Comparison of dynamic deuterium retention in single-crystal and poly-crystals of tungsten: The role of natural defects

Abstract

The time evolution of fuel retention in materials relevant for future fusion reactors is compared for two different tungsten microstructures: single-crystal versus recrystallized poly-crystals. The initial retention of both types of sample is similar. It decays exponentially with a time constant of ~18 h at 300 K (the so-called short-term retention). After 48 h at room temperature, a constant deuterium retention is measured (long-term retention) with the single-crystal containing systematically less deuterium than poly-crystals. Macroscopic rate equations models are built with density functional theory inputs to reproduce deuterium desorption observables with the MHIMS-R code. We found that the native oxide layer could explain the desorption peak located at ~450 K as well as most of the short-term retention in the single-crystal. The native oxide together with, dislocations for single-crystal and grain boundaries for poly-crystals, are responsible for the long-term retention. Dislocations should explain the desorption peak located at ~815 K for mechanically polished samples. The dual role of most of tungsten defects is related to their multi-trapping properties with filling-level-dependent detrapping energies. Finally, the use of an effective diffusivity of deuterium through the native oxide layer, i.e. its diffusion barrier character, is evaluated.