This study provides an inter-unit common cause failure (CCF) analysis method utilizing the international CCF database. The inter-unit CCF is one of the possible major risks at a site with multiple units. The conventional CCF analysis is based on the CCF database, in which inter-unit CCF events rarely occur in real nuclear power plants. The conventional approach cannot be directly applied to inter-unit CCF cases because of the lack of data. The method ignores the asymmetricity among units, so it may lead to overly conservative outcomes if it is applied to inter-unit CCF. We have proposed a new concept, “commonality factor,” which represents the degree of similarity among components in different units and showed a concept that the inter-unit CCF probability can be expressed in terms of the intra-unit CCF probability with the commonality factor. The commonality factor is approximated based on CCF coupling factors, which tie two or more failures together as a CCF. This study explains how to estimate the commonality factor by analyzing the conventional CCF database and judging commonality among components from different units. The proposed method is demonstrated through two preliminary examples.

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