The mechanism on retention of hydrogen in three representative tungsten nitride compounds in nuclear fusion reactors

ABSTRACT Nitrogen gas seeding is considered as an effective strategy to reduce the edge plasma temperature. The tungsten nitride compounds, which could form on the top of tungsten (W) first wall due to nitrogen gas seeding, may act as a barrier to hinder the diffusion of hydrogen (H) from the tungsten nitride layer to W first wall. By performing first-principles calculations, we investigate the dissolution, diffusion and retention behaviors of H in W2N1, W1N1 and W2N3, respectively. It is found that a H atom prefers to dissolve in tungsten nitride compounds than in W crystal. Hence the compounds can act as a barrier against H diffusion to W first wall, and probably reducing the hydrogen retention in W first wall. All of these behaviors are essentially correlated with the formation of strong N-H bonding in the compounds once H atoms dissolve in them. Moreover, our calculations indcate that H atoms do not form hydrogen molecules even within a vacancy, but stay separately from each other due to the formation of strong N-H bonding. Our work not only proposes that tungsten nitride compounds can significantly reduce hydrogen retention in W first wall, but provides a fundamental insight in understanding the interactions of H with tungsten nitride compounds.