Analysis of accident progression of Fukushima Dai-ichi Unit 1 with SAMPSON severe accident code

The severe accident in the Fukushima Daiichi and Daini Nuclear Power Plant started with the massive earthquake and tsunami of March 11, 2011. Since almost all external power and backup power supplies were lost, cooling of the fuel in the reactor core and spent fuel storage pool of Fukushima Daiichi Units 1 to 4 could not be carried out. In particular, Fukushima Daiichi Unit 1 had fuels rods exposed for a long time, and even now, it is thought to be in the most serious state. The SAMPSON code analyzes a series of severe accident phenomena (steady operation, in vessel phenomena and ex vessel phenomenon) combining 12 independent modules. However, in order to obtain more accurate analysis results, additional improvements of the main models are required. In this paper, we show two topics for simulation studies on the accident progression of Fukushima Daiichi Unit 1. First, the isolation condenser model was made more realistically than before using our model. Second, the pressure boundary damage assumption was made based on the SRM/IRM guide tube damage from the RPV decompression. We show simulation results that support this assumption. (author)