Deuterium diffusion in a chemical densified coating observed by NRA

Ceramic coatings on structural material are expected as a barrier for tritium permeation in fusion devices. A Cr2O3–SiO2–CrPO4 coating on stainless steel by use of a chemical densified coating (CDC) method was experimentally studied. The sample disk was exposed to a deuterium plasma and an analyzing beam of 3He ions irradiated the sample to observe deuterium concentration profiles. It was found that a very long time, typically 10 ks at 563 K, was needed for deuterium to diffuse from the surface to a few μm in depth. The profiles were analyzed with a diffusion model to obtain the diffusion coefficient of deuterium, which was expressed by 1.1 × 10−10exp(−0.71 eV/kT) [m2 s−1] between 470 and 723 K. The diffusion coefficient was about 5 orders of magnitude smaller than that in type 304 stainless steel at 563 K. The CDC coating is expected to significantly delay tritium permeation.