Development of probabilistic risk assessment methodology against strong wind for sodium-cooled fast reactors

For nuclear power plants, probabilistic risk assessment (PRA) should be performed not only against earthquake and tsunami, which are critical events especially in Japan, but also other external hazards such as strong wind. The aim of the present study is to develop a practical PRA methodology for sodium-cooled fast reactors (SFRs) against strong wind, paying attention to the final heat sink, ambient air, that removes decay heat under accident conditions. First, this study used Gumbel distributions to estimate hazard curves of the strong wind based on weather data recorded in Japan. Second, it identified important structures, systems and components (SSCs) for decay heat removal, and developed an event tree that results in core damage, focusing on the impacts of missiles (e.g., steel pipes) caused by strong wind. It also identified missiles that can reach SSCs at elevated places, and calculated the fragility of the SSCs against the missiles as a product of two probabilities. One is a probability of the missiles that would enter an inlet or outlet of the decay heat removal system, and another is a probability of failure caused by missile impacts. Finally, it quantified conditional decay heat removal failure probabilities by introducing the fragilities into the event tree. The core damage frequency (CDF) was estimated at about 5 x 10-10/y. The dominant sequence is that strong wind causes offsite power loss and missiles, the missiles penetrate the diesel fuel tank, cause a fire, and the fire increases air temperature around the reactor building where air cooler inlets of decay heat removal systems are installed, leads to loss of power for the diesel generator for forced circulation cooling, resulting in loss of decay heat removal. Through the above, this study has developed the practical PRA methodology for SFRs against strong wind.