Development of risk assessment methodology of decay heat removal function against external hazards for sodium-cooled fast reactors, 2; Probabilistic risk assessment methodology against strong wind

Nishino, Hiroyuki  ; Kurisaka, Kenichi ; Yamano, Hidemasa   

Probabilistic Risk Assessment (PRA) for external events has been recognized as an important safety assessment method after the TEPCO's Fukushima Daiichi Nuclear Power Station accident. The PRA should be performed not only for earthquake and tsunami which are especially key events in Japan, but also other external hazards (e.g. strong wind). The present study is intended to develop the PRA methodology against the strong wind. In this study, the methodology was developed for Sodium-cooled Fast Reactors paying attention to that the ambient air is their final heat sink for removing decay heat under accident conditions. First, this study estimated hazard curves of the strong wind by using Weibull and Gumbel distributions based on weather data recorded in Japan. Second, important structures and components for decay heat removal were identified and an event tree resulting in core damage was developed in terms of wind load and missiles (i.e. steel pipes, boards, cars and trees) caused by strong wind. Finally, conditional decay heat removal failure probabilities were quantified by introducing the fragilities into the event tree. The core damage frequency (CDF) was estimated about 4E-9 per year. A dominant sequence was led by the assumption that the operators could not extinguish tank fire caused by the missile impacts and the fire induced loss of the decay heat removal system. Through the above, this study developed the PRA methodology against the strong wind.