"Defense in Depth" is the basic philosophy for nuclear safety. Earthquakes are stochastic events and seismic probabilistic safety assessment (PSA) methodology has been much progressed. The strategy should be used for seismic safety. This paper investigates a risk-informed approach to attain optimum allocation of defense-in-depth measures to ensure adequate seismic safety of nuclear power plants. The safety-related structures, systems and components (SSC) are first grouped into categories by the characteristics of accident sequences, and ranges of required capacity of the SCC are determined in terms of High Confidence Low Failure Probability (HCLFP) according to the categories. Seismic PSA were conducted for a typical PWR plants to refine the strategy and to assure the adequacy, aiming at safety goals and appropriate risk balance among accident sequences. This approach will provide a guide to an effective seismic design of nuclear power plants.
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