Effect of Re-Deposition Layers in Plasma-Facing Wall on Tritium Retention and Tritium Depth Profile

Effect of the re-deposition layers formed on plasma-exposed stainless steel type 316L (SS316L) in QUEST on the retention and depth profile of tritium has been studied by both methods of tritium exposure experiments and numerical analyses of X-ray spectra observed by the β-ray-induced X-ray spectrometry (BIXS). Both samples of plasma-exposed and non-exposed SS316L were exposed to tritium gas under given temperature, time and pressure conditions. Surface of the former sample was covered with re-deposition layers after exposing to the plasma experiments. After tritium exposure, X-ray spectra induced by β-rays emitted from tritium atoms retained in the surface layers and/or dissolved into the bulk were measured using an ultra-low energy X-ray detector consisting of pure Ge semiconductor, and numerical analysis of the observed spectrum was conducted to estimate a tritium depth profile in the sample. As a result, it was found that the amount of tritium in surface layers of the plasma-exposed sample was about five times larger than that of the non-exposed sample, and the tritium depth profile for the plasma-exposed sample was about half depth in comparison with that for the non-exposed sample although the degassing temperature and tritium exposure conditions were the same for both samples. It was suggested, therefore, that the re-deposition layers played a role of diffusion barrier of tritium atoms formed on the sample surface.