Keywords:severe accident of a nuclear reactor, decommissioning of Fukushima-Daiichi NPP, MAAP, lower plenum debris
MAAP analysis was carried out based on the latest knowledge obtained through the 1F plant internal survey, plant data analysis, and computer model simulations. This study provides best-estimate evaluations for the Units 2 and 3 covering the core melting, core material relocation to the lower plenum, and relocated debris cooling behavior in the lower plenum. The fuel temperature history and the metal oxidation behavior during this period are provided, as well as the estimated composition of the materials relocated to the lower plenum. In this part, the overview and evaluated results for the Unit 2 are presented.