Seismic safety design and risk assessment

The adequacy and sufficiency of NPP seismic safety design are confirmed by the defense-in-depth safety performance and the risk profile. Seismic safety design and risk assessment are now inseparable for safety assurance of nuclear facilities. Applying the defense-in-depth principle to NPP seismic safety is a difficult issue with respect to the current seismic design philosophy in Japan. The defense-in-depth seismic safety of NPP requires not only resisting seismic events but also assuring safety when safety measures do not function as designed in the seismic events. The current deterministic design practice is not appropriate for the defense-in-depth principle when earthquake shaking demand increases because of new knowledge. Probabilistic risk assessment allows application of the defense-in-depth principle to NPP seismic safety design in these cases. The probability of failure, the maximum conditional probability of failure and the ground motion at the condition derived with the relationship between the realistic seismic load and seismic capacity are the three important indexes for safety design. Performing the safety performance analyses considering maximum conditional probability of failure and the risk assessment complying the probability of failure is essential to satisfy the necessary and sufficiently conditions for the defense-in-depth safety requirements of NPPs. (author)