抄録
Internal events as well as external events as initiators that may entail potential consequence have to be considered when we discuss the comprehensive risk of Nuclear Power Plant (NPP). Two or more NPPs that are located at a multi-unit site or close to each other could fail simultaneously at seismic event as one of external events, depending on the degree of seismic ground motion and its impact to units of the site. The seismometers at the Kashiwazaki-Kariwa NPPs recorded different time-histories of ground motion due mainly to the folding structure beneath the free field rock surface and to the installation condition of the embedded buildings, when the Niigata-ken Chuetsu-oki earthquake in July, 2007, impacted on the Site significantly. This suggested the different input ground motions on all plants at the site. Then the method is being enhanced to be applicable even if different input ground motions to respective plants at the site may happen to be generated. The interface of seismic hazard evaluation method, structure, system and component (SSC) fragility evaluation method, and accident sequence evaluation method were carefully and systematically considered. The enhanced method was preliminary applied to a hypothetical site where a highly-aseismatic and a lesser-aseismatic designed plant were located, and the applicability of the developed method to LWRs was discussed, applying typical correlation factor to the analysis model. Some insights on accident sequences for both plants failure and for either plant failure ("plant failure" is defined as "serious damage to the reactor core" in this paper.) were obtained from the analysis results. The developed method is robust to delineate the risk profile of multiple NPPs close to each other, being impacted by a strong seismic motion.
