A heat pipe reactor is ideal for underwater unmanned vehicles (UUV) because it is simple, is compact, and has high inherent safety.

A passive residual heat removal system that uses natural circulation to cool the adiabatic section of heat pipes was designed based on the characteristics of a new type of megawatt compact nuclear power plant with a heat pipe reactor.

Firstly, based on the characteristics of 3.5 megawatt compact nuclear power plant for UUV, natural circulation of water was utilized to cool the adiabatic section of heat pipes. Then, the computational fluid dynamics software STAR-CCM+ was used to simulate and analyze the heat removal capacity of the passive residual heat removal system with different geometric parameters, made it conservatively meeting the demand of maximum residual heat removal power.

The results show that a baffle around the adiabatic section of heat pipe bundle is beneficial to reduce the maximum temperature of the fluid. The widths of the inlet and outlet of the baffle have almost no effect on the heat removal capacity, while extending the lower part of the baffle is unfavorable to natural circulation. When the axial length of the emergency cooling chamber is 160 mm, it can conservatively meet the maximum residual heat power of 0.14 MW. The maximum fluid temperature is 288 ℃, which is lower than the boiling point under working pressure, and normal operation is possible in ambient temperatures ranging from 5 ℃ to 25 ℃.