Radioactive storage vessel of austenitic stainless steel is normally recommended for the long-term storage of hydrogen isotopes. The hydrogen isotopes could be fixed as a metal hydride in the storage vessel. In order to predict the possibility of deterioration of the mechanical properties from the diffusion of hydrogen, an evaluation program has been developed for diffusion analysis of hydrogen isotopes and helium systems in the stainless steel vessel. Numerical results showed that negligible tritium would be released by permeation through the vessel wall of 7 mm thick at the normal storage temperature of 40 ℃ for long-term storage. In hypothetical fire accidents, when the vessel was heated for 12 hours to temperature up to 800 ℃, permeation of hydrogen through the vessel wall was very rapid.