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.