The issue of using the most recent NOAA procedures for determining probable maximum precipitation (PMP) was raised[1] after an OL applicant disputed the NRC use of NOAA Hydrometeorological Report (HMR) Nos.
51[2] and 52,[3] published in June 1978 and August 1982, respectively. The PMP values are used in estimating design flood levels at reactor sites. It was the contention of the applicant that the use of HMR-52, which in general results in higher flood levels than those obtained using earlier reports, was inappropriate and constituted
an unauthorized backfit under NRC procedures. HMR-51[4] issued by NOAA in June 1978 expanded the
information previously presented in HMR-33[5] (cited in SRP[6] Section 2.4.2). This expansion extends the precipitation duration from 48 to 72 hours and increases the drainage areas from 1,000 to 20,000 square miles.
In addition to other provisions, HMR-52[7] provides techniques for analyzing PMP for drainage areas of 1 square mile and durations of 1 hour and less.
GDC-2 requires that design bases for floods reflect consideration of the most severe historical data with sufficient margin for the limited accuracy, quantity, and period of time in which data have been accumulated.
Guidance on what constitutes sufficient margin is contained in Regulatory Guides 1.59[8] and 1.102.[9] These documents state that the appropriate design basis for precipitation-induced flooding is the probable maximum flood (PMF) as developed by the U.S. Army Corps of Engineers. This PMF criterion has been used by NRC since 1970. Thus, in the case of floods, the PMF is the criterion that has been used to meet GDC-2.
Procedures for estimating PMFs are given in Appendices A and B of Regulatory Guide 1.59[10] (Appendix A has since been superseded by ANSI N170-1976). ANSI N170-1976 defines PMF as a hypothetical flood
that is considered to be the most severe reasonably possible, based on comprehensive hydrometeorological application of PMP and other hydrologic factors favorable for maximum flood runoff. Thus, PMP is an integral component of PMF determination. Section 5.2 of ANSI N170-1976 states that PMP estimates for the U.S. are available in generalized studies prepared by the National Weather Service (NWS); these estimates are presented in varying degrees of completeness. Specific PMP estimates for areas not adequately covered by these studies may be made by using techniques similar to those employed by NWS.
Recognizing the importance of using the most recent engineering technology in evaluating the potential impacts on reactor site safety, SRP[11] Section 2.4.2 was written to allow "...improvements in calculational methods..." With the publication of HMR-51[12] and HMR-52,[13] OL applicants were requested by the staff to assess the effects of their use on plant safety.
Improper drainage at reactor sites during heavy rainfalls can lead to flooding that can render safety-related equipment inoperable.
In all cases reviewed by the staff against HMR-51 and HMR-52, the issue has been resolved by the applicants taking the following actions: (1) site drainage has been designed to handle the increased design basis precipitation; (2) commitments were made to develop procedures to assure that critical entrances to buildings
will be closed; and (3) curbs were installed at critical entrances.[14] In order to clarify the staff's position and
remove ambiguities from the SRP,[15] it will be necessary to revise SRP[16] Sections 2.4.2 and 2.4.3. This solution will be a forward-fit and will incorporate the most recent technical advances for determining PMP that are known at the time that the SRP revision is made. Future technical advances in the determination of PMP will also require revisions to the SRP.
No quantitative analysis
of the issue was made because work on the final solution
was underway at the time the
issue was evaluated.
During the course of resolving this issue, the staff suspended[17] routine requests of NTOL applicants to review their site flooding assessments under the updated NWS guidelines of HMR-51 and HMR-52. After CRGR
review,[18] SRP[19] Sections 2.4.2 and 2.4.3 were revised in 1989 to incorporate the PMP procedures and criteria
contained in the latest NWS publications.[20] In addition, Generic Letter 89-22[21] was also issued to inform OLs and CPs of the resolution of the issue. Thus, this issue was RESOLVED and new requirements were
established.[22]
[1] Memorandum for W. Johnston from R. Ballard, "Disputed Procedures for Estimating Probable Maximum Precipitation," January 13, 1984. [8401260466]
[2] Hydrometeorological Report No. 51, "Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, June 1978.
[3] Hydrometeorological Report No. 52, "Application of Probable Maximum Precipitation EstimatesUnited States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, August 1982.
[4] Hydrometeorological Report No. 51, "Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, June 1978.
[5] Hydrometeorological Report No. 33, "Seasonal Variation of the Probable Maximum Precipitation East of the 105th Meridian for Areas from 10 to 1,000 Square Miles and Durations of 6, 12, 24 and 48 Hours," U.S. Department of Commerce, April 1956.
[6] NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981.
[7] Hydrometeorological Report No. 52, "Application of Probable Maximum Precipitation EstimatesUnited States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, August 1982.
[8] Regulatory Guide 1.59, "Design Basis Floods for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (Rev. 2) August 1977. [7907100225]
[9] Regulatory Guide 1.102, "Flood Protection for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (Rev. 1) September 1976. [7907100372]
[10] Regulatory Guide 1.59, "Design Basis Floods for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (Rev. 2) August 1977. [7907100225]
[11] NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981.
[12] Hydrometeorological Report No. 51, "Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, June 1978.
[13] Hydrometeorological Report No. 52, "Application of Probable Maximum Precipitation EstimatesUnited States East of the 105th Meridian," U.S. Department of Commerce, National Oceanic and Atmospheric Administration, August 1982.
[14] Memorandum for V. Stello from H. Denton, "Potential Generic Requirement Concerning Design for Probable Maximum Precipitation," June 25, 1984. [8407100105]
[15] NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981.
[16] NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981.
[17] Memorandum for V. Stello from H. Denton, "Generic Requirements Regarding Design for Probable Maximum Precipitation," October 10, 1984. [8503140522, 8410190029]
[18] Memorandum for H. Denton from V. Stello, "Generic Requirements Regarding Design for Probable Maximum Precipitation," August 8, 1984. [8408160442]
[19] NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants," U.S. Nuclear Regulatory Commission, (1st Ed.) November 1975, (2nd Ed.) March 1980, (3rd Ed.) July 1981.
[20] Federal Register Notice 54 FR 31268, "`Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants'; Issuance and Availability Revised SRP Sections 2.4.2 and 2.4.3," July 27, 1989.
[21] Letter to All Licensees of Operating Reactors and Holders of Construction Permits from U.S. Nuclear Regulatory Commission, "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. [8910180273]
[22] Memorandum for J. Taylor from E. Beckjord, "Close- out of Generic Safety Issue No. 103, `Design for Probable Maximum Precipitation,'" November 28, 1989. [8912180025]
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