In the late phase of severe accident in nuclear power plants, it is important to assure the coolability of the relocated corium in the reactor cavity. Under this circumstance, concrete ablation and over-pressurization caused by molten core concrete interaction (MCCI) may threaten the integrity of containment, the final barrier of the defense-in-depth, to prevent the release of radioactive material to environment. To ensure the long-term cooling of corium in the reactor cavity, it is important to ensure the coolant ingression into the internally heat generated corium debris bed which is affected by pressure drop in porous media (Fig. 10.). For this reason, it is necessary to understand the pressure drop mechanisms in the porous bed that can be characterized by physical parameters that include porosity, particle morphology, particle size distributions etc.