Seismic margin for small-magnitude earthquakes

Seismic margin analysis is currently receiving widespread attention in the reevaluation of existing nuclear power plants. In early 1984 the NRC convened a panel of experts (the Panel) to develop an approach for determining what margin of safety exists against core melt. The primary impetus for developing seismic margin methodology came from the Advisory committee of Reactor Safeguards (ACRS) who has for several years been requesting the NRC staff to determine how much margin exists in nuclear power plants for earthquakes larger than the safe shutdown earthquake (SSE) level. The seismic margin methodologies developed by the Panel and the EPRI Program provide, as a minimum, a list of components for which there is no concern for those earthquake magnitudes that would be considered as a lower-bound magnitude cutoff. In addition, components are identified which represent potential concern. Tables list those components and the concerns for higher level earthquakes which may be damaging. The Panel also believes that the recommendations given are conservative for earthquakes like the Leroy earthquake which occurred in January, 1986. Based on earthquakes like the Leroy event, the author believes that, except for possible relay chatter the concerns listed are not a problem. It is left to the geoscientists to verify that the engineering characteristics (in the sense of causing damage) of other small magnitude earthquakes at the lower-bound cutoff are essentially similar to the Leroy event. If this can be established, then from an engineering perspective, a M5.0 can be used a lower-bound magnitude cutoff