Interaction of hydrogen with actinide dioxide (011) surfaces

The corrosion and oxidation of actinide metals, leading to the formation of metal-oxide surface layers with the catalytic evolution of hydrogen, impacts the management of nuclear materials. Here, the interaction of hydrogen with actinide dioxide (AnO₂, An = U, Np, or Pu) (011) surfaces by Hubbard corrected density functional theory (PBEsol+U) has been studied, including spin-orbit interactions and non-collinear 3k anti-ferromagnetic behavior. The actinide dioxides crystalize in the fluorite-type structure, and although the (111) surface dominates the crystal morphology, the (011) surface energetics may lead to more significant interaction with hydrogen. The dissociative adsorption of hydrogen on the UO₂ (0.44 eV), NpO₂ (-0.47 eV), and PuO₂ (-1.71 eV) (011) surfaces has been calculated. It is found that hydrogen dissociates on the PuO₂ (011) surface; however, UO₂ (011) and NpO₂ (011) surfaces are relatively inert. Recombination of hydrogen ions is likely to occur on the UO₂ (011) and NpO₂ (011) surfaces, whereas hydroxide formation is shown to occur on the PuO₂ (011) surface, which distorts the surface structure.

Impact of Hydrogen on the Intermediate Oxygen Clusters and Diffusion in Fluorite Structured UO2+ x

Uranium dioxide is the most prevalent nuclear fuel. Defect clusters are known to be present in significant concentrations in hyperstoichoimetric uranium oxide, UO_{2+ x}, and have a significant impact on the corrosion of the material. A detailed understanding of the defect clusters that form is required for accurate diffusion models in UO_{2+ x}. Using ab initio calculations, we show that at low excess oxygen concentration, where defects are mostly isolated oxygen interstitials, hydrogen stabilizes the initial clustering. The simplest cluster at this low excess oxygen stoichiometry consists of a pair of oxygen ions bound to an oxygen vacancy, namely the split mono-interstital, which resembles larger split interstitials clusters in UO_{2+ x}. Our data shows that, depending on local hydrogen concertation, the presence of hydrogen stabilizes this cluster over isolated oxygen interstitials.

Phase Segregation, Transition, or New Phase Formation of Plutonium Dioxide: The Roles of Transition Metals

As impurities are virtually impossible to exclude from Pu oxides in realistic environments, understanding the roles of impurities is crucial for the applications and designs of Pu oxides. Here we perform a systematic first-principles DFT + U calculation to find the trends of transition-metal (TM) behaviors in PuO₂ in terms of energetics, atomic properties, oxidation states, and electronic structures. The results show that group IV-B elements Ti, Zr, and Hf are energetically and electronically favorable in PuO₂ and render the possibilities of forming Pu-TM-O ternary phases. In contrast, the remaining TMs tend to destabilize PuO₂ and whether phase segregation or transition occurs largely depends on the redox conditions: oxidation one induces segregation, whereas reduction one facilitates the transition from PuO₂ to Pu₂O₃. On the basis of the correlations between the properties of TMs and their relative stabilities in PuO₂, we conclude that the degree of electron match between TMs and Pu plays the decisive role in the stability, as established for the cases of tetravalent elements, whereas some electron-mismatched but energetically stable TMs such as III-B and V-B elements could drive the valence transition of Pu, resulting in the phase instability of PuO₂.

Theoretical prediction of some layered Pa2O5 phases: structure and properties

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure. Although the layered structure has been observed with the isostructural transition Nb and Ta metal pentoxides experimentally, the detailed structure and properties of the layered Pa₂O₅ are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa₂O₅ stoichiometry according to the existing M₂O₅ structures (where M is an actinide Np or transition Nb, Ta, and V metal) and replacing the M ions with protactinium ions. The structural, mechanical, thermodynamic and electronic properties including lattice parameters, bulk moduli, elastic constants, entropy and band gaps were predicted for all the simulated structures. Pa₂O₅ in the β-V₂O₅ structure was found to be a competitive structure in terms of stability, whereas Pa₂O₅ in the ζ-Nb₂O₅ structure was found to be the most stable overall. This is consistent with Sellers's experimental observations. In particular, Pa₂O₅ in the ζ-Nb₂O₅ structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb₂O₅-structured Pa₂O₅ is the most thermodynamically stable under ambient conditions and pressure.

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure.