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ISSUE 103: DESIGN FOR PROBABLE MAXIMUM PRECIPITATION (REV. 1)

DESCRIPTION

Historical Background

The issue of using the most recent NOAA procedures for determining probable maximum precipitation (PMP) was raised⁶⁸³ after an OL applicant disputed the NRC use of NOAA Hydrometeorological Report (HMR) Nos. 51⁶⁸⁵ and 52,⁶⁸⁴ 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-51⁶⁸⁵ issued by NOAA in June 1978 expanded the information previously presented in HMR-33⁶⁸⁶ (cited in SRP¹¹ 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-52⁶⁸⁴ 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 1.59⁶⁸⁷ and 1.102.⁶⁸⁸ 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Procedures for estimating PMFs are given in Appendices A and B of Regulatory Guide 1.59⁶⁸⁷ (Appendix A has since been superseded by ANSI N170-1976).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, SRP¹¹ Section 2.4.2 was written to allow "...improvements in calculational methods..." With the publication of HMR-51⁶⁸⁵ and HMR-52,⁶⁸⁴ OL applicants were requested by the staff to assess the effects of their use on plant safety.

Safety Significance

Improper drainage at reactor sites during heavy rainfalls can lead to flooding that can render safety-related equipment inoperable.

Possible Solution

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.⁶⁸⁹ In order to clarify the staff's position and remove ambiguities from the SRP,¹¹ it will be necessary to revise SRP¹¹ 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.

PRIORITY DETERMINATION

No quantiative analysis of the issue was made because work on the final solution was underway at the time the issue was evaluated.

CONCLUSION

During the course of resolving this issue, the staff suspended⁶⁹⁰ 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,⁶⁹¹ SRP¹¹ 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.¹²⁶¹ In addition, Generic Letter 89-22¹²⁶² was also issued to inform OLs and CPs of the resolution of the issue. Thus, this issue was RESOLVED and new requirements were established.¹²⁶³