A vacancy dynamic evolution model based on rate theory and its influence on hydrogen isotope retention behavior

In fusion environments, plasma-facing materials are subjected to the bombardment by high-energy neutrons and large fluxes plasmas, resulting in the formation of numerous irradiation defects. These defects act as trapping sites for fuel hydrogen isotopes, increasing the tritium retention. Under thermal loads at high temperatures, these defects undergo dynamic changes, leading to complex effects on hydrogen isotope transport behaviors. Understanding the evolution of defects at different temperatures and elucidating their impact on hydrogen isotope retention is essential. In this work, a dynamic evolution model of typical vacancy defects and their clusters was constructed based on rate theory and integrated into the hydrogen isotope transport model. The simulation results were compared with experimental data, showing a good agreement between the two. Further results indicated that increasing the annealing temperature reduces defect concentrations. At high temperatures, large vacancy clusters dissociate into smaller ones, with some smaller clusters annihilating and others associating to form larger clusters. In addition, the processes of vacancy clusters association and dissociation dominate this evolution. Regarding hydrogen isotope retention, defect evolution affects the concentration of deuterium (D) atoms within the material. After D is introduced into tungsten, the concentration of D corresponds to the concentration of vacancy clusters. It is found that during thermal desorption, the desorption peak corresponding to the largest vacancy clusters is more pronounced, while other clusters show less significant peaks. This is because some of the D atoms de-trapped from the vacancy clusters are released to the material surface, while others are re-trapped by larger vacancy clusters. The findings of this work will contribute to a more accurate assessment of hydrogen isotope retention under defect dynamic evolution in future fusion reactors.