A phenomena identification ranking table (PIRT) was constructed to identify important phenomena that should be considered in simulating the accident progression for the Fukushima Dai-ichi nuclear power station and predicting the molten core and debris distribution in the reactor system with the MAAP code. The accident scenario, starting from reactor scram to PCV failure, was divided into four time phases in which fuel rod degradation, molten core relocation, and reactor vessel failure were selected as the phase-changing events. Phenomena that would have occurred in the accident were identified in the plant system including the reactor vessel and the containment that consist of 16 sub-components, consistent with the MAAP nodalization for the BWR system. The importance of the identified phenomena was evaluated for each time phase in cooperation with the experts in the Atomic Energy Society of Japan. Eventually total of 1047 phenomena were identified, of which 386 were ranked as ‘highly’ important for the analysis code to evaluate the behavior of molten fuel rods and reactor internal materials. These important phenomena are compared with those considered in the current MAAP analysis models to make sure what should be enhanced to improve the analysis capability for the accident progression.

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