A methodology for assessment of nuclear power plant seismic margin: Final report

In recent years, increasing knowledge in the geoscience field has led to a better understanding that, although highly unlikely , it is possible for a nuclear power plant to be subjected to earthquake ground motion greater than that for which the plant was designed. There is a concern that there may be an upper shelf of seismic capacity, primarily for active equipment, which would render components inoperable if exceeded. For this reason, interest has developed in defining the margin above the Safe Shutdown Earthquake (SSE) that exists in operating plants. While defining this margin is a desirable goal, it is much more practical to select an earthquake level for which survivability is to be demonstrated for only those systems and components which are required to bring the plant to, and maintain, a safe shutdown condition following the seismic event. The earthquake level for which survivability is to be demonstrated is commonly referred to as the Seismic Margin Earthquake (SMW). This is not a new design earthquake. It is an earthquake for which existing plants may be required to demonstrate survivability. The principal objective of this research project was to define such new, practical approaches and criteria that are, for themore » most part, usable by utility staff to assess their plant for a seismic event larger than the SSE. A further objective was to develop approaches and criteria that are compatible with those being developed to resolve other generic safety issues.« less