AbstractRecently, we have shown that the ion-driven permeation through metal is basically controlled by hydrogen diffusion in both the front and back sides. The maximum permeation rate Φpmax is well represented by Φpmax = rφ/(d − r), (r is the range of the incident ion, φ the incident flux and d the specimen thickness), which is much smaller than the estimation given by the recombination theory.In the present work, therefore, we have re-estimated the tritium retention and permeation in the metallic first wall according to the diffusion model. In addition, we also demonstrate that the effect of residual hydrogen gas on the retention and permeation cannot be neglected, especially at high temperatures.In order to reduce the hydrogen permeation, we have investigated the usefulness of a protective layer. A fairly thick protective metal layer with small permeability is found to be effective. Unfortunately, the layer does not work for reducing the retention.