Development of Seismic PSA Methodology Considering Aftershock

When a large earthquake (main shock) has occurred around nuclear power plant, it is generally followed by a number of large aftershocks. Therefore, the safety of power plant against the main shock and aftershocks should be assessed. We have been developed the following seismic PSA methodology for nuclear power plant considering large aftershocks: 1) Seismic hazard analysis of aftershocks: It is possible to predict the aftershock activity by combining the evaluating the number of aftershock following main shock and the frequency of aftershock of a certain magnitude. The seismic motions due to aftershocks are evaluated by attenuation formula or fault model considering the spatial distribution of aftershocks in the aftershock region. 2) Seismic fragility evaluation: Structures and components fragility can be evaluated by considering seismic motions due to main shock and aftershocks that will occur within a short period of time. 3) System reliability analysis: The system reliability analysis of a nuclear power plant is taken into account for additional accidents by aftershocks while the reactor is establish cold shutdown after scram by a main shock. This paper is organized in two sections. In the first section, the seismic PSA methodology for aftershock is explained. In the second section, an example of the seismic hazard analysis for aftershock based on earthquake observation records is presented. In this analysis the prediction model of aftershock activity were verified and the influence of the seismic hazard of the aftershock was also examined by comparing with seismic hazard of the main shock. (authors)