Study on the interaction between molten material and concrete under serious accident in AP1000 nuclear power plant

When nuclear power plant has a serious accident which the core melts and penetrates the pressure vessel, molten material falls into the stack and interacts with the concrete (Molten core-concrete interactions, MCCI), threatening the integrity of the containment. Based on MELCOR's calculation model of AP1000 nuclear power plant, the failure of gravity injection in the refueling tank superposed by a large breach accident was set up in this paper to study the accident phenomenon after the pressure vessel was melted through and the cooling process of the molten material outside the reactor, and it analyzes the threat of MCCI to the integrity of the containment vessel, that is, the risk of overpressure, the risk of explosion and the risk of direct melting. The results show that after the molten material falls into the reactor cavity, the pressure inside the containment vessel will undergo initial rapid depressurization, and then reach the dynamic equilibrium of water evaporation and condensation, and the pressure inside the containment vessel will remain stable at 0.2 MPa. Combustible gas will be generated during MCCI process, which will accumulate to the explosion limit and trigger the ignition device to work. Compared with the side wall, the concrete bottom plate is more easily melted, and it is melted through about 1 m at 10⁵ s. After the safety shell space reaches the thermal hydraulic balance, the decay heat of the molten material is continuously exported outside the safety shell, and the accident is alleviated. (authors)