Clarification of key input parameters for site boundary dose due to criticality of fuel debris at the Fukushima Daiichi nuclear power plant

To proceed with the fuel debris removal at the Fukushima Daiichi Nuclear Power Station (1F-NPS), it is important to evaluate the potential impact of unexpected criticality events. The purpose of this study is to clarify parameters that can have a significant impact on the site boundary dose due to the criticality of the fuel debris at 1F-NPS. Criticality, kinetics, and dose calculations were performed using a set of analysis conditions for 1F-NPS. The results showed that the impacts of specific input parameters on the site boundary dose were much larger than others. Even when these high-impact parameters were assumed to be very large or small, the site boundary dose did not exceed about 0.1 mSv under conditions considered in this study. The findings contribute to confirming the effectiveness of criticality countermeasures and to supporting the planning of efficient debris removal. To further improve the generality of the conclusions, additional analyses that consider 1F-NPS-specific criticality scenarios, such as continuous inflow of water around the fuel debris, would be necessary.