CHEMICAL-RELEASE OF IMPLANTED DEUTERIUM IN GRAPHITE

Graphite is extensively being used in present-day fusion devices and has been selected as the reference material for plasma-facing components in the international thermonuclear experimental reactor, ITER. Graphite's major advantages are associated with its low Z and excellent thermomechanical properties. Its drawbacks include susceptibility to erosion and large capacity for H retention under hydrogen plasma exposure. H-retention affects fuel recycling and also presents a potential environmental risk when operating with tritium, during vacuum vessel maintenance and accidental vacuum loss. The objective of the present study was to investigate the removal of energetically implanted deuterium from pyrolytic graphite via chemical reaction with atmospheric gases (N2, O2, and H2O) for different graphite temperatures. N2 exposure at 300-570 K appeared to be ineffective for the removal of implanted D. Oxygen exposure, on the other hand, was most effective in removing the implanted D from graphite, most likely by the erosion of carbon through O2 + C reactions. For example, 10 h O2 or air exposure at 570 K resulted in an order of magnitude reduction in D content. At approximately 770 K, three orders of magnitude reductions were observed in less than an hour of O2 or air exposure. While H2O was also effective in reducing the implanted D concentration, the corresponding D-removal rates were observed to be lower than those for the case with O2. Comparisons of D-removal rates for ion-implanted D and chemisorbed thermal D0 atoms on graphite surfaces are also discussed.