Study on corium-concrete interaction, heat transfer, and concrete ablation: Impact of replacing Zircaloy cladding with accident tolerant silicon carbide cladding on corium characteristics

Accident-tolerant fuel (ATF) cladding is currently being developed to enhance nuclear reactors’ safety. Among the various materials considered, silicon carbide stands out due to its superior resistance to oxidation and low corrosion rate. Nevertheless, the generation of corium resulting from the use of these accident-tolerant fuel cladding materials, as well as the interaction between corium and concrete, remains uncertain and requires comprehensive investigation. This study represents the first exploration of the variation in corium behavior from a thermochemical perspective, specifically examining the transition in cladding composition from Zircaloy to silicon carbide applicable for all types of light water reactors (LWRs). The present sensitivity study involves utilizing the CORQUENCH code to incorporate an improved dry-out model for concrete with crust formation. The validity of the model is established by comparing its predictions to experimental data encompassing corium ablation, surface heat flux, and corium temperature using limestone-based common sand concrete (LCS). The findings from this study indicate that the adoption of silicon carbide cladding leads to significantly reduced concrete ablation during the interaction with molten corium. Furthermore, it is recommended to employ silicon carbide cladding in nuclear reactors due to the fact that the temperature of corium with SiC-based cladding decreases upon water injection. Conversely, corium with Zircaloy-based cladding experiences an increase in temperature, as the injected water reacts with the corium, generating substantial heat and explosive gases. Additionally, corium viscosity with silicon carbide cladding increases, impeding heat transfer to concrete, while the viscosity of corium with Zircaloy cladding decreases, intensifying heat transfer and ablation. Furthermore, a crucial aspect lies in the utilization of silicon carbide-based cladding, which effectively contributes to a notable reduction in the generation of highly combustible gases such as H₂ and CO, concurrently resulting in a considerable upsurge in the production of inert CO₂ gas. Conversely, contrasting outcomes are observed when employing Zircaloy-based fuel pin cladding material.