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Fig. 4-3 Methodology for seismic probabilistic safety assessment developed at JAERI The computer code SHEAT calculates the seismic hazard (exceedance frequency of earthquakes expressed as a function of seismic motion intensity) at a site taking into account the location, magnitude, and frequency of seismic sources and the attenuation of seismic waves generated from such sources. The SECOM-2 code calculates the failure probability of each component from its response and capacity. It then uses these probilities to evaluate the failure probability of systems and the frequency of core damage accidents caused by earthquakes.

The Kobe earthquake evoked public attention to the safety of nuclear facilities during seismic events. Nuclear facilities in Japan are designed to assure a sufficient level of safety during earthquakes. However, if the level of safety can be quantitatively evaluated, optimization and cost effective enhancement of seismic design and operation management would become feasible. In pursuit of this application methodology, the development of seismic risk assessment (seismic probabilistic safety assessment (seismic PSA)) is ongoing. A seismic PSA assesses the level of safety in terms of the frequency of a seismically induced core-damage accident by considering the earthquake motions that exceed the design basis. Since Japan is a country of high seismicity, it has rigorous design standards and an abundant accumulation of seismological data. It also has performed large-scale proving tests on equipment to demonstrate reliability under seismic loads. JAERI has developed a set of procedures and computer codes for seismic PSA that is suitable for incorporating such information in Japan (Fig. 4-3). A pilot application of the methodology to a BWR indicated that the loss of off-site power transient with failure of various components of safety systems would have relatively higher contributions to the risk than other scenarios. Such results demonstrated the usefulness of the methodology for understanding the safety margin of the current design and for identifying important factors for further reduction of seismic risk. The development and application of seismic PSA methodology will be continued to contribute to more rational and optimal design and operation of nuclear facilities. Examples of expected applications include the determination of design basis earthquakes and examination of the safety impact of new siting technologies.

Reference K. Muramatsu et al., Development of Seismic PSA Methodology at JAERI, The 3rd Int. Conf. on Nuclear Engineering (ICONE-3), Kyoto (1995).

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