title: Determination of Krypton Diffusion Coefficients in Uranium Dioxide Using Atomic Scale Calculations.
abstract: We present a study of the diffusion of krypton in UO2using atomic scale calculations combined with diffusion models adapted to the system studied. The migration barriers of the elementary mechanisms for interstitial or vacancy assisted migration are calculated in the DFT+U framework using the nudged elastic band method. The attempt frequencies are obtained from the phonon modes of the defect at the initial and saddle points using empirical potential methods. The diffusion coefficients of Kr in UO2are then calculated by combining this data with diffusion models accounting for the concentration of vacancies and the interaction of vacancies with Kr atoms. We determined the preferred mechanism for Kr migration and the corresponding diffusion coefficient as a function of the oxygen chemical potential μOor nonstoichiometry. For very hypostoichiometric (or U-rich) conditions, the most favorable mechanism is interstitial migration. For hypostoichiometric UO2, migration is assisted by the bound Schottky defect and the charged uranium vacancy, VU4-. Around stoichiometry, migration assisted by the charged uranium-oxygen divacancy (VUO2-) and VU4-is the favored mechanism. Finally, for hyperstoichiometric or O-rich conditions, the migration assisted by two VU4-dominates. Kr migration is enhanced at higher μO, and in this regime, the activation energy will be between 4.09 and 0.73 eV depending on nonstoichiometry. The experimental values available are in the latter interval. Since it is very probable that these values were obtained for at least slightly hyperstoichiometric samples, our activation energies are consistent with the experimental data, even if further experiments with precisely controlled stoichiometry are needed to confirm these results. The mechanisms and trends with nonstoichiometry established for Kr are similar to those found in previous studies of Xe.
authors.affiliation: CEA , DEN, DEC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France.
authors.forename: Emerson
authors.initials: E
authors.lastname: Vathonne
authors.affiliation: Materials Science and Technology Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Andersson
authors.affiliation: CEA , DEN, DEC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Freyss
authors.affiliation: Materials Science and Technology Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Romain
authors.initials: R
authors.lastname: Perriot
authors.affiliation: Materials Science and Technology Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Michael W D
authors.initials: MW
authors.lastname: Cooper
authors.affiliation: Materials Science and Technology Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Stanek
authors.affiliation: CEA , DEN, DEC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France.
authors.forename: Marjorie
authors.identifier: http://orcid.org/0000-0001-6598-5312
authors.initials: M
authors.lastname: Bertolus