The molten salt reactor (MSR) is one of the six advanced reactors identified by the Generation IV International Forum. The MSR exhibits unique characteristics, such as intrinsic safety, sustainable development, nuclear nonproliferation, natural resource protection, and economic efficiency. After a liquid-fuel molten salt reactor shuts down, residual decay heat in the reactor core is passively dissipated to the environment through a natural circulation loop of the molten salt. However, the decay heat from the molten salt in the main circuit can impact the thermal capacity of the overall system.

This study aims to determine the thermal characteristics of the passive system by establishing an analysis basis for a natural cycle model to examine the effects of natural circulation loop on the physical properties of the molten salt, the loop structure, and equipment resistance coefficient K.

Based on the direct reactor auxiliary cooling system (DRACS) for MSR, the natural circulation model of the passive residual heat-removal loop of a liquid-fuel molten salt reactor was established using self-developed Python analysis program, and the temperature distribution of the molten salt in the loop was explored using the numerical model. Then, the effects of different physical properties, loop structures, and core and heat exchanger resistance coefficients K on the heat transfer and flow characteristics of the residual heat-removal system were analyzed. Finally, the effects of critical factors on the residual heat-removal capacity of the reactor core were analyzed using self-developed Python program for natural circulation calculation equations code (NCCC) and validated by using the natural cycle experimental results of the DRACS circuit in CIET1.0.

The findings indicate that the presence of decay heat from molten salt in a system loop decreases the natural circulation-driven heat transfer by the molten salt within the core. Based on the significance analysis results show that fuel salt density, specific heat capacity, and height difference between the hot and cold cores are parameters that significantly influence the natural circulation capacity. When these three parameter values are increased by 15% separately, the residual heat removal capacities increase by 26.02%, 15.00%, and 18.59%, respectively.

Results of this study demonstrate that the molten salt properties, circuit structure, and equipment resistance coefficient all affect the natural circulation heat-removal capacity.