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