Concrete ablation analysis for molten corium-concrete interaction mitigation strategy

Abstract

The radioactive materials were released by molten corium–concrete interaction (MCCI) in Fukushima Daiichi accident. In order to prevent this situation, it is necessary to perform a quantitative analysis of concrete ablation and establish a strategy to mitigate the MCCI. However, because the MCCI has inherent uncertainty and complexity, it is not fully understood yet. In this study, an analysis model was developed 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. 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.