Code to code model comparison analysis of Sampson with Melcor for core degradation progression against Fukushima Daiichi NPP UN-1 accident

The Fukushima Daiichi nuclear power station accident occurred in March 2011 and Unit 1, Unit 2 and Unit 3 are believed to have experienced severe accidents involving core melting due to the loss of cooling capability. The severe accident (SA) code [1] has been used to understand the accident. Each simulation presents different result because of the different physical models and calculation conditions and it makes difficult to interpret the simulation result. The purpose of this study is to investigate which and how the different models affect the simulation results following the approach of the MAAP-MELCOR crosswalk [2]. The analysis presented in this paper was performed using the SAMPSON code to use the same initial conditions in the MAAP-MELCOR crosswalk study which considers the events at the Fukushima Daiichi Unit 1 (1F1) following the tsunami up to the core degradation. During the in-vessel early degradation phase the main parameters and key event timings are compared. Analyzed parameters are the hydrogen generation, the reactor vessel and containment pressure, the water level in the reactor vessel and the fuel cladding temperatures. Similarities can be seen in the timing of the core degradation onset, while differences are seen in the debris relocation progression and peripheral structures degradation timing. These differences are mainly due to the different fuel collapse criteria and debris relocating model in the core region.