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Nuclear fusion offers the potential to provide a carbon-free, high-power, and sustainable energy source. Among the different fusion approaches, Inertial Confinement Fusion (ICF) relies on high-power laser beams irradiating a small spherical deuterium-tritium (DT) fuel capsule, inducing an ablation-driven implosion resulting from the extreme compression and heating of the fuel required for thermonuclear ignition. The subsequent fusion reactions release a large amount of energy carried by energetic neutrons, charged particles, and X-rays. This energy must be absorbed and converted into usable thermal power by the surrounding reactor structures. (1)

GenF, a spin-off of the Thales Group, is developing an accelerated pathway toward a direct-drive ICF reactor. Within the framework of the TARANIS project, GenF collaborates with CEA, CNRS, Thales and other research institutions to address key scientific and technological challenges associated with inertial fusion energy.

For commercial power production, an ICF power plant must repeatedly inject, ignite, and burn fusion targets at high repetition rates while efficiently recovering the energy deposited by fusion products and converting it into electricity. A surrounding breeding blanket is therefore required to absorb this energy and transfer it to the power conversion system. Furthermore, the limited availability of tritium requires future fusion power plants to achieve tritium self-sufficiency through in situ tritium breeding within the blanket. (2)

One of the breeding blanket concepts currently investigated by GenF is based on unenriched liquid lithium, which simultaneously acts as the tritium breeder, neutron energy absorber, and primary coolant. Since no liquid breeder blanket has yet been developed for a direct-drive ICF power plant, its design remains an open engineering challenge. The blanket must provide efficient heat removal while maintaining acceptable structural temperatures, ensuring adequate lithium circulation, and enabling effective thermal power extraction under reactor operating conditions. A key design question is whether the high thermal loads can be removed through natural circulation alone or whether an active pumping system will be required, with significant implications for reactor complexity and reliability.

To support the development of this concept, a thermo-hydraulic model is developed in COMSOL Multiphysics by coupling the Heat Transfer in Solids and Fluids interface with the Turbulent Flow interface within the liquid lithium and surrounding structures. A volumetric heat source, representative of neutron energy deposition, and a surface flux, representative of ions and X-rays deposition, are applied to the breeder. The simulations are used to evaluate temperature fields within the walls and the lithium, identify potential thermal hotspots, investigate heat transfer mechanisms and flow patterns, and quantify the contribution of natural convection to heat removal. Parametric studies are performed to assess the influence of reactor dimensions, heat deposition profiles and thermal boundary conditions on blanket performance. Different heat extraction strategies are investigated, including natural circulation, forced convection and the preliminary sizing and placement of heat exchangers. This multiphysics approach provides a framework for exploring innovative blanket configurations, identifying key thermal-hydraulic design parameters, and guiding the preliminary design of a viable thermal management system for the GenF reactor.