Design basis flooding at a site is determined by evaluating all potential causes that produce the maximum flood elevation. The flood hazard elements are: local intense precipitation, probable maximum precipitation in the site drainage area, stream flooding, surges due to hurricanes, seiche, tsunami, dam failure effects, landslide induced flooding, ice blockage, and the worst flooding at a site that may result from a reasonable combination of individual flooding mechanisms.

Hurricanes and tsunamis are complex natural phenomena and they can pose challenges to plant safety due to concomitant effects. Hurricanes can cause flooding due to precipitation and surge with concomitant loss of off-site power. Similarly, tsunami waves can inundate a site and create impact due to floating debris, loss of off-site power, etc. A tsunami induced flooding and seismic ground motion that precedes it, like that occurred in March 11, 2011 event in Japan requires considerations of both seismic and tsunami effects on the plant and emergency response.

Regulatory Guide 1.59, “Design Basis Floods for Nuclear Power Plants” dated 8. 1977 [8] has provisions for flooding evaluations at nuclear power plant sites. RG 1.59 is currently being revised. RG 1.102, “Flood Protection for Nuclear Power Plants,” dated September 1976 [9] has provisions for flood protections including those for design of protective structures for plants that need exterior flood barriers to protect plant safety.

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