Formal approval was required to permit tritium operation of the JET machine to proceed and a safety case was prepared in order to justify this. The standards and the methodology used are summarised both for the Active Gas Handling System and for the torus systems. The main design safety principles are described and accident sequences are reviewed. Both deterministic and probabilistic analysis was carried out and the main results in terms of source terms, on-site and off-site doses are presented. It is shown that operation of JET with deuterium-tritium plasmas complies with the ALARP principle.