A09150, Responds to 901115 Request for Addl Info Re Boraflex Degradation in Spent Fuel Pool Storage Racks,Per . Preliminary Results Indicate That 45 Panels Had Gap in Boraflex Matl

Responds to 901115 Request for Addl Info Re Boraflex Degradation in Spent Fuel Pool Storage Racks,Per . Preliminary Results Indicate That 45 Panels Had Gap in Boraflex Matl

ML20066C677
Person / Time
Site:	Millstone
Issue date:	01/04/1991
From:	Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
A09150, A9150, TAC-77726, NUDOCS 9101100414
Download: ML20066C677 (3)

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January 4, 1991

.l Docket No. 50-336 I A09150 Re: Boraflex Degradation U.S. Nuclear Regulatory Commission.

Attention: Document Control Desk

-Washington, DC 20555-Gentlemen:-

Millstone Nuclear Power Station,-Unit No. 2 Request for Additional Information Boraflex Dearadation in Soent Fuel Pool Storace Racks (TAC No. 77726)

In a letter -dated October 1, 1990,U) Northeast Nuclear Energy- Company (NNECO) _ submitted information to the NRC Staff regarding Millstone . Unit No. 2's Boraflex degradation in the spent--fuel o001 storage racks.- In ,

reviewing letter datedthis information,

November the S%f requested adcitional information in a 15, 1990.

The following are responses to the '

questions raised by the Staff. .

NRC Ouestion #1

' Preliminary results from the blackness tests indicate that '45 panels had= a gap- in the Boraflex material :with the largest- single measured gap

.approximately 1.8 inches- wide. What was the total accumulated gamma radiation -to these panels at the time .of l the - blackness tests?

What additional gamma dose will be accumulated by the-Boraflex panels before the next blackness testing and what additional shrinkage (gap size) could this cause?

_(1) E. J. Mroczka letter to U.S, Nuclear Regulatory Commission, " Millstone i Unit No. 2, Spent Fuel. Racks Boraflex Degradation," dated October 1,- 0 1990.

.(2)

G. S. Vissing -letter to E. J. Mroczka, " Request for Additional Information concerning Boraflex Degradation in Millstone 2 Spent Fuel Pool Storage Racks (TAC No. 77726)," dated November 15, 1990.

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January _4, 1991 NNECO Response The total accumulated gamma radiation to the panels containing - gaps was between 5.9 x 108 and 2.3 x 1020 rads gamma, depending on the service of the rack cell with the gaps. ,

The spent. fuel racks 1have recently experienced the discharge associated with end of Cycle 10 spent fuel. A conservative method of estimating the additional exposure is the average value at 1.1 x 107 rads / day for 1 year storage. NNECO is still evaluating the frequency criterion associated with the follow up blackness testing program.

NNECO contends that little or no further increase in gap sizes to the tested cell locations are expected. This is due to the fact that Boraflex shrinkage- significantly. reduces and/or ceases to occur at gamma saturation levels beyond 5 x 108 rads gamma. All of the cells tested, wherein gaps were observed, have seen this ' exposure level and beyond; therefore, they are at or approaching the saturation level.

NRC Ouestion #2 It is reported that the results of -the CE criticality analysis confirms that the -K eff'of the spent fuel pool is less than 0.95 for 2.7-inch gaps located ~at the same exial . elevation throughout Region 1 for the fuel assemblies enriched to 4.5 weight percent U Since the present fuel assemblies ~ stored in Region _1 are . arranged 2N.-a two-out of-four storage-pattern- (checkerboard), was this the configuration assumed ~in the CE criticality analysis? How much margin existed between the calculated K-eff and 0.95?: As a result of previous Question 1 above, could gaps larger than the 2.7 inches assumed- in the- criticality analysis occur with further irradiation?-

NNEC0 Response-iThe- CE criticality analysis assumed 4.5 ' w/o U _

fuel arranged in - a 4-out-of-4 storage configuration as originally rev$ed and licensed by the NRC. _ The- only difference _ in the analysis was the- incorporation of the axial gaps.-

The 2.7-inch gap criteria resulted in a K-eff = .95.

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,, it .is possible that larger tha_n 2.7-inch gaps could occur in the untested-locations with -further irradiation. However, of- the 420 panels inspected, only 45 panels contained gaps; 37 of which had gaps less than 1 inch, j 7 had _ gaps ' between 1 inch - l\ inches, and 1 panel contained a 1.8-inch a gap. All of the' gaps encountered .were randomly distributed axially l throughout the panels, j l

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?U.S. Nuclear Regulatory Commission 3 *

-.A09150/Page 3 January 4,-1991 Additionally, the average---gap s'ize' experienced by other nuclear utilities utilizing Boraflex has been less than 1 inch, consistip.t with our results.

The follow-up future blackness testing program will provide the

- confirmation- of gap formation and . size in the untested cells.. However, .

-should additional reanalysis be required, a- redevelopment of the -f

-criticality model to account for the different gap sizes, axial locations and panels;containing the gaps would eliminate the very conservative nature

._- of the current analysis- and- thereby permit incorporation of gaps ~1arger than 2.7 inches should they occur.

Please contact us if you have any-additional questions.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY f we /

E. J. M czka ~'f Senior Vice President ,

cc: T. T. Martin,l Region 1 Administrator G. S.-Vissing, NRC Project Manager, Millstone Unit No. 2 P. Habighorst, Resident--Inspector, Millstone Unit No.-2 ---'

W. J. Raymond, . Senior Resident Inspector, Millstone Unit Nos.1, 2, and :3 --

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