Development of Dependence Indexes for Multi-Unit Risk Assessment and its Estimation Using Copula

Since the Fukushima nuclear accident, the importance of evaluating risks for multiple units has been emphasized. With this motivation, surrogate metrics such as the core damage frequency (CDF) have been revised to represent those of a site, for instance, the site CDF (SCDF). While SCDF has been proposed to assess the risk of multiple units, it is desirable to provide an additional metric how much dependencies are contributed to the site. The conditional probability of multi-unit accident (CPMA) has also been proposed to describe dependency, however it handles a specific unit, not a site. Therefore, in this paper, the site CPMA and the site dependence index (SDI) are developed to describe inter-unit dependencies. The site CPMA is defined to apply to a site and uses the concept of CPMA. The SDI is derived from the concepts of banking stability used for financial linkages. As an alternative option, this paper also proposes a way to approximately estimate the proposed indexes using copulas and two-unit models for n-units in a site. The SDIs for four units at a site were evaluated using the copula-based model and the fully developed four-unit MUPSA model. The results from each model were comparable.