Numerical simulation on thermal-hydraulic-mechanical coupling of core corium migration during severe accidents

Core corium migration is one of the critical challenges of severe core accidents. The lower support plate is a critical load-bearing component within the pressure vessel among core corium migration. Investigating the thermal exchange and mechanical failure of the lower support plate during the core migration process holds significant practical value. The heat transfer between the corium and the support plate is complex, involving multiple phenomena such as fluid dynamics, thermal exchange, melting, and mechanical effects, making a comprehensive analysis of the failure process challenging. In this study, a migration heat transfer model has been established, incorporating radiation heat transfer, impact heat transfer, and direct contact heat transfer. The interaction between the corium and the support plate is modeled using a mechanical analysis approach, while the mechanical effects are analyzed through the formulation of a constitutive equation. A Thermal-Hydraulic-Mechanical (THM) coupling calculation method is also developed to address these interactions. The results show that the corium migration heat transfer is consistent with findings in the relevant literature. The majority of corium migrates close to the wall of the RPV lower head, causing the temperature at the edges of the lower support plate to exceed that at the center, leading to creep failure under thermal stress. As the corium continues to migrate, the cumulative mass of molten material and the convective heat transfer coefficient increase. At 60 s, the maximum total deformation of the support plate reaches 0.89025 mm, with a maximum total strain of 0.01636 mm/mm. The equivalent stress is concentrated at the upper surface edges, exceeding the yield limit, indicating fracture failure. Ultimately, the support plate fails within 1 min due to sustained radiation heat. These simulation results offer insights for the safe design of the lower head.