A heat pipe-cooled reactor (HPR) is a type of nuclear reactor cooled by heat pipes. Using passive heat pipes instead of a pumped nuclear coolant eliminates the need for complex coolant pumping systems. Because heat pipes are passive devices, HPRs can be made highly compact and passively safe.

The first heat-pipe cooled microreactor was the Demonstration Using Flattop Fissions conducted by NASA and the US Department of Energy. On September 13, 2012, the experiment produced 24 W of electricity using a stirling engine fed from the reactor via heat pipes.^[1]

Heat pipes are closed tubes that use an internal phase-change material to efficiently transfer heat. A working fluid absorbs heat and evaporates at one end of the pipe, then circulates naturally to the other end where it condenses and deposits its heat. The condensed fluid returns via capillary action to the evaporator via a wick structure running through the heat pipe. For nuclear applications, the working fluid is typically an alkali metal. The use of the latent heat of the working fluid enables highly efficient, passive heat transfer.^[2]

Nuclear reactors require some mechanism of heat removal, both to prevent overheating and to generate power. Using heat pipes instead of a pumped nuclear coolant eliminates the need for coolant pumping systems and significantly simplifies the core design. Heat pipe-cooled reactors are regarded as inherently safe, and are considered highly resistant to single-point failures and loss-of-coolant accidents.^[3] Heat pipes can be used to make highly compact microreactors.^[4] Heat pipes, while simple in their operating principles, incorporate highly complex phenomena combining phase change, capillary action, and two-phase turbulent flow. These phenomena create large uncertainty in heat transfer capabilities, and have been the subject of significant research.^[2]

Heat pipe-cooled reactors have been researched since the 1960s. Space reactors, including SNAP-10A, Romashka, and TOPAZ used heat pipes to aid heat transfer in their thermoelectric generators. Los Alamos National Laboratory (LANL) proposed in the 1960s to develop a reactor cooled with heat pipes, however HPR research was severely limited by issues in materials and the efficiency of thermoelectric generators. By the 1990s, research had been largely discontinued following the cancellation of space programs.^[3]

Research picked up again in the early 2000s. LANL proposed the Heatpipe-Operated Mars Exploration Reactor (HOMER) in 2002, which laid the foundation for modern HPR designs.^[3] NASA's Kilopower program shifted focus to simplified designs, leading to the Demonstration Using Flattop Fissions (DUFF) project. This experiment, conducted with LANL, demonstrated the first heat pipe-cooled reactor on September 13, 2012. DUFF used a stirling engine to generate 24 W of electricity from heat supplied from the reactor core by heat pipes.^[1][3] In 2015, NASA began the Kilowatt Reactor Using Stirling Technology (KRUSTY) to develop a full prototype HPR. The KRUSTY prototype completed a full power cycle in 2018.^[3] KRUSTY operated successfully for 24 hours at over 800 °C (1,470 °F), and demonstrated the ability of HPRs to respond to reactor conditions and transients.^[4]

Westinghouse developed the eVinci microreactor, a small modular, transportable HPR power system using TRISO fuel, that is considered the first commercial HPR design.^[3] The company intended to build its first plant in Canada and submitted an license review application to the Canadian Nuclear Safety Commission in 2023.^[3] The project was cancelled in 2025, as the company shifted to government applications. In 2024, Westinghouse was selected to build and operate a prototype eVinci reactor at Idaho National Laboratory.^[5] In 2026, US Air Force selected Westinghouse to construct an eVinci reactor at Malmstrom Air Force Base by 2030 as part of its Advanced Nuclear Power for Installations program.^[6][7]

• Liquid metal cooled reactor – Nuclear reactor where the coolant is liquid metal