The passive cooling system for the reactor cavity of the molten salt reactor (MSR) is an important guarantee to ensure the safe operation of the reactor, is one of the four engineered safety features for the MSR, and its structure design is an important part of the thermal hydraulic design.

This study aims to find out a suitable passive reactor cavity cooling system (RCCS) to meet the requirements of thermal shielding design for the lower reactor cavity of the MSR, and maximize the removal of reactor core decay heat under accident conditions.

Firstly, based on the design parameters of a 153 MWt MSR, a 1/4 geometric model of the lower reactor cabin of this MSR was established. Then, ANSYS FLUENT 20.1 software was employed to conduct three-dimensional numerical simulation of the flow filed and temperature filed for the lower reactor cabin, the influence of thermal shielding of the lower cavity was analyzed by changing the structure and layout of the passive RCCS, the structure sizes of the passive RCCS with double channel, the thickness of thermal insulation cotton on the intermediate thermal shielding plate and the position of the air inlet pipe. Finally, a new and suitable structure of passive RCCS was proposed after step-by-step improvements for a 153 MWt MSR.

The simulation results show that the optimized new-style passive air-cooling system with a double channel in the lower reactor cavity is the best among the three structures. Changing the width of the RCCS has little effect on the thermal shielding results of the lower reactor cabin whilst increasing the thickness of thermal insulation cotton on the intermediate thermal shielding plate of the RCCS can significantly reduce the temperature of the inner surface of the concrete wall. The closer the inlet position of the air inlet pipe is to the top of the RCCS, the better the thermal shielding effect. Based on above results, a new-style passive air-cooling system with double channel in the lower cavity is designed to completely dsatisfy the requirements for shielding cooling for the lower reactor cavity of a 153 MWt MSR.

The results of this study provide an important reference for the further engineering optimization design of passive residual heat removal system in the hundred megawatt-scale molten salt reactor.