The current status of heat pipe R&D
Idaho National Laboratory under the Department of Energy–sponsored Microreactor Program recently conducted a comprehensive phenomena identification and ranking table (PIRT) exercise aimed at advancing heat pipe technology for microreactor applications.
This exercise, held earlier this year, brought together experts from national laboratories, universities, and the Nuclear Regulatory Commission to prioritize key phenomena affecting heat pipe performance. By analyzing these phenomena, the PIRT exercise provided valuable insights for guiding future research and development efforts, supporting the successful integration of heat pipes into critical applications such as nuclear reactors.
Heat pipe technology is becoming increasingly prevalent in the nuclear industry with the Westinghouse eVinci heat pipe microreactor (HPMR) and fission surface power initiatives. Heat pipes have a broad use, including for nuclear reactors, space systems, and electronics cooling. As passive thermal management devices, they use phase change and capillary action to reach efficient heat transfer.
Their operation relies on an alkali metal working fluid that absorbs heat and evaporates in the evaporator. The heated vapor then travels to the condenser, where it deposits the heat via condensation. The condensed liquid then returns to the evaporator via capillary action in the wick structure. This cyclic evaporation and condensation inside heat pipes enables efficient heat transfer with minimal temperature gradients across the heat pipe length.
Heat pipes in HPMRs send core heat to the power conversion system, which provides mobile reactor designs with passive core cooling. This process eliminates the need for coolant pumping systems. Other uses of heat pipes in nuclear applications include heat pipe heat exchangers, long-term passive cooling systems, and spent fuel storage systems. A high-temperature heat pipe under operation is shown above.
Heat pipes are seemingly simple systems, essentially composed of an enclosed pipe that contains a wick structure and a small amount of working fluid. However, the complexity of the phenomena that couple in heat pipes—including capillary, phase change, turbulence, and compressibility effects—causes high uncertainties in the predictability of heat pipe operational regimes and performance.
The PIRT exercise
The PIRT exercise was organized into several phases that each built on the phase before, culminating in a comprehensive final report (see Phenomena Identification and Ranking Table [PIRT] for Heat Pipes, INL/RPT-25-84171, April 2025). The process began with the preliminary identification of phenomena by the DOE’S Microreactor Program at INL. These initial phenomena encompassed fundamental aspects of heat pipe operation, including heat transfer mechanisms, fluid dynamics, phase change dynamics, and material science, at a variety of operational conditions.
A diverse group of stakeholders, including experts in modeling and simulations, experiments, instrumentation, heat pipe fabrication, and reactor licensing, were then invited to contribute their expertise and insights. This collaborative approach ensured a comprehensive and balanced evaluation of critical phenomena. A preliminary PIRT document was circulated for review and feedback, followed by a full-day hybrid meeting. This meeting provided a platform for in-depth discussions and finalization of the PIRT, allowing for real-time collaboration and consensus-building.
Identified phenomena
The PIRT exercise identified several phenomena critical to heat pipe operation. These were categorized into high, medium, and low importance based on their impact on the predictability of heat pipe operational regimes and performance. A selection of high- and medium-importance phenomena are listed below; a more detailed discussion of these phenomena can be found in the associated PIRT report.
