Experiments on the release of tritium from the first wall of JET

Abstract

During the JET Preliminary Tritium Experiment (PTE), an estimated 2 X IO'* Bq (1.1 X 10'' atoms) of tritium were injected into the JET vacuum vessel. A series of experiments was performed whose purpose was to deplete the torus of tritium, to compare the effectiveness of different methods of tritium removal and to obtain a quantitative understanding of the processes involved. The effectiveness of the cleaning procedures was such that the normal tokamak programme was resumed one week after the PTE and the routing of exhaust gases to the atmosphere after 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 gas were also effective and have the advantage of operational simplicity. Helium discharges, on the other hand, resulted in low tritium release from the vessel walls. It was demonstrated that the tritium release rate could be predicted using data from hydrogen to deuterium changeover experiments. Using the superior quality of data available from the tritium cleanup experiment, the physical mechanisms necessary to describe the hydrogenic uptake and release from the JET torus were identified. The release of tritium is reproduced using a model that incorporates implantation into a thin surface layer as well as diffusion of tritium into and out of the bulk material.

Plasma-material interactions in current tokamaks and their implications for next step fusion reactors

“…In the first experiment at JET in 1991, tritium was introduced into two pulses by injection from two of the sixteen neutral beam sources. This followed a series of discharges when the two sources were fed with a mixture of 1% T in D to check operational procedures, diagnostics, and transport codes for T in D plasmas [739]; the total T entering the torus during these trace tritium experiments was ≈3x10 19 atoms. The JET machine at this time was fitted with two belt limiters, but the plasma contacted specially shaped tiles at the top of the vessel, which were used as an open divertor (the so-called "X-point" configuration).…”

Fundamental hydrogen interactions with beryllium : a magnetic fusion perspective.

“…5 Of particular interest for the present discussion are "refined wall models," which emphasize the details of the material response to plasma exposure. [6][7][8] Recombination, particle impact desorption of hydrogen on surfaces, and diffusion of hydrogen into the material all contribute to recycling, as illustrated in Figure 1. As discussed in more detail in Refs.…”

“…To aid in the interpretation of the experimental results, we developed a computational model to simulate ion scattering at grazing incidence [5]. For this purpose, we incorporated a simplified surface model into the Kalypso molecular dynamics code [6]. This approach allowed us to understand how the incident ions interacted with the surface hydrogen, providing confirmation of the preferred binding site.…”

Measurements of tritium retention and removal on TFTR

“…Results from JET had indicated that a deuterium 'soak' was effective in removing tritium from the torus that was maintained at 300" (Ref. 6). There was interest in measuring the efficacy of this technique for the ambient temperature TFTR limiter.…”