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

TL;DR

This paper introduces a computational model for hydrogen isotope diffusion and retention in irradiated tungsten and vanadium, accounting for multi-occupancy traps, and validates it against experimental data.

Contribution

It develops a multi-occupancy, multi-isotope diffusion model based on first principles and demonstrates its accuracy without fitting parameters.

Findings

Effective diffusivity varies with mobile fraction and trap density.

Non-monotonic diffusivity dependence in vanadium due to vacancy binding.

Model aligns well with isotope exchange experiments.

Abstract

We propose a computational scheme for the diffusion and retention of multiple hydrogen isotopes (HI) with multi-occupancy traps parameterized 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 generalisation of the Oriani equilibrium for multiple isotopes and multi-occupancy traps. The effective 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 non-monotonic 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.