Various types of reported seismic margins and their uses

Nuclear power plant structures and safety-related systems have been generally designed conservatively for a safe shutdown earthquake (SSE) and more conservatively for a smaller operating basis earthquake (OBE). Depending upon the relative conservatism of the design criteria, either the SSE or the OBE will control the design. In recent years, increasing knowledge in the geoscience field has led to a better understanding that, although highly unlikely, it is possible for the nuclear power plant to be subjected to earthquake ground motion greater than the ground motion for which the plant was designed. For this reason, interest has developed in demonstrating that nuclear plant structures and safety-related systems can safely withstand earthquake ground motion larger than their design earthquake ground motions (SSE and OBE). Within this paper, this larger-than-design earthquake ground motion will be called the seismic margin earthquake (SME) to distinguish it from the design earthquakes. The goal is to determine the performance of already-designed structures, components, and systems when subjected to the SME. Different and generally more liberal criteria should be used when evaluating the performance of structures, components and systems for the SME than were used in design. Retrofit, and redesign, should only be contemplated if one cannot show a seismic margin greater than unity for the SME using these more liberal criteria. In other words, the SME is not a design earthquake. It is not a replacement for the SSE and generally has nothing to do with design. The SME is a performance-check earthquake