Cooling of ex-vessel corium mixture is one of the most important issues of the severe accident for preventing the accident progress and mitigating the accident result. To avoid MCCI and secure the safety from the threat of radioactive material release to environment, the coolability of ex-vessel corium should be sufficiently secured. In the flooded cavity with coolant water, there must be FCI, and following fragmentation of the discharged melt jet. For massive discharge of melt during very short time, there must be corium melt pool which is harder to be cooled. However, for the condition of deep enough cavity flooding and long enough melt release time, the development of corium fragments debris bed can be expected, which is more advantageous for cooling due to its porous characteristics