Energy release characteristics of prompt criticality induced by molten fuel sloshing in fast reactors

During severe accidents in sodium-cooled fast reactors, such as core disruptive accidents (CDAs), a large amount of molten fuel may form within the core and accumulate at its center, leading to central sloshing. This behavior can be triggered by pressure surges caused by fuel–coolant interactions between molten fuel and liquid sodium, which drive the fuel outward, followed by an inward motion that leads to central accumulation. However, unlike ordinary engineering fluids, molten fuel may undergo prompt criticality when compressed during sloshing, resulting in rapid energy release that can threaten the structural integrity of the core and reactor vessel. Therefore, from the viewpoint of nuclear safety, it is essential to investigate the dynamic core response during central sloshing of molten fuel. To address this issue, this study proposes a novel theoretical model that uses the density-normalized pressure increase rate at the onset of prompt criticality to evaluate the potential energy release. Numerical simulations were performed using a safety analysis code to characterize the central sloshing process in a confined molten fuel pool formed under post-accident conditions. The dynamic responses of cores loaded with MOX fuel and metallic fuel are systematically compared for a pure fuel pool. Furthermore, a parametric study of the thermodynamic properties used in the proposed theoretical model is conducted to clarify their effects on fuel energy release during prompt criticality. Based on these results, quantitative relationships between the density-normalized pressure increase rate at the onset of prompt criticality and the released thermal energy are established for a pure fuel pool.