The Corrosion of UO2 Versus ThO2: a Quantum Mechanical Investigation

Quantum mechanical surface energy calculations have been performed on both uranium dioxide (UO2) and thorium dioxide (ThO2) (111), (110), and (100) surfaces to determine their relative reactivities. While UO2 and ThO2 both have the fluorite structure Fm3m, they differ in that uranium has two dominant oxidation states, U4+ and U6+, while thorium only has one, Th4+. Furthermore, UO2 is an intrinsically weak p-type semi-conductor with a band gap of 2.14 eV (Killeen, 1980), while ThO2 is an insulator. Dissolution and spectroscopic studies indicate that UO2 and ThO2 have different solubilities (Sunder and Miller, 2000). We use the quantum mechanical program, CASTEP (CAmbridge Scientific Total Energy Package) to perform surface and adsorption energy calculations on the (111) surface of both materials, with specific attention to O, H2O, and combined adsorption cases. UO2 and ThO2 bulk unit cells were optimized to find the most stable configuration of atoms. Surface slabs were ''cleaved'' from the relaxed bulk for each orientation, placed in a 10 (angstrom) vacuum gap in order to simulate a free surface and were optimized. Relative surface energy trends and atomic relaxation were compared between the surfaces of UO2 and ThO2. The (111) surface is found to have the most energetically stable configuration of atoms in both cases, although ThO2 has higher surface energy values than UO2 on all three surfaces. The (111) surface slab is doubled in width in order to increase the number of surface sites, and different starting positions for adsorbates are tested in order to calculate the most energetically favorable adsorption sites. Adsorption energy results indicate that adsorption is more favorable on the UO2 (111) surface than the ThO2 (111) surface. Adsorption calculations are accompanied by partial density of state (PDOS) and bandstructure analyses in order to understand the role of electrons during adsorption on semi-conducting versus insulating mineral surfaces