ML19256E601

Forwards Fire Protection Analyses of Alternate Methods of Spent Fuel Pool Cooling,In Response to Requirements of NRC 790906 Fire Protection Safety Evaluation

ML19256E601
Person / Time
Site:	Prairie Island
Issue date:	10/30/1979
From:	Mayer L NORTHERN STATES POWER CO.
To:	Office of Nuclear Reactor Regulation
References
TAC-11094, TAC-11095, NUDOCS 7911080126
Download: ML19256E601 (3)

Text

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NSM8 NORTHERN 5TATES POWER COMPANY M IN N E A PO L.I S. M IN N E S OTA 5 5401 October 30, 1979 Director of Nuclear Reactor Regulation U S Nuclear Regulatory Commission Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket No. 50-282 License No. OPR-42 50-306 DPR-60 Fire Protection Analyses-Alternate Methods of Spent Fuel Pool Cooling Attached for your review is an analyses of alternate methods for spent fuel pool cooling for the Prairie Island Nuclear Generating Plant.

The attached analysis satisfies the requirements of Section 3.2.4 of the NRC Fire Trotection Safety Evaluation for the Prairie Island Nuclear Generating Plant dated September 6, 1979 This report summarizes the methods available for cooling the spent fuel pool in the utilikely event that fire damage makes.both spent fuel cooling pumps inoperable.

Please contact us if you have ary questions related to this analysis.

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Manager of Nuclear Support Services LOM/DNH/jh cc:

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l October 30, 1979 Fire Protection Analysis - Alternate Methods of Spent Fuel Pool Cooling

System Description

The Prairie Island Nuclear Generating Plant spent fuel pool cooling system consists of two 1300 gpm pumps (1100 gpm with both pumps in opgration) and two heat exchangers. Each heat exchanger is rated at 7 9 x 10 btu /hr with 2200 gpm of 120 F pool water and 1800 gpm of 95 F component cooling water flowing through it.

The system is designed to maintain acceptable spent fuel pool tempera'.ures while sustaining a side variety of component malfunctions.

Spent fuel pool cooling pumps (elevation 735) and heat exchangers (elevation 71.) are located in Fire Area 4.

As noted in the Prairie Island Fire Hazards Analysie and Supplement, this fire area is classified as having a very light fire loading.

The area is protected by fire detectors (three zones), eight hose stations, and eight extinguishers. Periodic inspections of the area are made by the shift operating organization. No components essential for safe hot or cold shutdown are located in Fire Area 4.

Method of Analysis The method employed in this analysis is to assume complete failure of all spent fuel pool cooling components. In view of the fire loading in the area where they are located and the fire protection equipment available, this is an e-tremely unlikely event.

Pool Heatup Rate As noted in The NRC Safety Evaluation Relating to Modification to the Spent Fuel Pool, dated April 15, 1977, the worst case failure of the spent fuel pool cooling system will occur just af tar transferring a full core to the small pool (pool number one) which just fills all storage locations.

It will take about four hours to heat the water in the small pool to the boiling poin:.

Four hours is sufficient time to open the fuel pool transfer slots to the larger pool (pool number 2) if they were not already open. With communica-tion established between the two pools, about ten hours will be required before boiling begins. Ten hours is a sufficient period of time in which to make repairs or find a source of cooling water for the spent fuel pools to replace the water which is vaporized.

Under conditions whera a full core has not been disenarged to the spent fuel pool, and when fuel in the pools has decayed for at least several weeks, longer periods of time (rp to several days) are available.

This will be the usual operating condition of the plant.

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e Alternate Sources of Makeup Water On loss of the spent fuel pool cooling system, the pools will gradually heat up co saturation temperature and heat will be transferred by water vaporizing from the pool surface. Local boiling wil occur at the upper end of the stored fuel assemblies. Maximum clad temperature has been calculated to be less than 260 F and no fuel limits are exceeded.

It is assumed loss of the spent fuel pool cooling system occurs under worst case conditions. As discussed earlier, this~ occurs with all storage locations filled (including a full core which was just trgnsferred).

Maximum decay heat load under these conditions is 17 x 10 btu /hr. The nr.keup rate to replace water vaporized from the pool surfaces is 37 gpm.

The following sources of water can be used to supply at least 37 gpm:

1) Reactor makeup water storage tanks through the reactor makeup pumps.

2) Boric acid storage tanks through the boric acid transfer pumps.

3) Refueling water storage tanks through the refueling water purification pumps.

4) Chemical and volume control system holdup tanks through the recirculation pumps.

5) Demineralized water system through the reactor makeup pumps.

6) A fire hose (a hose station is located within 25 feet of the southwest corner of the spent fuel pool).

All of these sources provide the required makeup water. This makeup can be provided indefinitely.

Time available to reach saturation as well as required makeup flow rate have also been calculated for a planned expansion of the capacity of the fuel pools.

The results of the calculations are similar to those reported above for the current storage capacity.

Conclusion In the event of a fire which makes the spent fuel pool cooling system inoperable, at least ten hours are available to make repairs. Under most conditions, significantly more time would be available.

If repairs cannot bc completed within this period or auxiliary cooling equipment cannot be installed, pool heatup to saturation conditions will occur. With boiling in the pools, acceptable cooling of the stored fuel is assured and many sources of makeup are available.

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