THE MODEL DEVELOPMENT AND VALIDATION OF DEBRIS BED REMELTING AND MOLTEN POOL FORMATION AFTER SEVERE ACCIDENT IN PWR

抄録

After a severe PWR accident, the residual heat of the core cannot be discharged in time which results in an increase of the temperature of fuel rods. When the temperature is larger than the melting point, the reactor core would melt and fall to the lower head of the vessel. Severe reaction would occur between the melting reactor core and the residual coolant which make the melt change to debris bed. In the following process of severe accident, the remelting of debris bed may occur if the cooling ability is not sufficient. Finally, a molten pool may form in the lower head of reactor pressure vessel.

In this paper, Models are established for the remelting of debris bed and formation of molten pool separately based on the energy conservation equation. Inside, the convective and radiative heat transfer are considered among the debris bed, the double-layer molten pool structure and the reactor internal components. The model was validated using the IVR analysis results calculated by the ERI, INEEL and DOE models. The AP600 and AP1000 are tested under different conditions. The results show good performance in all the cases. In the azimuthal angle of the lower head, all the models show same results in the bottom of the vessel while the largest error is found in the side aspect. The reason may be the simplified module for the calculation of metal layer of molten pool. However, the largest error of proposed model is lower than 15% for all the parameters when compared with the other models. So, it is concluded that the analytical model can be used to analyze the process characteristics of debris bed remelting and molten pool formation through a simplified method, which provides technical support for in-depth understanding of the serious accident process of pressurized water reactors.