ML24197A061
| ML24197A061 | |
| Person / Time | |
|---|---|
| Issue date: | 07/15/2024 |
| From: | Hayes B NRC/NRR/DEX/EXHB |
| To: | |
| References | |
| Download: ML24197A061 (1) | |
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NRC's regulatory approach for LIP flooding hazards and lessons learned in recent licensing and oversight activities Nuclear Energy Agency (NEA) Working Group on External Events (WGEV) - Workshop on Local Intense Precipitation Toronto, Canada September 19, 2024 ABSTRACT Barbara Dean Hayes, PhD, PE Branch Chief, External Hazards Branch Division of Engineering, Office of Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Local intense precipitation as one of several flood causing mechanism considered in US Nuclear Regulatory Commission (NRC) safety reviews of power plant applications for license, certifications and permits under a variety of regulations. NRC develops guidance for applicants and staff safety reviewers which provides efficiency, clarity and consistency. Guidance is maintained, updated, and expanded based on needs and experience. A major revision to guidance occurred in 2007 in preparation for an anticipated increase in new reactor applications.
In 2012, NRC sent an information request to licensees for the fleet of operating nuclear power stations in connection with the 2011 accident at the Fukushima Dai-ichi nuclear power plant site.
Licensees were required to perform flood hazard reevaluations using present-day methods and regulatory guidance. The request required evaluation of eight of the flood-causing mechanisms described in NRCs standard review plan. Fifty-four licensees reported reevaluated flood hazard elevations higher than the design basis flood elevations previously approved in licensing. Those exceedances were due primarily to the local intense precipitation (LIP) flood-causing mechanism.
Upon review, it was learned that the LIP flood-causing mechanisms was not explicitly considered in the original design basis at about a third of the power station sites. Following additional reviews, the NRC staff concluded that each licensee demonstrated that effective flood protection exists for those flood-causing mechanisms that were now estimated to exceed the licensing basis.
In response to the 2012 flood hazard reevaluation findings, the staff has undertaken certain regulatory initiatives to improve its guidance framework bearing on LIP. The staff is in the process of updating Regulatory Guide 1.59 to reflect present-day methodologies used when evaluating intense rainfall due to thunderstorms. The staff is also developing a new chapter to its standard review plan that will be specifically dedicated to the LIP flood-causing mechanism. The staff found that more than 40 percent of the licensee relied on a site-specific probable maximum precipitation estimate as an alternative to the National Weather Services Hydrometeorological Reports for the purposes of their 2012 flood hazard reevaluations. The staff issued a knowledge management NUREG documenting lessons learned in the review of these site-specific probable maximum precipitation estimates.