Innovative Fusion Heat Sink | Dr. Yiran Mao

Fusion energy represents one of the most promising long-term solutions for clean and virtually limitless power generation. By fusing light nuclei such as deuterium and tritium, enormous amounts of energy can be released without producing long-lived radioactive waste or greenhouse gases. However, achieving controlled fusion on Earth requires materials and components that can withstand extreme plasma environments with surface heat fluxes exceeding tens of megawatts per square meter.

A central challenge in this context lies in developing plasma-facing components (PFCs) capable of efficiently managing these heat loads while maintaining structural integrity and long operational lifetimes. In present-day divertor and first-wall designs, water-cooled monoblock structures have nearly reached their performance limits due to localized hotspots, steep temperature gradients, and thermal fatigue. The motivation of the HPPFC project is to substantially improve heat-transfer efficiency in fusion devices by creating a new PFC architecture based on high-temperature liquid-metal heat pipes, enabling passive, uniform, and reliable thermal management.

A heat pipe is a passive two-phase heat-transfer device that transports large amounts of heat through the evaporation and condensation of a working fluid. In this project, alkali metals such as lithium or sodium are employed as the working fluids, operating at temperatures above 600 °C. Within a sealed tungsten-based envelope, the liquid metal evaporates at the plasma-facing surface and condenses on the cooler side, where the latent heat is released to a secondary coolant. The condensed liquid returns through a capillary wick structure, completing a closed circulation loop. This mechanism allows phase-change heat transport with minimal temperature differences, distributing the incoming heat over a much larger surface area. Consequently, the local peak temperatures are reduced, and the thermal load on the coolant becomes significantly lower—even when only radiative heat transfer is used on the condenser side.

The scientific challenges addressed in this project are threefold:

1. Thermal–fluidic understanding: Quantifying phase-change heat transport and capillary flow under high heat flux, large temperature gradients, and the influence of magnetic fields.
2. Materials and manufacturing: Developing refractory metal (W and W alloys) envelopes and porous wick structures that remain stable under cyclic thermal loading, recrystallization, and neutron irradiation.
3. System-level integration: Establishing a reliable radiative coupling between the heat pipe and a secondary cooling structure while maintaining structural integrity and minimizing interfacial thermal resistance.

Compared with conventional water-cooled designs, the heat-pipe-based PFC concept offers several advantages:

• High thermal efficiency: rapid heat spreading via phase-change transport and latent heat transfer.
• Completely passive operation: no pumps, no moving parts, and minimal risk of active system failure.
• Fully sealed configuration: eliminates corrosion, high-pressure systems, and hydrogen production.
• Simplified replacement: modular design without solid joining, enabling fast maintenance.
• Reduced tritium retention: small liquid-metal inventory and high operating temperatures minimize hydrogen isotope trapping.

The project is organized into several work packages that together address both the fundamental science and the engineering realization:

• WP1 – Engineering Design: multiphysics modeling of heat transfer, capillary flow, and mechanical stress; optimization of geometry, working fluid charge, and material configuration.
• WP2 – Secondary Cooling: development of a purely radiative coupling between the condenser and the secondary coolant body to ensure fail-safe and flexible operation.
• WP3 – Prototyping and Integration: fabrication of small-scale prototypes combining tungsten armor, liquid-metal heat pipes, and cooling structures; experimental validation of performance.
• WP4 – Tungsten-Based Heat Pipe Manufacturing: production of refractory envelopes via FAST, HIP, and CIP processes, complemented by CVD routes for fiber-reinforced structures.
• WP5 – Advanced Materials: exploration of tungsten alloys, Wf/W composites, and ODS-W materials to improve ductility, crack resistance, and radiation tolerance.
• WP6 – Characterization and Testing: comprehensive evaluation of thermal, mechanical, and microstructural behavior, as well as high heat-flux and plasma-exposure tests at JUDITH-2 (FZJ), GLADIS (IPP Garching), and PSI-2 facilities.

Through this coordinated approach, the HPPFC project will bridge fundamental science and engineering application—providing new insights into phase-change heat transfer in liquid metals under fusion-relevant conditions, while delivering tangible prototype components and validated manufacturing routes. The long-term vision is to establish a scalable, durable, and maintainable plasma-facing component technology, paving the way toward reliable, high-efficiency heat management for future fusion power plants.