Abstract: The severe accident progression within first 4 days for Fukushima Daiichi NPP unit 2 (1F2) was simulated with application of MELCOR code. Detailed modeling and system response of the whole plant were made to achieve it. The simulated results were compared with those monitored. The simulation results based on assumption agree well with those measured. When coupled with RCIC system, the TORUS room, which was initially flooded to half height of TORUS outside, acted as an additional external heat sink, could effectively remove reactor decay heat, as well as counteract the PCV pressure rise. Within 96 h, the PCV pressure did not reach the opening set point of containment filter and venting system. The RCIC system became ineffective about three days after the accident occurred. For 4.6 h after RCIC failure, operator started to depressurize the reactor system by manually stuck-opening of SRV. Nevertheless, the core was almost total uncovered when fire water began to be pumped into reactor, intense Zr-water reaction took place as a result. Within 6 h, nearly 800 kg of H₂ was generated and released from the core. While the reactor core channel still kept its coolable geometry in the late phase of accident. Finally, when second SRV was opened to depressurize the reactor to more extent, and allowed higher flowrate of fire water to be pumped into reactor, the reactor was reflooded and cooled down by the end of 4 days.