Effects of Thimble Leak on the Thermal Characteristics of Water-cooled Fuel Drain System in Molten Salt Reactor

[Background]: The liquid-fueled molten salt reactor is one of the focuses of nuclear energy research. The  fuel drain system is feasible and safety important.  [Purpose]: This study aims at the water-cooled fuel drain system of MSRE (Molten Salt Reactor Experiment) to analyze the effects of thimble leak on thermal characteristics and key factors.  [Methods]: First, a computational fluid dynamics simulation was conducted to establish a multiphase heat transfer flow model, which was employed to study the impact of leak on the heat transfer performance of bayonet tube. Then, sensitivity analyses of leak sizes (2 mm to 7 mm), leak locations (above the bottom of thimble, ranging from 0.01 m to 1.25 m), and the pressure difference between the annular gas space and the fuel drain tank (ranging from 20684 Pa to 48258 Pa) were conducted.  [Results]: The results indicate that when thimble leak occurs, the molten salt flowed into the annular gas space. The heat transfer capacity of a single bayonet tube increased more than 500%. Compared with the steady-state, the natural circulation mass flow rate also increased. When the leak size is below a critical threshold, the molten salt froze in the annular gas space, preventing further leak of the molten salt. Otherwise, the molten salt flows out of the annular gas space and the mass flow rate of cooling water in the bayonet tube oscillates periodically. In addition, the leak location and pressure difference also affect the mass flow rate of the cooling water and heat exchange capacity of the bayonet tube.  [Conclusions]: This study is valuable to the engineering design of the fuel drain system for MSRs.