MELCOR modeling and sensitivity analysis of Fukushima Daiichi unit 2 accident considering the latest TEPCO investigations

Fukushima Daiichi accident occurred on 11 March 2011 due to the Great East Japan Earthquake and the following tsunami. Recently, Tokyo Electric Power Company (TEPCO) published investigation reports pertaining to the status of unit 2 and other issues that were confirmed. The present study modeled unit 2 accident with MELCOR 2.1 code and performed sensitivity analysis, in order to provide information towards understanding severe accident. The two-phase flow rate and its void fraction in the steam line connecting to the turbine of reactor core isolation cooling (RCIC) system were calculated using the developed RCIC operation model, and the pump injection rate was obtained as well. The suppression chamber (S/C) was divided into three layers with flow path connection to model thermal stratification and the mixing flow at SRV discharge, thereby enabling to capture the measured dry well (D/W) pressure. Through sensitivity analysis of seawater injection, it indicates that the likely seawater injection rate was about 1-2% of the total flow rate through fire engines, and the corresponding reactor pressure vessel (RPV) lower head failure time was located in 92.84 h- 96.17 h. As a result, about 74% of total fuel debris discharged into the reactor pedestal, and the release fractions of noble gas, iodine and cesium to the environment were about 0.741, 0.0167 and 0.00331, respectively. Even though the plausible accident progression was tentatively given, there are still many uncertainties concerning models, boundary conditions and the accident progression.