Issues Associated with Codeposition of Deuterium with ITER Materials

Codeposition of fuel species with the various plasma-facing materials present in ITER is expected to be the primary source of tritium accumulation within the vacuum vessel. In this paper, the levels of retention in codeposited layers of each of the three ITER materials (C, Be and W) are compared. Scaling laws, based on the conditions during the codeposition process (surface temperature, incident particle energy and deposition flux), are presented to allow prediction of expected retention values under ITER conditions. Retention in carbon codeposits scales inversely with incident particle energy, whereas in the metallic codeposits the retention level scales proportional to increasing particle energy. The differing scaling of retention level with incident particle energy provides some insight into which material may impact the global retention in ITER depending on where it may form codeposits. Carbon codeposits formed in low-energy particle areas (for example, in the divertor regions) tend to retain more tritium, whereas beryllium codeposits formed in relatively higher-energy particle areas (for example, the first wall) tend to retain increased levels of tritium. In addition to the amount of retention, the release behavior of tritium from codeposits will influence the tritium accumulation rate within ITER. The thermal release behavior of T (or D) from codeposits can be used to evaluate the effectiveness of baking at different temperatures as a means of tritium removal. Finally, the desorption kinetics from Be and W codeposits are contrasted. In the case of W codeposits, the duration of the baking cycle is important in determining the removal efficiency, whereas with Be codeposited layers, it appears that the maximum achievable bake temperature plays the leading role in determining removal efficiency.