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While ITER is a full-tungsten (W) tokamak, a substantial surface area of water-cooled stainless steel remains exposed to charge-exchange neutrals (CXN) originating from the plasma. Hydrogen isotopes implanted in steel can permeate rapidly and subsequently be released into the cooling water, where inventory limits are imposed by regulatory requirements. This work presents a sensitivity study of permeation through the stainless-steel (SS) 316L-IG ITER diagnostic first wall (DFW), which comprises ~30 m² of upper and equatorial port surface area.
The analysis employs the FESTIM modelling framework [1], which implements the McNabb–Foster reaction–diffusion model to resolve mobile and trapped hydrogen isotopes, while incorporating additional physics such as the Soret thermo-diffusion phenomenon. CXN wall fluxes are obtained from recent SOLPS-ITER and SOLEDGE3X plasma backgrounds using simulation grids extended to the wall contour, providing uncertainty ranges for particle flux and heat loads at the DFW surfaces. Tritium permeation to the water coolant loops occurs predominantly during baking phases. The influence of baking duration, temperature, and steel thickness is assessed.
Results indicate that permeation rates during baking are most sensitive to the impacting particle fluxes and heat loads during plasma operation. To reduce conservativeness, plasma exposure conditions are set as closely as possible in terms of heating power, pulse durations, and tritium content according to the ITER Research Plan. Depending on material properties, the inventory limit in the torus cooling water circuit may be reached by the end of the first DT operation phase (DT-1).
Finally, the application of permeation barriers at the DFW plasma-facing surface is evaluated. These may be implemented if early measurements during the initial DT-1 campaigns indicate permeation rates exceeding acceptable limits. For tens of micrometre thin coatings, assuming continuity of chemical potential, double layers such as SS/W on SS or SS/alumina on SS are highly effective in mitigating permeation. However, since such layers may not withstand disruption-induced thermal loads, a more robust solution—2 mm W cladding on SS—is proposed, reducing permeation by two orders of magnitude. The potential influence of material interface properties on barrier effectiveness is assessed through parameter scans.
[1] R. Delaporte-Mathurin et al., Int. J. Hydrog. Energy, vol. 63, pp. 786–802, Apr. 2024