EXPERIMENTS ON THE RELEASE OF TRITIUM FROM THE 1ST WALL OF JET

During the JET Preliminary Tritium Experiment (PTE), an estimated 2 X 10(12) Bq (1.1 X 10(21) atoms) of tritium were injected into the JET v acuum vessel. A series of experiments was performed whose purpose was to deplete the torus of tritium, to compare the effectiveness of diffe rent methods of tritium removal and to obtain a quantitative understan ding of the processes involved. The effectiveness of the cleaning proc edures was such that the normal tokamak programme was resumed one week after the PTE and the routing of exhaust gases to the atmosphere afte r two weeks. The release of tritium from the vessel was found to scale with the deuterium release from the vessel, suggesting that dilution and mixing of the hydrogen isotopes in the vessel walls is important. High density, disruptive tokamak discharges were found to be the most successful plasma pulses for tritium removal. Purges with deuterium ga s were also effective and have the advantage of operational simplicity . Helium discharges, on the other hand, resulted in low tritium releas e from the vessel walls. It was demonstrated that the tritium release rate could be predicted using data from hydrogen to deuterium changeov er experiments. Using the superior quality of data available from the tritium cleanup experiment, the physical mechanisms necessary to descr ibe the hydrogenic uptake and release from the JET torus were identifi ed. The release of tritium is reproduced using a model that incorporat es implantation into a thin surface layer as well as diffusion of trit ium into and out of the bulk material.