MD simulation of the He bubble effect on H retention in BCC iron

Abstract

Understanding the interaction between hydrogen (H) and helium (He) bubbles in reduced activation ferritic-martensitic (RAFM) steels is of vital importance for future fusion reactors. Molecular dynamics (MD) simulations have been used to evaluate the evolution of H near nano-sized He bubbles in body-centered cubic (BCC) iron. The effects of He-to-vacancy (He/V) ratio (0.5 to 1.5), bubble size (1 to 4 nm), H concentration (664 to 2641 appm), and temperature (300 to 723 K) on the H evolution were studied in detail. The simulation results suggest that H prefers to the reside at the bubble interface, with only a small fraction of trapped H inside the bubble. H capture and trapping at He bubbles was found to be a reaction rate-controlled process. H2 molecules were observed inside the He bubbles and the relation between H2 formation and the He/V ratio within the bubble is discussed. The binding energy of atomic H to a 2 nm He bubble with a He/V ratio of 1.0 was calculated to be 0.73 eV by molecular statics (MS), and the corresponding de-trapping energy was 0.8 eV. These results can inform a quantitative estimate of the H trapping capacity of He bubbles for a variety of situations.