The Experts below are selected from a list of 4257 Experts worldwide ranked by ideXlab platform
Dario Manara - One of the best experts on this subject based on the ideXlab platform.
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Melting behaviour of americium-doped Uranium Dioxide
The Journal of Chemical Thermodynamics, 2016Co-Authors: Damien Prieur, Florent Lebreton, Marie Caisso, Philippe Martin, Andreas C. Scheinost, Thibaud Delahaye, Dario ManaraAbstract:Abstract (Uranium + americium) mixed oxides are considered as potential targets for americium transmutation in fast neutron reactors. Their thermophysical properties and notably their melting behaviour have not been assessed properly although required in order to evaluate the safety of these compounds under irradiation. In this study, we measured via laser heating, the melting points under inert atmosphere (Ar) of U 1− x Am x O 2± δ samples with x = 0.10, 0.15, 0.20. The obtained melting/solidification temperatures, measured here, indicate that under the current experimental conditions in the investigated AmO 2 content range, the solidus line of the (UO 2 + AmO 2 ) system follows with very good agreement the ideal solution behaviour. Accordingly, the observed liquidus formation temperature decreases from (3130 ± 20) K for pure UO 2 to (3051 ± 28) K for U 0.8 Am 0.2 O 2± δ . The melted and quenched materials have been characterised by combining X-ray diffraction and X-ray absorption spectroscopy.
Robin W. Grimes - One of the best experts on this subject based on the ideXlab platform.
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Swelling due to the partition of soluble fission products between the grey phase and Uranium Dioxide
Progress in Nuclear Energy, 2014Co-Authors: Michael W.d. Cooper, Simon C. Middleburgh, Robin W. GrimesAbstract:The change in volume associated with the partition of soluble cations from Uranium Dioxide into the (Ba,Sr)ZrO3 grey phase has been investigated using atomic scale simulations. Here past work on the thermodynamic drive for the segregation of trivalent and tetravalent cations from Uranium Dioxide is built upon in the context of fuel swelling. Only small tetravalent cations segregate into the grey phase and this is predicted to result in an overall reduction in fuel volume. Individual trivalent cations that segregate, can cause either a contraction or an expansion of the overall fuel volume. Cr2O3 doped UO2 promotes co-partition forming mixed cation clusters in the grey phase and causing an overall reduction in fuel volume for all trivalent cations. This may have implications for fuel performance and may alter other fuel swelling mechanisms.
Simon C. Middleburgh - One of the best experts on this subject based on the ideXlab platform.
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Thermal conductivity variation in Uranium Dioxide with gadolinia additions
'Elsevier BV', 2020Co-Authors: Mj Qin, Simon C. Middleburgh, E Y Kuo, Cooper Mwd, Rushton Mjd, Puide M, Rw Grimes, Gregory R. LumpkinAbstract:By combining experimental observations on Gd doped fuel with a theoretical understanding, the variation in thermal conductivity with Gd concentration and accommodation mechanism has been modelled. Four types of Gd accommodation mechanisms have been studied. In UO2−x, isolated substitutional Gd3+ ions are compensated by oxygen vacancies and [equation] defect clusters. In UO2, isolated substitutional Gd3+ ions are compensated by U5+ ions and [equation] defect clusters. The results indicate that defect clusters can be considered as less effective phonon scatterers and therefore result in less thermal conductivity degradation. The thermal conductivity predicted for UO2 with [equation] defect clusters is in good agreement with experimental data for UO2 with 5 wt% Gd2O3. This supports the previous theoretical results that Gd is accommodated through [equation] defect clusters in UO2 in the presence of excess oxyge
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Swelling due to the partition of soluble fission products between the grey phase and Uranium Dioxide
Progress in Nuclear Energy, 2014Co-Authors: Michael W.d. Cooper, Simon C. Middleburgh, Robin W. GrimesAbstract:The change in volume associated with the partition of soluble cations from Uranium Dioxide into the (Ba,Sr)ZrO3 grey phase has been investigated using atomic scale simulations. Here past work on the thermodynamic drive for the segregation of trivalent and tetravalent cations from Uranium Dioxide is built upon in the context of fuel swelling. Only small tetravalent cations segregate into the grey phase and this is predicted to result in an overall reduction in fuel volume. Individual trivalent cations that segregate, can cause either a contraction or an expansion of the overall fuel volume. Cr2O3 doped UO2 promotes co-partition forming mixed cation clusters in the grey phase and causing an overall reduction in fuel volume for all trivalent cations. This may have implications for fuel performance and may alter other fuel swelling mechanisms.
