Flow-based temperature regulation during unprotected fuel salt overcooling transient in molten salt fast reactors

Molten Salt Fast Reactors (MSFRs) present unique safety and control challenges due to the absence of conventional mechanical reactivity control mechanisms and the circulation of fuel salt. In particular, unprotected fuel salt overcooling (UFSOC) transient can lead to power excursion and increased temperature gradients within the core. In this study, steady state and transient computational fluid dynamics (CFD) simulations were performed to investigate MSFR behavior. A UFSOC transient was analyzed, and a flow-based control strategy using a proportional–integral–derivative (PID) controller was implemented to regulate the primary pump speed based on deviations in the core inlet temperature. Steady state results reveal the formation of a recirculation zone near the blanket region, leading to localized temperature peaks exceeding the core outlet temperature. During the UFSOC transient, enhanced heat extraction in the intermediate heat exchanger induces a rapid power increase while maintaining a relatively stable average core temperature, but significantly increases temperature gradients. The proposed control strategy effectively limits excessive cooling, restores the core inlet temperature toward nominal conditions, and reduces thermal gradients. Increased flow enhances mixing and suppresses localized hot spots, resulting in a more uniform temperature field. These results demonstrate that the flow-based control can significantly enhance the stability and safety of MSFRs under UFSOC transient conditions, supporting its application in future MSFR designs.