During the severe accident in the nuclear power plant, the reactor core will melt due to the decay heat. In the late phase, the molten core material starts to relocate to the lower head of reactor pressure vessel (RPV). After the RPV wall is melted through, the molten corium can be diluted by a sacrificial material and then spreads to the cooling compartment. During the relocation of the molten material, the decay heat is transported with the corium material. One of the key phenomena is the process of phase change between solid as well as the liquid state of different involved materials.
Here is a simulation result of the hypothetical accident scenario achieved with shonDy:
The boundary conditions used for this case reveal the massive size.