Simulation Analysis of Core Melt Migration Behavior Under Severe Accident

The simulation analysis of melt migration process in severe accidents is of great significance for understanding the accident mechanism and evaluating the prevention and mitigation strategies of severe accidents. Due to the limitations of experimental research on severe accidents, computer simulation analysis is usually used to simulate the process of melt migration. At present, the calculation model of the severe accident analysis program mainly focuses on the formation and cooling process of the melt pool in the initial stage of core melting and the steady state of the lower chamber. However, there are few quantitative analysis studies on the secondary migration stage of the melt during the formation of the melt pool, and the influence of the support structure in the simulation analysis of the core melt migration process has not been effectively identified. The core support structure is an important channel for the melt to enter the lower chamber from the active zone, and the main source of metal mass in the final steady-state melt pool. Its failure mechanism has an important influence on the melt migration process and the effectiveness of IVR mitigation measures. Based on the design characteristics of the three types of core support structures of the third generation nuclear power unit in China, in this paper the severe accident analysis program was used to carry out the analysis of the melt migration process, and deeply study the phenomenon of different stages of the melt migration process and the influence of the core support structure on the migration process. The results show that in the early stage of the severe accident, due to the limited water level drop rate of the pressure vessel, the melt is retained and stratified in the core melting stage, forming a stable and cooling oxide hard shell, resulting in the continuous radial expansion of the internal melt pool and the initial lateral migration. Subsequently, the melt continues to undergo secondary expansion and migration along both the lateral and vertical directions. The three types of core support structures have different hindrance characteristics to the secondary migration of melts. After the initial lateral migration of the type A support structure, the melt has not yet reached the support plate, and the mass flow of the subsequent lateral migration melt is small. Most of the melts are successively melted through the core plate, the support column and the support plate into the lower chamber along the vertical direction, thereby prolonging the time of the melt migration process. After the initial migration of the type B and type C support structures to the pressure vessel before evaporation, the melt can basically contact and melt the support plate to form a steady-state melt pool in the lower chamber. Finally, most of the melts in the steady-state melt pool, especially the metal melts, need to enter the lower chamber through secondary vertical migration. In addition, the MAAP program is limited by the simplified model and empirical relationship, and lacks the consideration of the stress distribution of the support plate structure. As a result, there may be a certain deviation between the calculation results in the secondary vertical migration stage of the melt and the serious accident mechanism, which affects the final penetration position of the support plate.