First-Principles Study of the Hydrogen Resistance Mechanism of PuO₂

The in-depth investigation of hydrogen behaviors in Pu-oxide overlayers (mainly PuO₂ and α-Pu₂O₃) is critical for modeling the complex induction period of Pu hydriding. Within density functional theory (DFT) + U + D3 schemes, our systematic first-principles calculations and ab initio thermodynamic evaluations reveal that the hydrogen incorporation, dissolution behaviors, and diffusion mechanism in PuO₂ are quite different from those in α-Pu₂O₃, among which the highly endothermic incorporation and dissolution of hydrogen are the primary hydrogen resistance mechanism of PuO₂. Since its difficult recombination, atomic H is the preferred existence state in PuO₂, but H will recombine spontaneously in α-Pu₂O₃. In PuO₂, H diffusion is always clinging to O anions, whereas in α-Pu₂O₃, H₂ prefers to migrate along O vacancies with higher barriers. H dissolution in intact PuO₂ is very difficult, which can only be driven by extremely high pressure P_H and temperature. Based on a series of theoretical studies, we conclude that the main interactions between hydrogen and Pu-oxide overlayers are not involved with chemical reactions, and intact PuO₂ can effectively inhibit hydrogen permeation.