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partition of soluble fission products between the grey phase zro2 and Uranium Dioxide
Journal of Nuclear Materials, 2013Co-Authors: Michael W.d. Cooper, Simon C. Middleburgh, Rw GrimesAbstract:Abstract The energies to remove fission products from UO 2 or UO 2+ x and incorporate them into BaZrO 3 , SrZrO 3 (grey phase constituent phases) and ZrO 2 have been calculated using atomistic scale simulation. These energies provide the thermodynamic drive for partition of soluble fission products between UO 2 or UO 2+ x and these secondary oxide constituents of the fuel system. Tetravalent cation partition into BaZrO 3 , SrZrO 3 and ZrO 2 was only preferable for species with smaller radii than Zr 4+ , regardless of Uranium Dioxide stoichiometry. Under stoichiometric conditions both the larger and the smaller trivalent cations were found to segregate to BaZrO 3 but only the smaller fuel additive elements Cr 3+ and Fe 3+ segregate to SrZrO 3 . Partition from UO 2+ x was always unfavourable for trivalent cations. Additions of excess Cr 3+ (as a fuel additive) are predicted make the partition into BaZrO 3 and SrZrO 3 more favourable from UO 2 for the larger trivalent cations. Trivalent fission products with radii smaller than or equal to that of Sm 3+ were identified to segregate into ZrO 2 only from UO 2 . No segregation to SrO or BaO is predicted.
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swelling due to fission products and additives dissolved within the Uranium Dioxide lattice
Journal of Nuclear Materials, 2012Co-Authors: Simon C. Middleburgh, Rw Grimes, K H Desai, Paul Blair, Lars Hallstadius, K Backman, P Van UffelenAbstract:Abstract Simulations using empirical inter-atomic potentials have been used to predict the change in volume of the Uranium Dioxide lattice due to the accommodation of soluble fuel additives and fission products. The incorporation of divalent, trivalent and tetravalent cations are considered. The change in accommodation mechanism for aliovalent cations between UO 2 and UO 2+ x gives rise to markedly different defect volumes. Experimental data is in good agreement with the predictions made in this work, particularly swelling as a function of dopant concentration under different conditions. The predicted defect volumes have been combined to predict the change in lattice volume with burnup (fission product inventory) due to incorporation of these soluble species, which agrees well with swelling data from irradiated fuel.
Damien Prieur - One of the best experts on this subject based on the ideXlab platform.
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Melting behaviour of americium-doped Uranium Dioxide
The Journal of Chemical Thermodynamics, 2016Co-Authors: Damien Prieur, Florent Lebreton, Marie Caisso, Philippe Martin, Andreas C. Scheinost, Thibaud Delahaye, Dario ManaraAbstract:Abstract (Uranium + americium) mixed oxides are considered as potential targets for americium transmutation in fast neutron reactors. Their thermophysical properties and notably their melting behaviour have not been assessed properly although required in order to evaluate the safety of these compounds under irradiation. In this study, we measured via laser heating, the melting points under inert atmosphere (Ar) of U 1− x Am x O 2± δ samples with x = 0.10, 0.15, 0.20. The obtained melting/solidification temperatures, measured here, indicate that under the current experimental conditions in the investigated AmO 2 content range, the solidus line of the (UO 2 + AmO 2 ) system follows with very good agreement the ideal solution behaviour. Accordingly, the observed liquidus formation temperature decreases from (3130 ± 20) K for pure UO 2 to (3051 ± 28) K for U 0.8 Am 0.2 O 2± δ . The melted and quenched materials have been characterised by combining X-ray diffraction and X-ray absorption spectroscopy.
Boris Dorado - One of the best experts on this subject based on the ideXlab platform.
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Linking atomic and mesoscopic scales for the modelling of the transport properties of Uranium Dioxide under irradiation
Journal of Nuclear Materials, 2015Co-Authors: Marjorie Bertolus, Michel Freyss, Boris Dorado, Philippe Garcia, C. Valot, Guillaume Martin, Kiet Hoang, Serge Maillard, Richard Skorek, Alain ChartierAbstract:Abstract This article presents a synthesis of the investigations at the atomic scale of the transport properties of defects and fission gases in Uranium Dioxide, as well as of the transfer of results from the atomic scale to models at the mesoscopic scale, performed during the F-BRIDGE European project (2008–2012). We first present the mesoscale models used to investigate Uranium oxide fuel under irradiation, and in particular the cluster dynamics and kinetic Monte Carlo methods employed to model the behaviour of defects and fission gases in UO 2 , as well as the parameters of these models. Second, we describe briefly the atomic scale methods employed, i.e. electronic structure calculations and empirical potential methods. Then, we show the results of the calculation of the data necessary for the mesoscale models using these atomic scale methods. Finally, we summarise the links built between the atomic and mesoscopic scale by listing the data calculated at the atomic scale which are to be used as input in mesoscale modelling. Despite specific difficulties in the description of fuel materials, the results obtained in F-BRIDGE show that atomic scale modelling methods are now mature enough to obtain precise data to feed higher scale models and help interpret experiments on nuclear fuels. These methods bring valuable insight, in particular the formation, binding and migration energies of point and extended defects, fission product localization, incorporation energies and migration pathways, elementary mechanisms of irradiation induced processes. These studies open the way for the investigation of other significant phenomena involved in fuel behaviour, in particular the thermochemical and thermomechanical properties and their evolution in-pile, complex microstructures, as well as of more complex fuels.
