The Simulation of Fukushima unit 2 Accident with MELCOR based on the boundary conditions from TEPCO

This paper shows the calculation results of the severe accident of the Fukushima Daiichi unit 2 that occurred in March 2011 in Japan. The simulation was performed with MELCOR version 1.8.5 and the available plant data. The input deck was made based on the 'Peach bottom'. However, the exact boundary conditions such as water injection rate, its timing, leak size, and location were still not identified clearly. The first type of boundary conditions proposed from the TEPCO was applied for this first trial calculation. Sensitivity studies were performed for four parameters, 'RCIC injection rate', 'sea water injection rate.', 'leak area' and whether a rupture of the wet-well occurred or not. The calculation scope was covered from the time of the reactor trip to the end of the transient. The results from a sensitivity calculation on the injection rate predicted the some differences in the degree of core damage but the lower vessel head was always predicted as being intact. The effect of the leak rate and a consideration of the wet well rupture showed a large difference on the pressure for the wet well and primary containment