The severe accident of Fukushima Daiichi occurred on March 11, 2011, which originated from an earthquake and tsunami. There were six units in the Fukushima Daiichi nuclear power station. Units 1, 2 and 3 were operating and Units 4, 5 and 6 were shutdown at that time. It was estimated that partial or full core degradation occurred in all three operating units, according to the results of Phase I of OECD/NEA BSAF Project. Furthermore, it was estimated that a RPV lower head penetration failure might occur in the case of Units 1 and 3. Understanding the situation of the accident and tracking of the current molten corium locations are essential to conduct decontaminations and to control the site situations hereafter. Cooling of the reactor vessel and corium debris is one of the most important works to prevent or mitigate a severe accident. In the case of Units 1 and 2, it was well known that external water injection started after about 15 hours and 80 hours from the reactor scram, respectively. The amount of water reaching the reactor pressure vessel and the start time of the external water injection are crucial factors to remove the decay heat in the vessel and are directly related with the degree of core degradation. In this research, some relationships were observed between the core damage progression and the injection mass flow rate of water or the timing of the injection. Moreover, the actual injection water flow rate and the extent of fission products release were estimated by comparing the measured data (for instance, pressures of reactor vessel and drywell). MELCOR 2.1 was used in this study.