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
.