State of SAMPSON code development for the analysis of Fukushima Daiichi severe accident

Severe accident computer codes historically treat the accident with coarse correlations in order to connect main variables to major events, resulting in a fast running code to perform a large number of simulations and sensitivity analyses. After the three core melt-down at the Fukushima power station it was acknowledged that the complex phenomena involved into a long station black-out must be studied in more detail, in order to provide meticulous information of accident progression to increase plant reliability and operator's resilience in the unlike case that the accident occurs. SAMPSON in this direction is a severe accident computer code developed at the Institute of Applied Energy with a mechanistic approach to treat every phenomenon with deeper accuracy compared to historical codes. SAMPSON, due to the very different time and length scales between phenomena composing the accident, is composed of hierarchical modules to account for the diverse physics involved in the various phases of the accident evolution. The paper shows SAMPSON first phase of analysis of Fukushima Daiichi units 1, 2 and 3, in which the code has proven not only realistic agreement with measurement data but has also provided important information for scenario reconstruction and given fundamental support in the decommissioning phase required by the Japanese government.