The permeation of tritium through 316L stainless steel with multiple coatings

TiN + TiC + TiN and TiN + TiC + SiO2 films were deposited on the surface of 316L austenitic stainless steel by means of physical vapor deposition (PVD). The thickness of the films was about 2–3 μm. The film is compact, oxidation-resistant, and has good adherence with the substrate below 500°C. Tritium gas permeation of 316L with multiple films was examined, and it was found that the tritium permeability in 316L with a TiN + TiC + TiN film was 4–5 orders of magnitude lower, and in 316L with a TiN + TiC + SiO2 film was 4–6 orders of magnitude lower than that in 316L with a Pd film at about 200–500°C. At about 600°C, the permeability of 316L with the multiple coating was 3–4 orders of magnitude lower than that in 316L with a Pd film. The result shows that the tritium permeation barrier is formed by multiple coating above 300°C, and it is stable below 500°C. However, the barrier is partly destroyed at about 600°C because of oxidation; although this results in degradation of the barrier, it still plays a positive role. These films may be useful as coatings for the first wall, tritium blanket, and heat exchanger in fusion reactors for tritium permeation resistance.