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effect of the cascade energy on defect production in Uranium Dioxide
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2011Co-Authors: G. Martin, P. Garcia, L. Van Brutzel, Boris Dorado, S. MaillardAbstract:Abstract The primary damage induced by a displacement cascade in a pure Uranium Dioxide matrix was investigated using classical molecular dynamics simulations. Cascades were initiated by accelerating a Uranium primary knock-on atom (PKA) to a kinetic energy ranging from 1 keV to 80 keV inside a perfect UO2 lattice at low temperature (300 K and 700 K). There is little effect of temperature in the temperature range studied. Following the cascade event, the damage level, defined as the total number of defects irrespective of whether they form clusters or not, is proportional to the initial kinetic energy of the PKA, in agreement with the literature relating to other materials. The linear dependence of damage upon initial PKA energy results from the formation of subcascades at high energy and constitutes a simple law which can be applied to any material and used in order to extrapolate molecular dynamics results to high energy PKAs. The nature of irradiation induced defects has also been studied as a function of the cascade energy.
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First-principles calculation and experimental study of oxygen diffusion in Uranium Dioxide
Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2011Co-Authors: Boris Dorado, Philippe Garcia, Gaëlle Carlot, C. Davoisne, M. Fraczkiewicz, B. Pasquet, M. Freyss, C. Valot, Gianguido Baldinozzi, David SimeoneAbstract:This work provides an illustration that density functional theory (DFT) + U calculations may quantitatively describe transport phenomena in Uranium Dioxide. Oxygen diffusion mechanisms are investigated using both ab initio calculations and experimental approaches mainly involving self-diffusion coefficient measurements. The dependences of the experimental data upon oxygen potential and sample impurity content demonstrate, by comparison with basic point defect and diffusion theory, that oxygen migration occurs via an interstitial mechanism. The temperature study provides an estimate of interstitial formation and migration energies which compare very favorably to energies calculated using the DFT+U approximation relating to the interstitialcy mechanism. Also, vacancy migration and Frenkel pair formation energies are shown to agree well with existing data.
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dft u calculations of the ground state and metastable states of Uranium Dioxide
Physical Review B, 2009Co-Authors: Boris Dorado, Michel Freyss, Bernard Amadon, Marjorie BertolusAbstract:We report a study of the ground state and metastable states of Uranium Dioxide using ab initio $\text{DFT}+\text{U}$ calculations. We highlight that in order to avoid metastable states and systematically reach the ground state of Uranium Dioxide with $\text{DFT}+\text{U}$, the monitoring of occupation matrices is crucial, as well as allowing the $5f$ electrons to break the cubic symmetry. For this purpose, we use a method based on the monitoring of the occupation matrix of the correlated orbitals on which the Hubbard term is applied. We observe the presence of numerous metastable states in calculations both with and without taking into account the symmetries of the wave functions. We investigate the influence of metastable states on the total energy, as well as on the electronic and structural properties of Uranium Dioxide. We show that the presence of metastable states induces large differences in the total energy and explain the origin of the discrepancies observed in the results obtained by various authors on crystalline and defect-containing ${\text{UO}}_{2}$. Finally, in order to check the consistency of the procedure, we determine the structural and electronic properties of the ground state of Uranium Dioxide and compare them with results obtained in previous studies using the $\text{DFT}+\text{U}$ approximation and hybrid functionals, as well as experimental data.
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gga u study of the incorporation of iodine in Uranium Dioxide
European Physical Journal B, 2009Co-Authors: Boris Dorado, Michel Freyss, Guillaume MartinAbstract:Ab initio calculations based on the Density Functional Theory are carried out in order to investigate the incorporation of iodine in Uranium Dioxide. The GGA+U approximation is used to describe the strong correlations of Uranium 5f electrons. We studied several defects that are likely to accommodate the incorporation of iodine in the material, such as Uranium and oxygen vacancies, divacancy and Schottky defects. We find the iodine atoms to be stable in a neutral Schottky defects, with an incorporation energy of -1.3 eV. This result may account for the solubility of iodine in Uranium Dioxide observed experimentally. We also notice that the incorporation of iodine involves steric and electronic contributions. The larger the defect iodine is incorporated in, the lower is its incorporation energy. Besides, we find iodine to be charged -1, thus getting the stable electronic configuration of rare gases. We also highlight the fact that the use of GGA+U increases the number of metastable states (non global energy minima), compared to the LDA/GGA approximations. Consequently, special care has to be taken on the 5f electronic occupancies in order to ensure that the absolute energy minimum has been reached.
