(563a) 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.