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Potential for Increased Roof Loads and Plant Area Flood Runoff Depth at Licensed Nuclear Power Plants Due to Recent Change in Probable Maximum Precipitation Criteria Developed by the National Weather Service (Generic Letter 89-22)
October 19, 1989 TO: ALL LICENSEES OF OPERATING REACTORS AND HOLDERS OF CONSTRUCTION PERMITS* SUBJECT: POTENTIAL FOR INCREASED ROOF LOADS AND PLANT AREA FLOOD RUNOFF DEPTH AT LICENSED NUCLEAR POWER PLANTS DUE TO RECENT CHANGE IN PROBABLE MAXIMUM PRECIPITATION CRITERIA DEVELOPED BY THE NATIONAL WEATHER SERVICE (GENERIC LETTER 89-22) This letter is to inform you that the NRC staff has adopted for future plants the latest probable maximum precipitation (PMP) criteria published by the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS) to establish acceptable design configurations for safety-related nuclear power plant facilities. The staff has been using the PMP concept in plant flood design for well over 15 years. The criteria appear in Regulatory Guides, ANSI Standards, and Standard Review Plans (NUREG-0800) and were based primarily on procedures established in the 1940s and 1950s by the U.S. Army Corps of Engineers and National Weather Service (NWS). Since 1977, more recent PMP criteria have been published by the National Oceanic and Atmospheric Administration (NOAA), National Weather Service. These criteria are to be used to evaluate the degree of licensee compliance with the General Design Criterion #2 App A to 10 CFR Part 50. The new criteria are contained in NOAA/NWS Hydrometerological Reports (HMR) No. 49 (1977), No. 51 (1978), No. 52 (1982), No. 53 (1980) and No. 55 (1984). One of the more significant changes in the new HMRs is that they now provide PMP estimates for drainage areas as small as 1 square mile and for durations as small as 5 minutes. The previous reports generally only provided PMP estimates for areas of 10 square miles or greater and durations of 6 hours or more. There were empirical methods to subdivide the 6 hour duration to smaller increments, but there was never any methodology to estimate PMP for areas less than 10 square miles. Thus, the 10 square mile PMP values were used for site and roof drainage design. Current 1 square mile short duration PMP estimates are larger than those previously used. Staff has revised (Revision 3) Sections 2.4.2, "Floods," and 2.4.2, "Probable Maximum Floods (PMF) on Streams and Rivers," of the Standard Review Plan (NUREG-0800) to formally incorporate the latest NWS reports into the licensing 1* Five plants, Byron, Braidwood, Vogtle, South Texas, and River Bend, fully meet the new PMP Criteria and the licensees for these five plants may disregard this letter. 8910180273 . Generic Letter 89-22 - 2 - October 19, 1989 review process. In general, the latest NWS criteria call for higher rainfall intensities over shorter time intervals and smaller areas than have been previously considered. In some cases, such events could result in higher site flooding levels and greater roof ponding loads than have been used in previous design studies. This information is provided for your review and determination of appropriate action, if any. No submittal is required. Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure: List of Most Recently Issued Generic Letters Technical Contact: S. Chan, ESGB/NRR 492-0830 .
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