The Mobility of Small, High-pressure Helium Bubbles in Tungsten via Frenkel Pair Nucleation and Annihilation*

Fusion reactor environments invariably lead to the formation of helium bubbles for which the nucleation, growth, and diffusion strongly impact plasma-facing component performance. This research characterizes a diffusion mechanism of small, over-pressurized bubbles in tungsten, which proceeds via successive Frenkel pair nucleation and annihilation, as a function of helium content, vacancy cluster size, and number of self-interstitials. Molecular dynamics was used to directly simulate the diffusion of bubbles at high-temperature with helium-per-vacancy ratios of 4.5-7. It is found that bubbles are most mobile when the Frenkel pair nucleation/annihilation rates are nearly equal and when the bubbles nucleate and annihilate a single self-interstitial. Peak diffusivity can be as high at 10^-11 m²s^-1, and decreases with size and temperature. The kinetics and energetics of bubble Frenkel pair nucleation/annihilation are described and a model for bubble diffusivity is developed which produces diffusion coefficients in good agreement with those calculated from molecular dynamics. These results provide valuable insight into the mobility of helium bubbles in plasma-facing components and input for higher-order modeling of the near-surface region of materials during plasma-surface interactions (LA-UR-20-30368).