Modeling the oxidation surface reaction on uranium metal

The chemistry of γ-uranium is of primary importance for metal-fueled fast reactors which have been suggested as a key component of future fuel cycles. However, despite the fascinating chemistry of uranium, experimental difficulties have limited the scope of studies until recent years when theoretical methods have begun to adequately describe the correlated f-shell electrons of the lower actinides. In particular, the surface reactions of γ-uranium have been little studied using theoretical methods. Prior work in surface reaction simulation is briefly reviewed and a possible reaction mechanism for γ-uranium surface oxidation and dissolution are discussed. Results for surface structure and chemical adsorption based on investigation using the plane-wave pseudopotential formulation of density functional theory are presented. Specifically, belying previous work which exclusively focused on the (1 0 0) surface of γ-uranium, the (1 1 0) surface is found to be more stable. The (2 1 1) surface is also of commensurate energy. The calculated surface energies are towards the low end of the experimental range (0.8–1.6 J·m^−2), but the trend seems solid: the energy of the relaxed (1 0 0) surface is found to be 0.981 J·m^−2; that of (1 1 0), 0.886 J·m^−2; that of (2 1 1), 0.952 J·m^−2. The shape of an equilibrium crystal is considered as well. Adsorption of atomic hydrogen and molecular oxygen were studied as well. The atomic hydrogen optimal adsorption energies on the (1 0 0), (1 1 0), and (2 1 1) surfaces, respectively, are 4.1 eV, 5.5 eV, and 4.7 eV. For molecular oxygen monolayer deposition, dissociative adsorption occurs on the (1 1 0) and (2 1 1) surfaces, with energies of 3.2 eV, 6.5 eV, and 5.5 eV on the (1 0 0), (1 1 0), and (2 1 1) surfaces, respectively. The relative differences in adsorption site minima on each surface are small, indicating that surface exposure is a more important factor in kinetics than absolute differences between adsorption sites on a given surface. A possible hydrogen diffusion mechanism on the (1 0 0) surface is identified, with a naive (unrelaxed) energy barrier of 6.9 eV. Finally, directions for further theoretical and experimental inquiry into γ-uranium surface chemistry are suggested