AbstractThe first tritium experiment was carried out successfully in the JET machine in November 1991. A new gas introduction system was constructed and an existing cryogenic pumping system was modified to permit collection of tritium in the exhaust gas. The total amount of tritium used was 0.12 g, out of a total available on-site inventory of 0.24 g.This paper describes the essential safety features of the modifications to the machine and how the safety and other approvals were obtained. The standards which were required to be met are discussed, and it is shown that the risk to employees and members of the public arising from the experiment was negligible. In addition, the measures taken to identify potential operational problems and to respond to incidents which might have occurred are described.