rack impact was simulated at the baseplate level by establishing the proper op element between the individual racks that make-up each super rack module. ine results indicate that the maximum load at the contact points between racks at the base plate level is 20,000 pounds. The evaluation of each structural component affected by this force indicates that their material strength far exceed the impact force.

The licensee has developed numerous time histcries in all three directions of excitation corresponding to discrete slab mode locations of the spent fuel pool. The time history near the center of spent fuel pccl slab was found to be the most severe and it was used to perform the ncn-linear rack dynamic analyses. The various time histories were derived directly from the applied ground motions to the building structure. These time histories were not developed from the pcol slab response spectra, and therefore, do not have the limitations of non-uniqueness associated with the synthetic time histories that are produced directly from repsone spectra.

This approach affects the prediction of displacements of the support legs, by slightly underpredicting them. However based on the large margins between the available rack to rack gap and the actual displacements, it is considered inconsequential. Also, the licensee dces not include the bending and shear spring eierents in the fuel assembly model. This condition was justified by the licensee utilizing results of studies of similar rack assemblies. The numerical results indicate that the stresses and displacerents are changed by less than 10%. The small effects and the uncertainty of whether the fuel assemblies should be considered as channelled or unchannelled storage, make the modelling assumption for the fuel as lumped masses, acceptable.

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The licensee has provided the applicable s'.te specific spectra utilized in a previous evaluation at Millstone 1 and the resulting time histories at the ground level and at elevation 65.75 feet, which is the alevation of the spent fuel pool floor. The staff has determined that the peak accelerations identified in these graphs agree with those utilized in the evaluation of the spent fuel rack modules, thus, resolving satisfactorily this staff concern.

The results are presented in terTr.s of stress ratios (R) of actual stress to allowable stress. There are six stress ratio categories, as follows:

R Ratio of direct tensile or compressive stress on a net section to its 3 = allowable value R2 = Ratio of gross shear on a net section to its allowable value R.,

= Ratio of maximum bending stress due to bending about the y-axis to its allowable value for the section P4 = Ratio of maximum bending stress due to bending about the y-axis to its allowable value.

RS = Combined flexure and compressive factor R6 = Combined flexure and tension (or compression) factor

• The results were reviewed to determine if they meet the acceptance requirements of less than one for nortnal condition and less than two for severe cccident conditions which include the seismic safe shutdown earthquake (SSE) condition. The highest stress ratio for all simulations occurs for R . Its value is 1.063 for the female (upper) support leg er.d 0.373forthhrackbase. Also, the results indicate that no impacts occur with either adjacent racks or with adjacent walls. The maximum corner displacements in either direction for the SSE event are 0.2249" and 0.4380" for the two horizontal directions. The gap element is 2.0".

The licensee has addressed the effects of the horizontal forces by the supports on the liner for the critical location and loading. NNECO has detertained that this condition occurs for the new racks only, at intersection of the new racks D-1, D-2, D-3 and E. This evaluation considered the therTr.al effects of 212*F and the reduced liner material stress allowable of 90% of the yield value. The results indicate that liner integrity is maintained with adequate margins.

Finally, the licensee has addressed our concern regarding the measures taken by the licensee to assure that all of the support leg loads of the various rack modules are transmitted to the spent fuel pool floor and that they exert an even distribution. NNECO has stated that leg location and height adjustrent will be part of the QA for Category I Plant Operation Review Comittee procedures. Also plumbing of the rack modules and assurance of leg-floor contact during installation will assure proper load distribution to the spent fuel pool floor.

1T.3 The staff has visited tAe site and has discussed these assumptions, rodels, and available results developed by NNEC0 and their consultants.

The results were found by the staff to ir,eet the acceptance requirements for the first phase of the spent fuel pool modifications, the removal of the seismic restraints and the utilization of the existing rack modules as free-standing racks.

3.0 C0NCLUSION, Based on the evaluation of the licensee submittal, the supplementary infonration provided by the licensee, discussions with the licensee at

' meaMngs and during the site visit, the staff concludes that the 1./.nsee's structural analyses of the existing spent fuel racks are in corcpliance with the acceptance criteria set forth in the FSAR and consistent with current licensing practice and therefore, are acceptcble.

Principal Contributor- F. Rinaldi Dated: August 22, 1988