Diffusion of oxygen in uranium dioxide: A first-principles investigation

Abstract

Results of ab initio density-functional theory calculations of the migration energies of oxygen vacancies and interstitials in stoichiometric UO 2 are reported. The diffusion of oxygen vacancies in UO 2 is found to be highly anisotropic, and the ͓1 0 0͔ direction is energetically favored. The atomic relaxations play an important role in reducing the migration barriers. Within the generalized gradient approximation ͑GGA͒, we find that the migration energies of the preferred vacancies and interstitials paths are, respectively, 1.18 and 1.09 eV. With the inclusion of the Hubbard U parameter to account for the 5f electron correlations in GGA+ U, the vacancy migration energy is lowered to 1.01 eV while the interstitial migration energy increases slightly to 1.13 eV. We find, however, that the correlation effects have a drastic influence on the mechanism of interstitial migration through the stabilization of Willis-type clusters. Indeed, in contrast to GGA, in GGA+ U there is an inversion of the migration path with the so-called "saddle-point" position being lower in energy than the usual starting position. Thus while the migration barriers are nearly the same in GGA and GGA+ U, the mechanisms are completely different. Our results clearly indicate that both vacancies and interstitials contribute almost equally to the diffusion of oxygen in UO 2 .

Orbital Specificity in the Unoccupied States ofUO2from Resonant Inverse Photoelectron Spectroscopy

“…It seems unlikely that a single effect can explain all of these observations. Now, we present evidence of novel satellites in the Resonant IPES of UO 2 , at yet again a different energy.…”

“…[1,2] Because of the important role that computational simulations play in the pursuit of safety within nuclear power generation, a detailed knowledge of the electronic structure of uranium dioxide is crucial for the benchmarking of these models and theories. [2] Hence, it is not surprising that uranium and uranium dioxide have been widely studied for many decades with an immense variety of theoretical models and electron spectroscopic techniques.…”

“…[1,2] Because of the important role that computational simulations play in the pursuit of safety within nuclear power generation, a detailed knowledge of the electronic structure of uranium dioxide is crucial for the benchmarking of these models and theories. [2] Hence, it is not surprising that uranium and uranium dioxide have been widely studied for many decades with an immense variety of theoretical models and electron spectroscopic techniques. [3] An incomplete sampling of the experimental studies includes Ultraviolet and X-ray Photoelectron Spectroscopy (UPS and XPS) [4][5][6][7][8][9][10], Bremstrahlung Isochromat Spectroscopy (BIS) [5], Inverse Photoelectron Spectroscopy (IPES) [11,12], Electron Energy Loss Spectroscopy (EELS) [8], X-ray Absorption Spectroscopy (XAS) [13 -16], Resonant Photoelectron Spectroscopy (ResPES) [17] and Resonant Inverse Photoelectron Spectroscopy (RIPES).…”

“…Here, a break-though experiment is discussed, which has allowed the direct determination of the U5f and U6d contributions to the unoccupied density of states (UDOS) Uranium Dioxide is the most widely used nuclear fuel for the generation of electrical power. [1,2] Because of the important role that computational simulations play in the pursuit of safety within nuclear power generation, a detailed knowledge of the electronic structure of uranium dioxide is crucial for the benchmarking of these models and theories.…”

“…Here, a break-though experiment is discussed, which has allowed the direct determination of the U5f and U6d contributions to the unoccupied density of states (UDOS) Uranium Dioxide is the most widely used nuclear fuel for the generation of electrical power. [1,2] Because of the important role that computational simulations play in the pursuit of safety within nuclear power generation, a detailed knowledge of the electronic structure of uranium dioxide is crucial for the benchmarking of these models and theories.[2] Hence, it is not surprising that uranium and uranium dioxide have been widely studied for many decades with an immense variety of theoretical models and electron spectroscopic techniques. A common thread through the studies of the core levels, particularly those using Photoelectron Spectroscopy [4][5][6][7][8][9][10], has been the observation of satellite structure in Uranium Dioxide.…”