ML20053D270

Safety Evaluation Supporting Proposed Expansion of Spent Fuel Storage Facility

ML20053D270
Person / Time
Site:	Dresden
Issue date:	05/26/1982
From:	Office of Nuclear Reactor Regulation
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ML20053D263	List: ML20053D262 ML20053D270
References
NUDOCS 8206040250
Download: ML20053D270 (3)

Text

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ENCLOSURE DRESDEN NUCLEAR POWER PLANT UNITS 2 AND 3 PROPOSED EXPANSION OF SPENT FUEL STORAGE FACILITIES 1

j SUPPLEMENTARY SAFETY EVALUATIOf. PCPCRT Commonwealth Edison Company, the licensee, originally proposed to increase the storage capacity of the spent fuel pools at Dresden 2 and 3.

This proposal involved replacing the existing spent fuel racks with 33 high-density poisoned racks. A Safety Evaluation Report was issued on June 6, 1980, the essential conclusion of which was that the proposal was acceptable to the staff. The

~~ 5EP'Brant.h review of TOPIC IX-1 for Dresden 2 revealed thtt the seismic analysis of the new rack installation conducted by Commonwealth Edison did not adequately address several issues. Subsequently, aLL but one issue was resolved and the Licensee was allowed to install five new racks in the Dresden 2 pool in order to support a refueling outage.

The.. single issue which remained unresolved was that it was not adequately demonst' rated that the spent fuel pool floor could withstand the loads, including

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impact, which would be imparted snould aLL 33 racks rock (tip) during a seismic event and fall back sihvitaneously. This supplemen'tary safety evaluation addresses that issue.

On October 2, 1981, Cormonwealth' Edison submitted to the staff a report entitled

" Evaluation of the Effects of Postulated Rocking of Racks on Spent Fuel Pool Structures of Dresden Station Units 2 and 3."

In that report the results of a non-linear time history analyr,'s of the potential effects of a seismic event, includirg the modeling of rack impact on the pool floor due to tipping, were presented.

The development of the pool floor motion time-histdry~used in this evaluation consisted of the following s_teps:

1.

The building structural model developed by Lawrence Livermore Labor & tory in NUREG/CR-0891. (Seismic Review of Dresden 2 for SEP) was modified for theas-builtbondition.

s 2.

USNRC Regulatory Guide 1.60 resnonse spectrum scaled to O Jg was selected as the basis for the input motbr at tne ground Level.' A synthetic tie.e7 history catching this response spectrum was developed.

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, 3.

A time-history response analysis was performed using the building model (Item "1" above) and the synthetic time-history (Item "2" above). A seven percent building structural damping was used per. USNRC Regulatory Guide 1.61.

4.

A response spectrum at the spent fuel pool floor level was developed using "

a two percent equipment damping. The use of two percent damping is con-servative since Regulatory Guide 1.61 recommends four percent damping, and NUREG/CR-0098 reccemends five to seven percent damping.

5.

The floor response spectrum developed as outlined in Item "4" above was smoothened and peak-broadened by i 15 percent to account for building

- modeling and response uncertainties.

6.

A synthetic time-history was developed matching the peak-broadened floor response spectrum (Item "5" above). The peak acceleration for the syn-thetic time-history is about 20 percent higher than the actually computed peak acceleration. This provided additional conservatism in the input motion.

The" mathematical model of the rack /poot floor used in the anal) sis accounted for the non-linear effects of fuel bundle " rattling" within the cells and of rack tipping on the pool floor. The results of the analysis indicated that the pool floor structure was well within its capacity under simultaneous loading from 33 racks. On December 14r 1981, the Licensee sub'mitted additional argu-ments and responses to previous staff questions which supported his choice of-seismic loadir,g and hethod of analysis. It was found that these presentations demonstrated suitable conservativism in the Licensee's analysis.

The staff has had a non-linear analysis of the pool and rack system for Dresden 2 performed by an independent consultant. His findings indicate that the sper.t fuel pool structure is conservatively capable of withstanding the postulated loads imposed by the full 33 rack installation.

(G. Harsted; Report dated 5/19/82.)

The staff has also made a comparison of the proposed Quad Cities spent fuel pool expansion seismic analysis with that used for Dresden 2.

Both plants are MKI BWR's founded on Yock in'the same tectonic province. Although the Quad Cities plant is arranged somewhat differently than at Dr'e'sd'en, the spent

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fuel pool structures are identical in dimensions and very similar in construc-tion. The seismic event postulated for Quad Cities was a 0.24g maximum event

, based on the 1957 Golden Gate Park earthquake time history. The floor response spectra at the Quad Cities pool floo'r was developed from this event and used for construction of the plant. The Latest seismic event used for the Dresden

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analysis is as described previously.

Although comparisons are somewhat complex and judgemental, it was considered that useful conclusions could be made regarding the adequacy of the proposed 33 rack Dresden installation by studying the results of the analysis performed for Quad cities. The Quad Cities 2 pool structure was found to be conservatively capable of withstanding the Loads imposed by 3970 fuel modules which wiLL be

" ~arringed in 20 racks. The Dresden pools wiLL each hold 3537 storage Locations arranged in 33 racks. Since the Quad Cities structures were found to the acceptable, it is considered that this provides some additional assurance,in the form of yet another analysis that the Dresden spent fuel pool structures 3

are adequately constructed to withstand the loads which are postulated for their seismic event under full 33 rack Load.

Acc rdingly, we find that the spent fuel pool expansion as proposed by the Licensee for a full 33 rack installation is acceptable and satisfies the appli-cable requirements of the General Design Criteria 2, 4, 61, and 62 of Appendix A to Part 50 of 10 CFR.

e see l