In Japan, the Seismic Design Guideline for Nuclear Power Plants revised in 2006 requires that residual risk for earthquakes beyond design base be considered. Moreover, in the Niigata-ken Chuetsu-oki earthquake (NCOE:2007), the earthquake motion exceeded the seismic design condition of the Kashiwazaki-Kariwa nuclear power plants (KK NPPs). In response to these issues, there is a growing demand for quantitative clarification of seismic safety margin. The Japan Nuclear Energy Safety Organization (JNES) started a study on the seismic safety margin. JNES defined the term “seismic safety margin” in this study. The seismic safety margin is based on the probability distribution of seismic response and seismic capacity of equipment. Regarding the seismic capacity, JNES has carried out seismic capacity tests on various types of equipment whose malfunction would significantly affect core damage frequency in terms of seismic probabilistic safety assessment (seismic PSA). Our seismic safety margin is effective to understand the quantitative margin related to the failure of equipment. JNES applied the concept of seismic safety margin to thin wall cylindrical tanks utilizing ultimate strength test results, and compared it with the design margin. This paper reports examples of the seismic margin evaluation of cylindrical tank.

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