Synergistic effect of transmutation rhenium/osmium and vacancy clusters on hydrogen retention in tungsten

Tritium (T) self-sustainability in fusion reactors demands minimal T retention in plasma-facing materials (PFMs) tungsten (W). While vacancy clusters in W act as strong trapping sites for hydrogen (H) isotopes T, their behavior is significantly altered in a fusion environment where transmutation elements rhenium (Re) and osmium (Os) segregate around these defects. Using systematic ab initio and Object Kinetic Monte Carlo simulations, we investigate the segregation behavior of Re/Os near vacancy clusters and their synergistic impact on H retention in W. Our results reveal that both Re and Os energetically favor segregation around vacancy clusters, with Os exhibiting stronger binding and a more persistent segregation tendency. Crucially, Re/Os decoration significantly reduces the capacity of vacancy clusters to trap H atoms, lowering both the total and incremental binding energies of H. This effect is concentration-dependent, becoming more pronounced at higher Re/Os levels. Moreover, Re/Os segregation reduces the desorption temperature of H from vacancy clusters, facilitating H release and thereby decreasing overall H retention in W. These findings explain the experimental phenomena why H retention in W–Re alloys is orders-of-magnitude lower than in pure W after high-temperature irradiation. These results provide atomic-scale insights into the H retention mechanism in W and offer valuable guidance for designing advanced W-based PFMs with low H inventory.