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Transition of the mission success probability under severe accident conditions: analysis by the GO-FLOW methodology and the consideration of uncertainty   MATSUOKA Takeshi1,2   1. College of Nuclear Science and Technology, Harbin Engineering University, Harbin, 150001, China 2. Center for Fundamental Education, Utsunomiya University, Utsunomiya City, Tochigi Prefecture, 321-8505, Japan  (mats@cc.utsunomiya-u.ac.jp)   Abstract: In case of a nuclear power plant accident, the plant configuration drastically changes and required missions also change. The GO-FLOW is applied to estimate mission success probabilities of nuclear power plant system under severe accident conditions. A hypothetical sequence of accident conditions has been settled based on the Fukushima Daiichi accident. Effects of loop structures in nuclear power plant system and common cause failures are considered in reliability analysis. The situation of "phased mission problem" is considered for the operation change from reactor core isolation cooling system (RCIC) to high-pressure coolant injection (HPCI)  and during the change of water sources from water tank to sea water for core injection by fire protection pump. Uncertainty analysis is performed and the results are expressed in graphical style with uncertainty ranges. It corresponds to a display window of risk monitor system, which visualizes risk state intuitively. Discussions are given for uncertainty ranges which are important information for judging the safety level of a system. Keyword: uncertainty analysis; loop structure; phased mission problem; common cause failure; GO-FLOW
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