Effect of cationic chemical disorder on defect formation energies in uranium-plutonium mixed oxides

At the atomic scale, uranium-plutonium mixed oxides (U,Pu)O_2 are characterized by cationic chemical disorder, which entails that U and Pu cations are randomly distributed on the cation sublattice. In the present work, we study the impact of disorder on point-defect formation energies in (U,Pu)O_2 using interatomic-potential and Density Functional Theory (DFT+U) calculations. We focus on bound Schottky defects (BSD) that are among the most stable defects in these oxides. As a first step, we estimate the distance R_D around the BSD up to which the local chemical environment significantly affects their formation energy. To this end, we propose an original procedure in which the formation energy is computed for several supercells at varying levels of disorder. We conclude that the first three cation shells around the BSD have a non-negligible influence on their formation energy (R_{D} = 7.0 \{AA}). We apply then a systematic approach to compute the BSD formation energies for all the possible cation configurations on the first and second nearest neighbor shells around the BSD. We show that the formation energy can range in an interval of 0.97 eV, depending on the relative amount of U and Pu neighboring cations. Based on these results, we propose an interaction model that describes the effect of nominal and local composition on the BSD formation energy. Finally, the DFT+U benchmark calculations show a satisfactory agreement for configurations characterized by a U-rich local environment, and a larger mismatch in the case of a Pu-rich one. In summary, this work provides valuable insights on the properties of BSD defects in (U,Pu)O_2, and can represent a valid strategy to study point defect properties in disordered compounds.Comment: 33 pages, 20 figure

Lattice constant in nonstoichiometric uranium dioxide from first principles

International audienceNonstoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, in both the hypostoichiometric and the hyperstoichiometric regimes. Based on first-principles calculations within the density functional theory (DFT)+$U$ approximation, we have developed a point defect model that accounts for the volume of relaxation of the most significant intrinsic defects of UO$_2$. Our point defect model takes special care of the treatment of the charged defects in the equilibration of the model and in the determination of reliable defect volumes of formation. In the hypostoichiometric regime, the oxygen vacancies are dominant and explain the lattice constant variation with their surprisingly positive volume of relaxation. In the hyperstoichiometric regime, the uranium vacancies are predicted to be the dominating defect,in contradiction with experimental observations. However, disregarding uranium vacancies allows us to recover a good match for the lattice-constant variation as a function of stoichiometry. This can be considered a clue that the uranium vacancies are indeed absent in UO$_{2+x}$ , possibly due to the very slow diffusion of uranium

Electron polarons and donor point defects in americium dioxide AmO 2

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Structural, electronic and energetic properties of uranium–americium mixed oxides U1−yAmyO2{\rm U}_{1-y}{\rm Am}_y{\rm O}_2 using DFT+U+{U} calculations

International audienceIn this paper, we determine for the fjrst time the electronic, structural and energetic properties of U 1−y Am y O 2 mixed oxides in the entire range of Am content using the generalized gradient approximation (GGA) +U in combination with the special quasirandom structure (SQS) approach to reproduce chemical disorder. This study reveals that in U 1−y Am y O 2 oxides, Am cations act as electron acceptors, whereas U cations act as electron donors showing a fundamental difference with U 1−y Pu y O 2 or U 1−y Ce y O 2 in which there is no cation valence change in stoichiometric conditions compared to the pure oxides. We show for the fjrst time that the lattice parameter of stoichiometric U 1−y Am y O 2 follows a linear evolution which is the structural signature of an ideal solid solution behavior. Finally, using two approaches (SQS and parametric), we show by assessing the enthalpy of mixing that there is no phase separation in the whole range of Am concentration

Atomic-scale modeling of 1/2 (110){001} edge dislocations in UO2: Core properties and mobility

