Abstract

During the Fukushima Daiichi and Daini accidents, the interactions of multiple units at the same site made accident mitigation more difficult compared to single unit sites. The accidents revealed important multiunit risk sources that are not identified by risk assessments for single-unit sites. Therefore, it’s important to obtain an integrated risk evaluation for multiunit sites. However, multiunit accident scenarios are difficult to analyze due to the complexity of multiunit interactions. In the work reported here, a framework capable of analyzing multiunit accident scenarios involving inter-unit interactions is presented. Our framework provides a structured method to analyze accident propagation events, which are not being studied much currently. In addition, our framework is capable of providing accident mitigation and site improvement suggestions that can help improve site safety.

The accident scenarios and risk contributors analyzed in our framework are developed based upon our interviews with Tokyo Electric Power Company (TEPCO) engineers concerning their experiences during the 2011 Fukushima accidents. This first-hand information helps us to better understand multiunit accident scenarios and difficulties in multiunit accident mitigations. In this work, the major steps of our framework are first explained by a simplified two-unit site. The simplified site structure is constructed such that the distractions from overly complex systems are minimized. Additionally, analyses of more risk contributors are illustrated using a relatively complex two-unit site, which illustrates the capability of our framework to analyze complex sites.

Even though only a limited number of accident scenarios and risk contributors are illustrated in our work, the capability of the framework goes beyond that. With proper input information, our framework can be adapted to sites and accident scenarios more complex than those illustrated in our work. Analyzing multiunit risks using our framework can help sites to refine expertise and data and to identify hidden multiunit vulnerabilities and eliminate them in advance. In addition, the risk assessment groups developed during this process can support emergency trainings and risk communications as well as provide risk assessment leadership for utilities.