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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. [ABSTRACT FROM AUTHOR]

The interaction of atomic and molecular hydrogen with actinide dioxide (AnO2, An = U, Np, Pu) (111) surfaces has been investigated by DFT+U, where noncollinear 3k antiferromagnetic behaviour and spin-orbit interactions are considered. The adsorption of atomic hydrogen forms a hydroxide group, coupled to the reduction of an actinide ion. The energy of atomic hydrogen adsorption on the UO2 (0.82 eV), NpO2 (−0.10 eV), and PuO2 (−1.25 eV) surfaces has been calculated. The dissociation of molecular hydrogen is not observed, shown to be due to kinetic rather than thermodynamic factors. As a barrier to the formation of a second hydroxyl group, an unusual charge distribution has been shown. This could be a limitation of a (1·1) unit cell method or an artefact of the systems. The recombination of hydrogen ions on the AnO2 (111) surfaces is favoured over hydroxide formation. [ABSTRACT FROM AUTHOR]

The presence of cubic PuH2 and PuH3, the products of hydrogen corrosion of Pu, during long-term storage is of concern because of the materials' pyrophoricity and ability to catalyse the oxidation reaction of Pu to form PuO2. Here, we modelled cubic PuH2 and PuH3 using Density Functional Theory (DFT) and assessed the performance of the PBEsol+U+SOC (0 ≤ U ≤ 7 eV) including van der Waals dispersion using the Grimme D3 method and the hybrid HSE06sol+SOC. We investigated the structural, magnetic and electronic properties of the cubic hydride phases. We considered spin–orbit coupling (SOC) and non-collinear magnetism to study ferromagnetic (FM), longitudinal and transverse antiferromagnetic (AFM) orders aligned in the <100>, <110> and <111> directions. The hybrid DFT confirmed that FM orders in the <110> and <111> directions were the most stable for cubic PuH2 and PuH3, respectively. For the standard DFT, the most stable magnetic order is dependent on the value of U used, with transitions in the magnetic order at higher U values (U > 5 eV) seen for both PuH2 and PuH3. [ABSTRACT FROM AUTHOR]

Thorium dioxide (ThO2) is a promising alternative to mixed-oxide nuclear fuels due to its longer fuel cycle and resistance to proliferation. Understanding the thermal properties, in particular the thermal conductivity, under reactor conditions is critical to the success of any candidate fuel material. ThO2 has a higher thermal conductivity and thus a lower operating temperature than other fuel systems. However, the presence of defects in real materials directly influences the structural dynamics and physical properties, and the impact of defects on the properties of ThO2 is largely unexplored. We have employed density-functional theory calculations to study the structure and energetics of the intrinsic Schottky and Frenkel defects in ThO2 and their impact on the thermophysical properties. We identify the anion Frenkel defect to be the most stable, and we identify characteristic spectral signatures of the defects in the phonon dispersions and infrared spectra. We further employ two approximate models to assess the impact of the defects on the thermal transport and find that both types of defect are predicted significantly to reduce the thermal conductivity. The methodology we present facilitates the prediction of the thermophysical and transport properties of defective materials at an atomistic level, and should be readily transferrable to existing and in-development nuclear fuel systems. [ABSTRACT FROM AUTHOR]

The results of a combined study into a new class of kinetic inhibitor of gas hydrate formation are presented. The inhibitors are based on quaternary ammonium zwitterions, and are identified from a computational screen.

The magnetic structure of the actinide dioxides (AnO2) remains a field of intense research. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic ground-state, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3̅m (No. 166) has been established for NpO2. Band structure calculations have been performed to analyse these results. [ABSTRACT FROM AUTHOR]

Oxidation of UO2 in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U2O5 and U3O8 using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U3O8, P21/mβ-U3O8 and P6̅2mγ-U3O8 by including C222 α-U3O8, Cmcmβ-U3O8 and Pnmaδ-U2O5. All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U2O5 contains exclusively U5+ ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U3O8 polymorph modelled we predict U5+ ions in pentagonal bipyramidal coordination and U6+ in octahedral coordination, with no U4+ present. The elastic constants of each phase have been calculated and the bulk modulus is found to be inversely proportional to the volume per uranium ion. Finally, a number of thermodynamic properties are estimated, showing general agreement with available experiments; for example α- and β-U3O8 are predicted to be stable at low temperatures but β-U3O8 and γ-U3O8 dominate at high temperature and high pressure respectively. [ABSTRACT FROM AUTHOR]

Oxidation of UO2 in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U2O5 and U3O8 using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U3O8, P21/mβ-U3O8 and P6̅2mγ-U3O8 by including C222 α-U3O8, Cmcmβ-U3O8 and Pnmaδ-U2O5. All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U2O5 contains exclusively U5+ ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U3O8 polymorph modelled we predict U5+ ions in pentagonal bipyramidal coordination and U6+ in octahedral coordination, with no U4+ present. The elastic constants of each phase have been calculated and the bulk modulus is found to be inversely proportional to the volume per uranium ion. Finally, a number of thermodynamic properties are estimated, showing general agreement with available experiments; for example α- and β-U3O8 are predicted to be stable at low temperatures but β-U3O8 and γ-U3O8 dominate at high temperature and high pressure respectively. [ABSTRACT FROM AUTHOR]

The magnetic structure of the actinide dioxides (AnO2) remains a field of intense research. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic ground-state, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3[combining macron]m (No. 166) has been established for NpO2. Band structure calculations have been performed to analyse these results.