Depth distribution of tritium in fusion reactor materials

A huge amount of gaseous tritium is used as fusion fuel in the fusion reactors. Fusion reactor materials are exposed to various chemical species of molecular, atomic, ionic tritium. A part of the tritium species adsorbs on the surface of materials and it diffuses into bulk. On the contrary, it diffuses from bulk toward the surface and releases from the surface of materials, when the chemical potential of tritium species in gas phase lowers. From the economical and safe view points, it is important to examine a tritium depth distribution caused by elemental processes such as adsorption, absorption, diffusion, and reemission. From this view point, the depth profile and total amount of tritium charged thermally into stainless steel were examined by -ray-induced X-ray spectrometry (BIXS) and a chemical etching method. The former method is able to measure nondestructively tritium trapped in surface layers of materials and it is very sensitive for changes in the surface tritium concentration, while the later method is a conventional method and it enable to evaluate the total distribution and amount of tritium in materials. When stainless steel was exposed to tritium gas under the given temperature and pressure conditions, both analysis Methods indicated that tritium concentration in the bulk was almost uniform, but the enrichment of tritium appeared in surface layers thinner than 200 nm. The tritium concentration in surface layers was higher 100 times and more than that in bulk. Namely, the former tritium distribution indicates that an equilibrium state of dissolution was achieved, while the latter one indicates that a large amount of trapping sites existed in surface layers. Such a different tritium distribution has a possibility affecting the dynamic behavior of tritium in materials. (authors)