In severe accident, the ex-vessel corium
coolability is one of risk-significant issues and there are still significant
uncertainties in the coolability in terms of the debris bed characteristics
even in the case the cavity flooding system is utilized. The coolability of
debris particle bed which is important because of the possibility of molten
core concrete interaction is related with debris size, distribution and so on.
These components depend on the earlier phase such as melt-jet injection, break
up, fragmentation and so on, so the investigation of jet break up phenomena is
needed to understand the whole process of ex-vessel corium coolability.