International audienceThe dislocation properties of UO2 , the main nuclear fuel material, are important ingredients to model the mechanical properties and predict nominal and accidental operations of nuclear plant reactors. However, the plastic behaviour of UO2 is complex with little known about dislocations and other extended defects. In this study, we use a combination of interatomic potentialbased atomistic simulations and ab initio calculations to investigate the core structure and mobility of the 1/2 (110){001} edge dislocation, which controls the plasticity of UO2 single crystals. Various dislocation cores are obtained and compared, including the classical asymmetric Ashbee core and a so-far unreported core made of an alternation of both variants of the Ashbee core along the dislocation line. This new core, called here zigzag, is ubiquitous in molecular dynamics simulations at high temperature in the nominal-toaccidental transient regime (1600 to 2200 K). Molecular dynamics is also used to determine the velocity of the edge dislocation as a function of temperature and stress. A dislocation mobility law is adjusted from the simulations and provides an up-scaling ingredient central to the multi-scale modeling of UO2 nuclear fuel mechanical properties

Experimental and ab initio study of the O3_3 detection at the CuO (111) surface

International audienceCombining experiments and first-principles calculations, we present in this paper a detailed study of the O$_3$ detection mechanism on the CuO (111) surface. The exchange-correlation functional is treated within both the LDA and the GGA including the spin polarization. In order to better take into account the on-site electronic interactions between $3d$ electrons of Cu atoms a Hubbard term U has to be added in all calculations. We show that the O$_3$ molecule is reduced to a O$_2$ molecule with an enthalpy of reaction of −1.11 eV (−1.15 eV) within LDA+U (GGA+U). Along the reaction path, the O$_3$ molecules are first physisorbed with a large adsorption energy of −1.83 eV (−1.03 eV) and a significant charge transfer from the surface to the molecule. The p-doping strengthening is compared to the electrical response of a CuO based sensor under O$_3$ exposure

Qualification of the MEXIICO loop dedicated to nuclear power transients: An experimental and modelling approach

International audienceThe MEXIICO project carried out by the CEA and supported by EDF and AREVA has the objective to characterize the behaviour of irradiated fuel pellet issued from nuclear water reactor during power transients. The MEXIICO experimental loop has been recently implemented in the LECA-STAR facility of the CEA Cadarache. It will allow studying the fuel fragmentation of nuclear fuels submitted to various temperature and pressure histories (up to1600 °C and 1600 bar) using the monitoring of released $^{85}$Kr activity during the experiment. Since the fission gas release is measured thanks to a gamma detector located in the rear cell, while the nuclear fuel sample is located inside the MEXIICO furnace in the hot cell, it is necessary to take into account the residence time of the gas in the loop to accurately correlate fission gas release to the local temperature and pressure conditions of the sample, which are also time dependent. In this paper, we will compare a thermal hydraulic approach mixing both an analytical method and a numerical CFD simulation to experimental test results. This modelling of the MEXIICO loop will support the interpretation of future tests, and will allow, more precisely, to determine the fuel fragmentation thresholds for various stress conditions

Atomic scale investigation of thermodynamic and defect properties of (U,Pu)O2 mixed oxide

International audienceOne way of increasing significantly the efficiency in designing and qualifying innovative fuels is to enhance the predictive capability of fuel behaviour simulation by developing a more physically based description of nuclear fuels. Basic research approaches combining multiscale modelling and separate effect experiments can bring significant insight into materials properties and key phenomena involved in the evolution of fuels in reactor.We will show the results obtained using state-of-the art electronic structure and empirical calculations on the uranium-plutonium mixed oxide. In particular, the thermal expansion, enthalpy increments and specific heat of (U,Pu)O2 as a function of Pu content will be presented. The defect properties of (U,Pu)O2 and the impact of the disorder on the cationic sublattice will also be discussed

GGA+U study of uranium mononitride: A comparison of the U-ramping and occupation matrix schemes and incorporation energies of fission products

International audienceUranium mononitride is studied in the DFT + U framework. Its ground state is investigated and a study of the incorporation of diverse fission products in the crystal is conducted. The U-ramping and occupation matrix control (OMC) schemes are used to eliminate metastable states. Beyond a certain amount of introduced correlation, the OMC scheme starts to find a lower total energy. The OMC scheme is chosen for the second part of this study. Furthermore, the influence of the magnetic ordering is studied using the U-ramping method, showing that antiferromagnetic order is the most stable one when the U parameter is larger than 1.75 eV. The effect on the density of states is investigated and elastic constants are provided for comparison with other methods and experiments. The incorporation energies of fission products in different defect configurations are calculated and these energies are corrected to take into account the limited size of the supercell