| Selected high-importance phenomena | Selected medium-importance phenomena |
|---|---|
| 1. Liquid pressure drops: The flow resistance to capillary pumping along the working fluid path within the heat pipe is critical for heat pipe operation. Capillary limits are often the limiting factor for alkali metal heat pipes under nominal operating conditions, where the capillary action induced by the wick is insufficient to drive the fluid circulation. | 1. Thermal contact between core and heat pipe: Good thermal contact is crucial for transferring heat from the reactor core to the heat pipe efficiently. |
| 2. Vapor pressure drops: Understanding of vapor pressure drops is essential for effective design and operation of systems involving vapor flow. | 2. Liquid and vapor advection: These processes are needed for maintaining the cyclic operation of heat pipes and ensuring continuous heat transfer. |
| 3. Wick de-wetting: De-wetting of the wick can result in significant temperature increases at the heat pipe wall in the evaporator, causing performance degradation. | 3. Compressible flows: Important during startup and for understanding pressure and temperature distributions. |
| 4. Evaporation and condensation: The phase change processes within the heat pipe are central to heat pipe function. | 4. Geyser boiling: Can cause significant wall temperature fluctuations during transient conditions or startup. |
| 5. Nucleate boiling: Occurrence of nucleate boiling in the wick (boiling limit) can be severe for alkali metal heat pipes, as nucleating bubbles can prevent liquid return to the evaporator. | 5. Underfilling: Can cause serious performance degradation and dry spots in the evaporator. |
| 6. Critical heat flux: This condition can cause rapid spikes in wall temperatures, potentially damaging the heat pipe body or wick structures. | 6. Dimensional tolerances and fabrication methods: Variations can result in performance deviations and uncertainties, especially during transient scenarios. |
| 7. Capillary action: The driving force that enables liquid return to the evaporator from the condenser via a capillary pressure difference between the vapor and liquid supplied by the pores in the wick structure. | 7. Frozen startup and solidification post-shutdown: Critical for ensuring reliable startup and efficient restart of the heat pipe. |
| 8. Wettability: The contact angle determines the maximum interfacial curvature and the maximum capillary pressure provided by the wick. | 8. Successive startups and shutdowns: Important for quantifying the effects of thermal cycling on heat pipe performance. |
| 9. Pressure dynamics: Pressure dynamics during startup, shutdown, and thermal power transients are important for the study of flow dynamics and limit conditions. | 9. Thermal stresses: Induced by transient conditions and can compromise the structural integrity of the heat pipe. |
The SPHERE facility is INL’s flagship high-temperature heat pipe test facility for nuclear applications. (Photo: INL)
R&D technician Travis Neumann inspects thermocouples before a test at SPHERE. (Photo: INL)
Modeling and simulation needs
Addressing the low modeling knowledge for phenomena such as wick de-wetting, critical heat flux, contact angle, and pressure dynamics requires a multifaceted approach. Developing advanced multiphase and multiscale models, coupled with high-resolution simulations and detailed experimental validation, will enhance our understanding and predictive capabilities for these critical phenomena. As an example, a snapshot of state-of-the-art computational fluid dynamics (CFD) results for a copper-water heat pipe operation are shown above. This will ensure the reliable and efficient operation of heat pipes under various conditions, supporting their successful integration into critical applications.
Experimental and instrumentation needs
Reliable techniques for internal temperature, pressure, or velocity measurements are essential to address knowledge gaps associated with the complex flow dynamics and heat transfer within heat pipes. Extending current X-ray radiography techniques to resolve the wick can enable the study of evaporation and condensation mechanisms during operation. Additionally, experiments investigating factors affecting the life of heat pipes, such as corrosion, oxidation, and wick degradation, would be beneficial in establishing heat pipes as effective cooling solutions for nuclear applications. These experimental needs will drive future tests and upgrades in SPHERE (Single Primary Heat Extraction and Removal Emulator), INL’s flagship heat pipe test facility.
QA and standards
Implementing robust quality assurance processes and establishing industry standards are crucial for ensuring the reliability, safety, and performance of heat pipes. Addressing QA-related issues such as fluid contamination, underfilling, surface roughness, dimensional tolerances, and manufacturing defects through stringent QA protocols and regular inspections will help mitigate these issues and improve overall reliability.
Conclusion
The PIRT exercise successfully identified and prioritized key phenomena affecting the operational regimes and performance of heat pipes. By addressing critical knowledge gaps in modeling and experimental techniques and implementing robust QA processes, we can enhance the reliability and efficiency of heat pipes under various conditions. This systematic evaluation and prioritization process serves as a valuable resource for guiding future research efforts, supporting the successful integration of heat pipes into critical applications such as nuclear reactors, and contributing to the advancement of heat pipe technologies in safety-critical industries.
Ilyas Yilgor is a postdoctoral research associate at INL.
Mauricio Tano is a modeling/simulation professional at INL.
Katrina Sweetland is an R&D engineer at LANL.
Joshua Hansel is a computational scientist at INL.
Piyush Sabharwall is a distinguished staff scientist and DOE Microreactor Program technical area lead at INL.