Paper TF-TuP6
Study of Point Defects in Uranium Oxide by Ab-Initio and Semi-Empirical Calculations

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Uranium oxide is used as the standard nuclear fuel in pressurized water reactors. Point defects have an important effect on the physical properties of the fuel as they can cause swelling of the material and change the crystal structure thereby reducing the fuel performance. The aim is to understand the stability of these defects while including their correct electronic structure. Here, density functional theory calculations using the local spin density approximation with the Hubbard U correction term, or the L(S)DA+U method, is used in combination with thermodynamic approaches to calculate the formation energy of point defects present in UO₂. The nudged elastic band method is used to calculate the migration energies. These calculations are supported by semi-empirical simulations using two different potentials and larger supercells. We have been able to predict the correct electronic structure of UO₂ which allows us to consider charged defects. We predict the defect formation energies of neutral uranium vacancy and interstitial to be 4.2 eV and 7.29 eV respectively. For the oxygen vacancy and interstitial, we predict the values to be 6.33 eV and -2.71 eV respectively. The calculations thus predict that the oxygen Frenkel pair complex is the dominant defect in UO₂, which is also what is observed experimentally. We observe a similar trend with our semi-empirical calculations. We will discuss the effect of temperature, pressure and microstructural features, such as grain boundaries, on the defect formation energies.