The increasing demand for diversified space missions necessitates addressing the extreme heat transfer challenges in space nuclear power systems, such as spatial constraints, long-distance unpowered cycles, and zero-gravity environments. Thus, the stable transfer of the substantial heat generated by the reactor core to the energy conversion device is crucial. High-temperature heat pipes, characterized by high heat flux, high operating temperatures, minimal temperature differences in heat transfer, and strong adaptability, are ideal for the key components of nuclear thermal energy conversion in space power systems. Traditional nuclear reactor power systems using coolants are less competitive in space because of their complexity, leakage risk, and stringent material strength requirements. Conversely, space heat-pipe-cooled reactors do not require auxiliary equipment such as high-temperature pumps. The phase change of the working fluid and the diffusive transport of vapor in a high-temperature heat pipe form a natural cycle that transfers heat from the core to the thermoelectric converter, thereby reducing system complexity, enhancing safety and reliability, and thus providing an effective solution for efficient heat transfer and energy conversion in space nuclear power systems.

The research and development of high-temperature heat pipe technology are pivotal for improving energy conversion efficiency and heat transfer performance in space nuclear power applications. Research on high-temperature heat pipes encompasses working fluid flow and heat transfer mechanisms, model establishment, experimental verification of frozen startup and heat transfer limits, steady-state heat transfer experimental analysis, and failure mechanisms, yielding promising results. As this technology advances, its applications and research in space nuclear thermal energy conversion systems have become more extensive and in-depth. Studies have focused on the startup and safety performance of space nuclear power systems, the coupling performance of high-temperature heat pipes in nuclear thermal energy conversion systems, and the research and design of high-temperature heat pipes in the radiators of space reactors. However, high-temperature heat pipes face challenges in achieving efficient thermal energy transfer and management, structural design of heat pipes and wicks, and adaptability to extreme space environments when applied to space nuclear power systems for thermal energy conversion. In response to these challenges, new research and attempts have been conducted. Studies on heat pipe performance under microgravity conditions have demonstrated their feasibility. In addition, research on shaped high-temperature heat pipes designs more flexible and efficient structures to meet complex heat transfer requirements. Furthermore, studies on additive manufacturing, aerogel insulation, and advanced testing techniques provide theoretical support and a technical foundation for the space application of high-temperature heat pipes. The current research on the space applications of high-temperature heat pipes still has some limitations. Ground-based tests of high-temperature heat pipes have advanced but cannot match the demands of real space scenarios. This mismatch prevents us from knowing their true performance in space reactors. Moreover, studies on the performance of high-temperature heat pipes coupled with nuclear reactors and thermoelectric converters are scarce. Thus, the overall coupling performance is largely unknown.

High-temperature heat pipe technology shows promise, but still faces challenges in space applications. Currently, most space heat-pipe-cooled reactors are in the design and feasibility exploration stages. Future research will focus on optimizing high-temperature heat pipe models and their applicability, exploring the heat and mass transfer mechanisms of working fluids in space environments, conducting theoretical research and complex wick design and manufacturing studies for shaped heat pipes, and ensuring reliable coupling of high-temperature heat pipes with other components in space nuclear power systems.