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

Zou, Z.*; 守田 幸路*; 石田 真也 ; 山野 秀将  

Zou, Z.*; Morita, Koji*; Ishida, Shinya; Yamano, Hidemasa

During severe accidents in sodium-cooled fast reactors, 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.