[en] X-ray absorption spectroscopy experiments were conducted in order to characterise plutonium dioxide oxidation. It is shown that the sample preparation adopted does not enable elaboration of hyperstoichiometric plutonium dioxide. These results could mean that plutonium dioxide oxidation only occurs under very specific conditions

[en] The stability of point defects and the behaviour of rare gases in uranium dioxide have been studied using electronic structure calculations. Krypton atoms are found to be insoluble in UO₂ whatever the trapping site considered. Their presence induces a swelling of the lattice when they are located in interstitial sites or in oxygen vacancy sites. Due to its smaller atomic size, the predicted helium behaviour is very different. Indeed, helium is found to be soluble in stoichiometric and hyperstoichiometric uranium dioxide in the presence of uranium vacancies or divacancies. Moreover helium atoms induce a lattice contraction except in interstitial sites for which a slight expansion is found. Some preliminary results concerning xenon are also given

[en] In the original article, a mistake was made in the figure 7 presenting the clustering energies of Di-Schottky (green) and 2V_U (light blue) clusters. The corrected figure 7 is given. (authors)

[en] Highlights: • Using MEXIICO furnace to perform slow depressurisation. • Characterisation of the behaviour of fuel pellet during the pressure and temperature ramps. • Study of fission gas release by the fuel pellet. • Analytical and CFD study of fission gas entrained by the Argon flow. - Abstract: The 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 ⁸⁵Kr 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.

[en] We report an investigation of the ground-state properties of americium dioxide and its sesquioxides using first-principles calculations. In order to take into account strong 5f electronic correlations, we apply the generalized gradient approximation (GGA)+U as well as the hybrid functionals to these compounds. We present a systematic study of several bulk properties such as structural, elastic, energetic, electronic, and magnetic properties as a function of the effective Coulomb U and exchange J GGA+U parameters in the range of 0.0-8.0 and 0.00-0.75 eV, respectively. The values (U, J) = (6.00 eV, 0.75 eV) are those providing a correct description of bulk properties by comparison to the available experimental data. We discuss the effect of loss of symmetry caused by DFT+U on several properties. We also discuss the effect of the spin-orbit coupling on these properties. Furthermore, we highlight that in order to reach the ground state of americium dioxide and sesquioxides using GGA+U the monitoring of the occupation matrices of 5f correlated orbitals is crucial. Our computational scheme provides reliable and accurate results concerning several bulk properties that have not been studied experimentally yet. For instance, our elastic constants calculated for AmO₂ follow the same trend as those of other actinide oxides (UO₂, NpO₂, and PuO₂) and can be used as reference. We have also computed the electronic and static dielectric constants, as well as the Am and O Born effective charge of americium dioxide. Finally, we find the A-type hexagonal structure to be the most stable structure at low temperature for americium sesquioxides. For this phase, we predict the internal structural parameters as well as the bulk modulus. We provide data concerning the formation enthalpy of the Am₂O₃ bcc cubic structure. (authors)

[en] We present a physically justified formalism for the calculation of point defects and cluster formation energies in UO₂. The accessible ranges of chemical potentials of the two components U and O are calculated using the U-O experimental phase diagram and a constraint on the formation energies of vacancies. We then apply this formalism to the DFT + U investigation of the point defects and cluster defects in this material (including charged ones). The most stable charge states obtained for these defects near stoichiometry are consistent with a strongly ionic system. Calculations predict similarly low formation energies for V_U⁴⁻ and I_O²⁻ in hyperstoichiometric UO₂. In stoichiometric UO₂, V_O²⁺ and I_O²⁻ have the same formation energy in the middle of the gap and in hypostoichiometric UO₂, V_O²⁺ is the most stable defect. (paper)

[en] UO₂ and (U,Pu)O₂ are the most used fuels in the current nuclear reactor fleet. Under irradiation in a reactor, new chemical elements are created in these materials by the fission of uranium and plutonium nuclei. Some of these fission products, especially iodine, caesium and tellurium may chemically react with one another to form compounds that are potentially corrosive for the cladding. One way to avoid the cladding corrosion consists of preventing the formation and migration of such corrosive species within the fuel. To that end, the determination of the most stable chemical forms of iodine, caesium and tellurium in the fuel as well as the trapping and diffusion mechanisms of these elements in UO₂ and (U,Pu)O₂ is of first interest. The gain of Cr-doped UO₂ fuel on the fission gas retention has already been demonstrated experimentally. However, no data are yet available on the release of corrosive fission products in Cr-doped UO₂. With a view to simulating the behaviour of these species within Cr-doped UO₂, one has to prior identify the most favourable oxidation state and location site of Cr in UO₂, which is a much debated topic in the literature. We perform electronic structure calculations, using the Hubbard-corrected density functional theory (DFT+U) to evaluate the preferred trapping site of I, I₂, Cs and Te in UO₂ and (U,Pu)O₂ crystals, as well as the preferred oxidation state and location of Cr in UO₂. We first determine the stability of I, I₂, Cs, Te and Cr in various point defects and then compute their XANES spectra in each considered site, using the FDMNES code and the DFT+U atomic configurations. The comparison of the computed spectra with the experimental ones contributes to the identification of the chemical forms and the trapping sites of iodine, caesium, tellurium and chromium in the actinide oxides. The use of a Hubbard term (GGA+U) allows us to take into account the strong correlations of the actinide 5f electrons. To avoid the metastable states inherent to this method, we use the occupation matrix control (OMC) procedure, which also allows us to control the valences of each species in the simulation. This particular point makes our approach reliable with respect to the determination of fission products incorporation energies in the various studied defects. (authors)

[en] In this paper, we determine for the first time the electronic, structural and energetic properties of mixed oxides in the entire range of Am content using the generalized gradient approximation (GGA) in combination with the special quasirandom structure (SQS) approach to reproduce chemical disorder. This study reveals that in oxides, Am cations act as electron acceptors, whereas U cations act as electron donors showing a fundamental difference with or in which there is no cation valence change in stoichiometric conditions compared to the pure oxides. We show for the first time that the lattice parameter of stoichiometric 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. (paper)

[en] Nonstoichiometric uranium dioxide experiences a shrinkage of its lattice constant with increasing oxygen content, in both the hypo-stoichiometric and the hyper-stoichiometric 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 UO₂. 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 hypo-stoichiometric regime, the oxygen vacancies are dominant and explain the lattice constant variation with their surprisingly positive volume of relaxation. In the hyper-stoichiometric 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. (authors)

[en] We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV α particles. The positron lifetime was measured as a function of the temperature in the 15-300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in UO_2. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the DFT + U method to take into account strong correlations between the 5f electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + U results and the positron trapping model indicates that neutral V_U + 2V_O tri-vacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV α irradiated UO_2 samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in UO_2. (authors)