Response of the UO2 (111) Surface to Varying Oxygen Partial Pressure

            UO₂ corrosion is a significant concern for the nuclear power industry as well as for the weapons industry.  Oxidation of UO₂ can lead to significant expansion of the material, which in turn, can cause cracking and spallation.  While limited studies of oxidation and reactivity of bulk UO₂ and the UO₂ (111) surface have been performed using periodic density functional theory (DFT), surface and subsurface oxidation as a function of temperature and pressure has not been explored.  We present periodic DFT calculations of the clean and oxidized UO₂ (111) surfaces with and without subsurface oxidation.  Generally, the generalized gradient approximation (GGA) formulation used in this work gives highly exothermic oxidation energies, which is consistent with experimental data and in contrast to previous GGA+U studies, demonstrating that GGA describes the energetics of this system better.  Using ab initio thermodynamics, the temperatures and pressures needed to oxidize the surface and perform initial subsurface oxidation have been calculated.  We also analyze the uranium magnetic and electronic structure as well as local geometry in response to oxidation.