Effect of multi-occupancy traps on the diffusion and retention of multiple hydrogen isotopes in irradiated tungsten and vanadium

We propose a computational scheme for the diffusion and retention of multiple hydrogen isotopes (HI) with multi-occupancy traps parametrized by first principles calculations. We show that it is often acceptable to reduce the complexity of the coupled differential equations for gas evolution by taking the dynamic steady state, a generalization of the Oriani equilibrium for multiple isotopes and multi-occupancy traps. The gas diffusivity varies most with mobile fraction when the total gas concentration approximates the trap density. We show HI binding to a monovacancy in vanadium produces a nonmonotonic dependence between diffusivity and gas concentration, unlike the tungsten system. We demonstrate the difference between multiple single occupancy traps and multi-occupancy traps in long-term diffusion dynamics. The applicability of the multi-occupancy, multi-isotope model in steady state is assessed by comparison to an isotope exchange experiment between hydrogen and deuterium in self-ion irradiated tungsten. The vacancy distribution is estimated with molecular dynamics, and the retention across sample depth shows good agreement with experiment using no fitting parameters.