ML20086P036

Forwards Response to NRC 911101 Request for Addl Info Re Criticality Aspects of single-failure-proof Crane Mods, Provided in Util 911004 License Amend Request

ML20086P036
Person / Time
Site:	Prairie Island
Issue date:	12/16/1991
From:	Parker T NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9112260153
Download: ML20086P036 (4)

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- Northem States Power Company l

414 Nicollet Mall Minneapolis. Minnesota 55401-1927 Telephone (612) 330-5500 December 16, 1991 U S Nuclear Regulatory Commission Attn: Decument Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Response to Request For Additional Information Related to Criticality Aspects of the Sincle-Failure-Proof Crane Urinrade License Amendment Reauest The attached information is provided in response to the November 1,1991 NRC letter which transmitted NRC Staff questions related to the criticality aspects of the single-failure proof crane modifications provided in our License Amendment Request dated October 4, 1991.

Please contact us if you have any questions related to the attached responses.

&i ^ 'u Thomas M Parker Manager Naclear Support Services c: Regional Administrator - Region III, NRC Senior Resident Inspector, NRC NRR Project Manager, NRC J E Silberg

Attachment:

Response to Request For Additional Information Related to Criticality Aspects of the Single-Failure-Proof Crane Upgrade License Amend;aent Request 11_1 m11 m ,

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PRAIRIE ISLAND NUCLEAR GENERATING PLANT Response to Request For Additional Information 1 ,

Related to Criticality Aspects of the Sincle-Failure-Proof Crane Uncrade License 6mpndment Reouest Ouestign li The criticality analysis for the flooded Transnuclear 1H-40 spent fuel storage cask assumes that the cask contains water borated to 1800 ppm to meet the K-eff no greater than 0.95 limit. However, the surveillance requirement to verify that 1800 ppm of boron exists while a spent fuel cask containing fuel is located in the spent fuel pool has been increased from daily to weekly. Explain why a weekly boron surveillance is sufficient to enable an inadvertent boron dilution event in the spent fuel pool to be-terminated before the 0.95 limit could be exceeded.

Responre:

The' current Prairie Island Technical Specifications specify that the spent fuel pool boron concentration be sampled monthly. The daily boron sampling

-requirements in Table TS.4.1 2B of the Prairie Island Technical Specifications are actually requirements for sampling the recctor coolant system boron concentration during refueling. The reactor coolant system boron

. concentration is sampled daily during refueling to ensure that adequate shutdown margin is alwaysimaintained during refueling operations.

In order-to keep the cask surface contamination to a minimum, the time the cask spends immersed in the pool will be minimized. It is expected, based on experience with storage and shipping' casks at other locations, that the cask

. vill be removed from the pool the same day that it is placed into the pool.

With this type of operating procedure, daily boron concencration surveillance would not result in any increased sampling over'a weekly surveillance.

A weekly _ boron concentration surveillance is sufficient to ensure the 0.95 k,g requirement during an inadvertent dilution event because of the pool monitoring instrumentation which would detect any' spent fuel--pool dilution event before any significant dilution of the boron concentration would occur.

The pool low level alarm is at 39.00 feet'above the floor of the pool, the high level slarm is at 40.16 feet 'above the floor of the pool. Assuming that spent fuel pool water level starts at the low level alarm and 1800 ppm, the dilution would be detected at 41.166~ feet which would result in a dilution to:

1803 ppm x 40.16 ft - 1748 ppm 39.00 ft This small dilution would have an insignificant effect on the spent fuel pool k Additional assurance that a boron dilution event could be detected bYlo.re having a significant effect on spent fuel pool k,g is provided by the normal spent fuel pool boron concentration which is always maintained significantly above 1800 ppm.

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• Attrchment USNRC 4 - Deceenber 16, 1991 Page 2 of 3

- The criticality. analysis for the TN-40 spent fuel storage cask did not take

- credit for the burnup of the spent fuel stored in the cask, it conservatively assumed the cask contained 40 fresh unexposed fuel assemblies. A cask containing 40 spent fuel assemblies with substantial burnup will reach 0.95 k,,, at a substantially lower boron concentration. This worild provide additional margin to criticality during a dilution event.

- Ouestion 2:

The Fuel Handling TS 5.6.A as well as the Refueling and Fuel Handling TS Bases state that criticality analysis has shown that the use of a three-out of-four storage configuration will assure that the k-eff will remain

'less than 0.95, including uncertainties, when fuel with a maximum enrichment of 4.25 weight percent U 235 and average assembly burnup of less than 5,000 MWD /MTU is stored in the spent fuel pool. If this criticality analysis also assumed moderation by unborated water, perhaps this should also be stated in these specifications.

Resnonse:

The spent fuel rack criticality analysis did assume an unborated water condition. As written, the bases for these specifications do not detail all of the assumptions used in that analysis. A complete description of the anslysis may be found in the NRC Staff Safety Evaluation for License Amendment

- Nos. 90 and 83 to the Frairie Island operating licenses transmitted by letter dated August 8, 1989.

Because the details of the spent fuel rack criticality analysis are well documented in other docketed correspondence, we don't believe it is necessary to incorporate.further details into the Prairie Island Technical L Specifications at this time.

l Ouestion 3:

The maximum enrichment of fuel allowed to be stored in the spent fuel

_ pool is 4.25 weight percent U-235. If 4- 25 weight percent fuel can be L

stored in the. spent fuel pool , why were the casks only analyzed for'3.85 weight percent fuel?

Restonse-L j- The license amendment which allowed storage of 4.25 weight percent fuc1 in the spent fuel pool was'only recently granted (License Amendment Nos. 90 and 83,.

August 8,.1989.). The first 4.25 weight percent fuel was utilized in Unit 2 Cycle 1-4 which was loaded in October 1990. There are approximately 1300 spent fuel assemblies et Prairie Island that meet the 3.85 weight percent criteria.

The storage of those 3.85 weight percent spent fuel assemblies in the TN-40 i spent fuel storage casks will allow for more than fifteen years of additional l plant operation.

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At tachtrent USN2C

. December 16, 1991

.- Page 3 of 3 After that time, if the Department of Energy has not yet begun to accept fuel at a monitored retrievable storage facility or a repository, the TN 40 cask would have to be re-evaluated and licensed for higher enrichment fuel, or another storage cask.with higher allowable enrichment would have to be utilized.

-The decision to license a cask designed for 3.85 weight percent fuel and not to pay for additional flexibility to store fuel with enrichments we are just beginning to utilize was strictly an economic decision on the part of NSP.

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