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Description
The tritium fuel cycle is an important aspect for the design of a fusion reactor, since the initial on-site amount of tritium must ensure self-sufficient reactor operation with required fusion power and tritium breeding efficiency, complying at the same time with safety limits on in-vessel tritium accumulation. Retention of hydrogenic species inside plasma-facing components (PFCs) represents a challenge for the fuel cycle: although ion and neutral fuel fluxes impinging on the PFCs only penetrate some nanometres into the wall upon impact, diffusive processes of light fuel atoms can drive significant amounts of fuel deeper into the wall structure when constant bombardment by the fuel is considered. For a reactor-class device, with some hundred square metres of PFCs exposed to the plasma and charge-exchange neutral fluxes, accumulation of tritium trapped at material defects, especially in the presence of neutron-induced material damage represents a significant safety concern and may cause issues related to economic operability of the device.
FESTIM [1] is a simulation code designed for fusion applications, which calculates the diffusive transport and trapping of hydrogenic species in materials like tungsten (W), which is the preferred material for PFCs in high-power fusion devices. In this work, the FESTIM code is applied to assess the retention of tritium in a hypothetical full-tungsten fusion device with Wendelstein 7-X (W7-X) stellarator geometry. The approach follows the HISP framework [2]. Taking into account 3D-resolved wall plasma and heat fluxes from an existing EMC3-EIRENE solution for a W7-X plasma, a set of 1D FESTIM simulations evaluates the local fuel retention for selected representative wall locations. The total tritium inventory in the fusion device after $10^6$ seconds of plasma operation is estimated by scaling the local results with the corresponding total surface areas. This result can be used to upscale to larger geometries of potential fusion power plants.
[1] R. Delaporte-Mathurin et al., “FESTIM: An open-source code for hydrogen transport simulations”, Int. J. Hydrog. Energy 63 (2024)
[2] K. Dunnel et al., “Hydrogen Inventory Simulations for PFCs (HISP) in ITER”, presented at the 31st IEEE Symposium on Fusion Engineering (SOFE2025), June 23-26, 2025, Cambridge, MA USA