ESTIMATION OF THE 1F2 CORE THERMAL STATUS DURING THE CORE HEAT-UP PHASE WITH MELCOR-2.2

For decommissioning of the Unit 2 of Fukushima Daiichi Nuclear Power Station (1F2), more supportive information is necessary regarding the debris distributions and characteristics as complementary to what has been gained through internal investigations. The current debris status is expected to have been greatly affected by the peak core energy (temperature) during the core heat-up phase, before collapsing (slumping) to the lower plenum of the reactor pressure vessel (RPV), at the time of the accident. In another word, whether the oxidic fuel melted in the core region or not. Such supportive information can be gained through detailed analyses of the plant data and the corresponding accident progression analyses. In this study, accident progression analyses of 1F2 were performed considering uncertainty of the core oxidation and the impact of the leakage from the RPV to the D/W on the estimated core energy with the severe accident analysis code MELCOR-2.2. The best estimate case successfully reproduced RPV and D/W pressure histories and showed that the peak core temperature was about 2700 K. This indicated that large-scale melting of the oxidic fuel in the core region was unlikely in 1F2, although some local oxidic fuel melting may have taken place.