Study on melt stratification and migration in debris bed using the moving particle semi-implicit method

In the severe accident of nuclear reactors, the melt stratification and migration in debris bed directly affect the melt heat transfer characteristics and the heat flux distribution on the lower head wall, which in turn affect the follow-up accident progressions including the pressure vessel failure mode, melt leakage, and molten corium concrete interaction. They are the key issues in nuclear reactor severe accident research. In the present study, the MPS method was applied to analyze melt stratification and migration in debris bed by including the models of radiation heat transfer among debris, melt phase change and surface tension. To validate the numerical method, the experiment was performed using the metal tin and ternary nitrate as the core melt simulant. Then, the stratification behavior of oxide and metallic core melt was investigated under varied parameters and conditions. The results indicated that the crust formed at the oxide and metal melt interface could block the stratification, which had important influence to the layered configuration. The melt contact area increased in the conditions of crust formation prior to the stratification completion. The high melt and debris temperatures and surface tension could contribute to the stratification, but the high viscous melt when undergoing phase transition needed a long time. Moreover, the comparison of melt stratification under adiabatic and isothermal boundary conditions showed that the oxide melt solidification trapped some metal melt in the voids. The penetration distance of metal melt in the oxide debris bed increased rapidly in the initial moment and gradually in the following moment, and finally the migration was terminated by melt crust. The melt penetration distance increased as the increase of debris bed porosity and melt and debris temperatures.