Influence of helium bubbles location on hydrogen isotope retention and exchange behavior in plasma-facing materials: a numerical simulation investigation

Tritium (T) is a costly radioactive element that, when retained in plasma-facing materials (PFMs), not only results in fuel loss but also raises issues of radioactive contamination. Hydrogen isotope exchange is a potential method for T removal in future fusion devices. However, in the nuclear fusion environment, PFMs will be subjected to low-energy and high-flux helium (He) plasma irradiation, forming a He bubble layer near the material surface. This greatly impacts the diffusion and retention behavior of hydrogen isotopes in PFMs. In this work, a multi-component hydrogen isotopes exchange model was developed considering the presence of He bubble layers. The influence of He bubbles location on hydrogen isotope retention and exchange behavior was investigated. It is found that when the hydrogen (H) particles implanted outside the He bubble layer, H entering the material would diffuse across the He bubble layer, resulting in a decrease of H amount entering the bulk. In other words, a thicker He bubble layer leads to lower H retention. As to isotope exchange, when there are deuterium (D) retained in the material, implanted H has a possibility to exchange with D, making D release from the materials. However, the D atoms exchanged out by H also need to diffuse across the He bubble layer, further reducing the D release rate. The results showed the barrier effect of the He bubble layer can have two distinct effects on hydrogen isotope exchange. One is that with enhancing the effect of the He bubble layer, the number of H entering the bulk will increase, resulting in an increase in the D removal. The other effect is that with strengthening the barrier effect of the He bubble layer, the D atoms exchanged out will be more inclined to diffuse into the bulk, leading to a decrease in the D removal. As a result, the D removal efficiency exhibits a peak considering the barrier effect of He bubble.