Concrete ablation analysis for molten corium-concrete interaction mitigation strategy

Abstract

The molten corium-concrete interaction (MCCI) has been receiving increased attention because it can cause the direct release of core melt into the environment via basemat melt-through (BMT). The MCCI analysis has high uncertainty and it is not fully understood according to the current level of knowledge. However, since there is a possibility that MCCI may occur, nuclear power plants (NPPs) must take action to mitigate the MCCI based on quantitative analysis. Therefore, in this study, an analysis model was proposed and MCCI analysis was conducted for NPPs using the model to provide database for establishment of MCCI mitigation strategy of real NPPs. First, the CORQUENCH code was selected as the analysis tool to conduct the MCCI analysis for NPPs, which was released in August 2018 (version 4.1b). The analysis model was proposed and validated for both small and large MCCI experiments. The validation results confirmed that the analysis model predicts the experimental results well regardless of experimental scale within a reasonable range. Second, MCCI analysis was conducted for the NPPs under a conservative assumption to consider the uncertainties in the initial conditions. The results showed that continuous ablation occurs when water is not injected into the reactor cavity and the containment liner plate is damaged by the core melt at 916 min after the melt pouring time. To verify the analysis results, sensitivity analyses were conducted for the initial corium temperature, total corium mass, melt eruption model, void fraction model, and time step. Third, possible MCCI mitigation strategies were proposed based on additional analyses. One strategy is adding new concrete at the bottom of the reactor cavity. In this case, the analysis results showed that the additional concrete should have a thickness of 40 cm or more to secure extra time for the core melt cooling. The other is injecting water into the reactor cavity. The analysis results confirmed that the core melt is effectively cooled down when the water is injected in the early phase.