The immobilized hydrogen as a titanium hydride could be used in a stainless steel vessel for the long-term storage of hydrogen isotopes. In order to predict the safety concerns of storage vessel at elevated temperature, a comprehensive mathematical model has been developed for diffusion analysis of hydrogen isotopes in the stainless steel vessel. Numerical results showed that significant amount of hydrogen isotopes could be released by permeation through the vessel wall of 7mm thick at the 800 ℃ of hypothetical fire accident condition. A rapid permeation of tritium gives that steady state permeation is achieved in approximately 5 hours at this temperature.