Mechanism analysis of flow reversal phenomena in research reactor cores

An analytical solution for the coolant flow reversal process in the core of the Tsinghua High Flux Reactor (THFR) following the loss of forced circulation is developed based on thermal-hydraulic feedback. In this scenario, the coolant reverses direction driven by buoyancy forces generated within the core. Key factors influencing this process are systematically analyzed. Both theoretical analysis and computational fluid dynamics (CFD) simulations indicate that the time required for flow reversal is primarily influenced by the coolant channel width and coolant viscosity. The final natural circulation flow velocity is determined by the coolant channel width, coolant viscosity, and reactor power level. The additional driving head provided by the residual heat removal system responds more slowly and with lower magnitude compared to the core's inherent thermal-hydraulic feedback. Among the influencing parameters, the coolant channel width has the most significant impact on both the final flow rate and the reversal speed. Additionally, the presence of unheated regions in the fuel assemblies slightly reduces the final circulation flow rate. For future reactor designs, it is essential to tightly control the machining and assembly tolerances of fuel assemblies and minimize the rotational inertia of emergency pumps to facilitate faster flow reversal in the core.