Abstract: In the paper the simulation of severe accident progression within first 3 days for Fukushima Daiichi NPP unit 3 was performed with application of MELCOR code. The detailed modeling of the whole plant system was made to achieve it. The resulted parameters were compared with those monitored. The major physical phenomena from the accident initiation to reactor core degradation until hydrogen explosion were reproduced in simulation. The simulation results based on assumption as defined agree well with those measured. The results indicate that the reactor water level drops down to top of core active part at 36 h since the earthquake. Operators fail to depressurize the containment and reactor at an earlier time, by the time water injection into reactor through fire pump, the core claddings are already severely oxidized, and ruptures of claddings bring up release of volatile fission product at 40.7 h. Suspended supply of fire pump water (55.5-63.2 h) leads to further degradation and relocation of core materials. Upon slumping of debris into lower plenum, more H₂ from Zr-water (or metal-water) reaction releases and accumulates over upper space of reactor building, finally results in H₂ detonation; up to 72 h，around 50% of zircalloy is oxidized, and the lower head of RPV is intact from rupture.