Effects of nitrogen on hydrogen retention in tungsten: First-principles calculations

While the use of nitrogen seeding to reduce the edge plasma temperature has been successfully applied in many tokamak experiments, questions remain as to effects of the nitrogen-enriched layer on hydrogen retention in tungsten. In this paper, we investigate the influence of nitrogen on hydrogen dissolution and diffusion behavior in tungsten using the first-principles. The nitrogen has little effect on the dissolution of hydrogen in the interstitial site but significantly impedes the accumulation of hydrogen in the vacancy, leading to the decrease of hydrogen retention in nitrogen-enriched layer. Meanwhile, both the interstitial nitrogen and the vacancy-nitrogen complex can significantly reduce the hydrogen effective diffusivity. This suggests that the nitrogen-enriched layer forming on the tungsten surface can act as a diffusion barrier for the re-emission of implanted hydrogen, enhancing hydrogen diffusion into the bulk, and consequently, increasing of hydrogen retention in bulk. These results provide a sound explanation for the recent experimental results.