Electronic and Geometric contributors to Hydrogen Binding in Uranium Oxide Grain Boundaries

Hydrogen induced corrosion of uranium, which leads to the formation of toxic and pyrophoric \ce{UH3}, raises significant safety concerns for long-term storage of nuclear materials. Previous work suggests hydrogen diffuses through the top passivating oxide layer to initiate hydriding reactions within the underlying grain boundaries (GB). However, the atomistic mechanisms underlying this phenomenon and the structural factors that control its initiation are not well understood. To address this knowledge gap, here we use a high-throughput density function theory (DFT) workflow to investigate the adsorption of H and \ce{H2} in the defective bulk \ce{UO2}. Specifically, we exhaustively investigated the adsorption of H (107 sites) and \ce{H2} (26 sites) in three different coincidence site lattice (CSL) GBs: $\Sigma3$, $\Sigma5$, and $\Sigma9$. Compared to the binding energies in pristine \ce{UO2}, we observe significantly stronger hydrogen adsorption at these GB sites. Interestingly, we find that the trends in H and \ce{H2} adsorption vary considerably across the three GB models. In particular, while a small number of sites in $\Sigma5$ and $\Sigma9$ show exothermic adsorption of H and \ce{H2}, respectively, no such sites are found in $\Sigma3$. Further statistical analysis of these trends suggests that H adsorption, which is adsorbed as a hydride anion (i.e., \ce{H-}), is positively correlated with the value of its negative charge and depends on the number of neighboring oxygen and uranium atoms. Together, these results provide fundamental atomistic insights that could guide the development of future corrosion mitigation strategies for the storage of nuclear materials.