Elucidating the initial steps in {\alpha}-uranium hydriding using first-principles calculations

TL;DR

This study uses first-principles calculations to explore the molecular mechanisms of hydrogen embrittlement in { extalpha}-uranium, focusing on surface adsorption, subsurface absorption, and diffusion, with implications for safety and mitigation.

Contribution

It provides a detailed atomic-level investigation of hydriding mechanisms in { extalpha}-uranium, highlighting the roles of surface coverage and strain, which were previously overlooked.

Findings

Hydrogen adsorption and absorption are influenced by surface facets.

Tensile strain accelerates hydriding kinetics.

Surface and subsurface processes are critical in initial hydriding stages.

Abstract

Hydrogen embrittlement of uranium, which arises due to the formation of a structurally weak pyrophoric hydride, poses a major safety risk in material applications. Previous experiments have shown that hydriding begins on top or near the surface (i.e., subsurface) of a-uranium. However, the fundamental molecular-level mechanism of this process remains unknown. In this work, starting from pristine {\alpha}-U bulk and surfaces, we present a systematic investigation of possible mechanisms for formation of the metal hydride. Specifically, we address this problem by examining the individual steps of hydrogen embrittlement, including surface adsorption, subsurface absorption, and the inter-layer diffusion of atomic hydrogen. Furthermore, by examining these processes across different facets, we highlight the importance of both (1) hydrogen monolayer coverage and (2) applied tensile strain on hydriding kinetics. Taken together, by studying previously overlooked phenomena, this study provides foundational insights into the initial steps of this overall complex process. We anticipate that this work will guide near-term future development of multiscale kinetic models for uranium hydriding and subsequently, identify potential strategies to mitigate this undesired process.