The catastrophic Fukushima nuclear accident reminded the nuclear community about potential extreme accident scenarios, including those involving multiple reactor units on the same site. In response to the Fukushima accident, the nuclear power industry developed and implemented a series of strategies, including Diverse and Flexible Coping Strategies (FLEX), to enhance the capacities of nuclear power plants (NPPs) to cope with extreme accidents. This study examines the impact of FLEX strategies on the overall risk from all reactor units located at the same NPP site, including risks from accidents involving either a single unit or multiple units. The fundamental, methodological element of this study is Multi-Unit Probabilistic Risk Assessment (MUPRA) requiring a shift in Probabilistic Risk Assessment (PRA) from a one-reactor-at-a-time mindset to a consideration of all reactors sharing a site. An integrated modeling approach for multi-unit event sequence development is leveraged to develop the MUPRA model and address intra-unit and inter-unit dependencies. Systems Analysis Programs for Hands-on Integrated Reliability Evaluations (SAPHIRE), a PRA software developed and maintained by Idaho National Laboratory for the United States (U.S.) Nuclear Regulatory Commission, serves as the platform for MUPRA modeling. This study selects loss of offsite power (LOOP) as a representative initiating event potentially occurring on a generic two-unit NPP site and impacting both reactor units. First, an MUPRA model, including multi-unit event trees, is developed to obtain single-unit and multi-unit accident scenarios. Next, different FLEX strategies are assumed; for example, whether FLEX equipment for multiple units can be used in a cross-connected manner. Lastly, the effectiveness of each postulated FLEX strategy is evaluated by incorporating the corresponding FLEX equipment and deployment logic into the MUPRA model.

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