Development of margin assessment methodology of decay heat removal function against external hazards. (4) Event sequence assessment based on continuous Markov chain Monte Carlo method with plant dynamics analysis

Margin assessment of a nuclear power plant against external hazards is one of the most important issues after Fukushima Dai-ichi Nuclear Power Plant Accident. In general, an event tree (ET) method is applied to investigate the margin and the effectiveness of countermeasures against the hazard in which a scenario of the plant status and the timing of the countermeasures are determined and a success or failure probability of each event or countermeasure is assumed mainly based on an engineering judgment. In order to investigate the plant dynamics during the scenario examined in the ET method, a numerical simulation is carried out in accordance to the sequentially-presented events. However, a number of scenarios are depicted potentially in a real situation. In the present paper, a new approach has been developed to assess the plant status during external hazards and countermeasures against them in operation quantitatively and stochastically by taking possible scenarios into account. For this purpose, a Continuous Markov chain Monte Carlo (CMMC) method is applied. Furthermore, a preliminary event sequence assessment has been carried out under the condition of deep snow in a loop type sodium cooled fast reactor. (author)