Papers by Keyword: Uranium Dioxide

Abstract: UO₂ transforms into a superionic conductor at temperatures in excess of 2000 K with oxygen ions becoming mobile and exhibiting collective diffusive dynamics. While the response of UO₂ to irradiation is of current interest, the possible impact of superionic characteristics on defect dynamics and recovery following radiation has not been explored yet. In the current work, we use atomistic simulations to elucidate the short-time dynamical response of stoichiometric UO₂ subjected to low energy radiation knocks. We observe that the oxygen ions exhibit a collective behavior that is characterized by frequent hopping across their native lattice sites and forming quasi-one-dimensional string-like structures, which are typical of the superionic state. Approximately, a quarter of the displaced oxygen ions dynamically recover through concerted string-like displacements. Our simulations thus suggest a plausible correlation between defect recovery of irradiated UO₂ and the characteristic superionic hopping mechanism among the oxygen ions.

Abstract: Plasticity of oxide fuel based on uranium dioxide is one of the less-covered topics in nuclear materials science. A useful tool to study the deformation mechanisms in this material at elevated temperatures is hot pressing. In this work, UO$_{2.06}$ powder prepared via~ADU route was uniaxially compacted at temperatures within the~$250$--$600$~{\textdegree}C range under a compressive axial stress from~$95$ to~$220$~MPa applied for a time interval between~$10$ and~$60$~min. Examinations performed with scanning electron microscopy~(SEM) and density measurements revealed a temperature effect on densification when increasing the compaction temperature from~$400$ to~$600$~\textdegree{C} with other conditions being equal. By varying the loading duration for hot pressing at~$600$~{\textdegree}C under constant compressive stress~$95$~MPa, different stages of compactions were analyzed. In addition,~XRD measurements revealed a texture developing in the material during compaction and a possible increase in dislocation density. It is argumented that dislocation-mediated plasticity contributes to densification at elevated temperatures.

Abstract: Uranium dioxide powders were surface pre-oxidation treated. Phases and properties of the powders after pre-oxidation treatment were investigated by DSC-TG、XRD、BET and Dilatometer. The results show that there is a little of U₃O₇ in uranium dioxide powders by pre-oxidation treatment at 240°C for 8h in air, UO₂ powders will transform into U₃O₈ at 382°C for 8h in air. The shrinkage temperature of the uranium dioxide pellets after pre-oxidation treatment decreased from 1200°C to 580°C, densification rate (ΔL/L) increased also from 1.52×10^-4/K to 3.08×10^-4/K. The mechanism of low temperature sintering to pre-oxidation UO_2+x pellets was explained by simplified point defect model and densification equation. The UO_2+x diffusion coefficient, , is much higher than which of UO₂. The densification equation is expressed by , and the factor A is according to , a=16.23658, b=0.04247 and c=-2.18802×10^-5.

Abstract: Manufacture of fuel tablets from UO₂ powder includes many processes, among which shrinkage without application of external loading till now requires explanations. Particles of UO₂ are brittle up to ~ 700°C. By cold pressing due to the porous structure of powder billets separate particles are pressing one into another with accompanying rise of residual elastic stress. Under following heating and sintering plasticization of UO₂ occurs, its yield stress decreases and conditions for creep and resulting shrinkage of tablets takes place.

Abstract: The presence of iodine in UO₂ has a great impact on nuclear fuel behaviour, both during-in reactor operation or under long-term repository conditions. Here the volume diffusion coefficient of iodine is studied using a methodology which involves ion implantation, annealing under different oxygen potentials and SIMS (Secondary Ion Mass Spectroscopy) for the concentration profile characterization of samples. The changes in the initial concentration profile induced by annealing are interpreted using Ficks second law to determine the diffusion coefficient. The first part of this paper is devoted to the description of the applied methodology. The iodine diffusion coefficients in the bulk are shown to depend upon the annealing temperature and oxygen potential. In smaller grained polycrystalline samples, the SIMS signal is averaged out over several grains and therefore, the analysis of the depth profile changes in samples due to annealing may be efficiently compared to depth profiles in as-implanted samples. By contrast, in Cr-doped UO₂ samples in which the grain size is larger than the zone analysed by SIMS, a particular methodology has to be developed. To this end, large grained material was examined using EBSD (Electron Back Scattering Diffraction) in order to determine the relative crystalline orientations of the grains. Following iodine implantations, various grains with different orientations were studied using SIMS. An attempt is presented at correlating the different sputtering rates with the various grain orientations.

Abstract: Transmission electron microscopy (TEM) characterizations were carried out on a set of UO₂ thin foils previously implanted at room temperature with 400 keV Xe²⁺ and 250 keV Kr²⁺ ions at the fluence 7.10¹⁵ at.cm^-2 (equivalent to 1 at.%/at. UO₂). The experiment was devoted to the study of the evolution of the fission gases bubbles populations with increasing temperature. Annealings were performed in the laboratory furnace at 600°C, 800°C, 1000°C for 12h, 1400°C for 4h and 1500°C for 2h under Ar-5%H₂ atmosphere. For each annealing condition and for as-implanted specimens the bubble population has been characterized in size and number density. A comparison between Xe and Kr has been done that showed a similar behaviour. Globally, from the as-implanted sample to the 1500°C annealed, the bubbles growth phenomenon and the microstructure evolution with temperature was put in relieve.

Abstract: High helium contents will be generated within minor actinide doped uranium dioxide blankets which could be used in fourth generation reactors. In this framework, it is essential to improve our understanding of the type of damage which a pellet could incur as a result of extensive helium build-up. This paper is an attempt at tackling this issue. Sintered uranium dioxide disks have been implanted with helium ions then annealed at various temperatures. Above a concentration of 0.4 at.% and above 1000°C, optical images of the sample surface revealed swollen grains and extensive areas which have exfoliated. Nuclear reaction microanalyses and atomic force microscopy observations were performed to demonstrate that helium has substantially precipitated within the swollen grains. Massive precipitation of the gas leads under these conditions to sample surface blistering which appears to precede flaking. Deuterium ion irradiations have also been performed at ambient and a direct flaking of the sample surface was observed, but for this phenomenon to be observed required much higher doses than in the He study, indicating that temperature could be an essential ingredient for gas to migrate and cause extensive precipitation. Such phenomena could possibly lead to degradation of the fuel.

Abstract: In this paper, we describe two experimental set-ups which enable the measurement of electrical properties and intrinsic diffusion coefficients in UO2. Electrical conductivity measurements are insured by a standard four point Kelvin-Bridge method. In parallel, the gas-solid isotopic exchange method is used to load the samples with 18O tracer atoms, the concentration profile of which are subsequently characterized using SIMS and chromatic confocal microscopy. An application of both types of measurements on a UO2 single crystal is given. The diffusion study was carried out at 750°C, and the electrical conductivity study was performed between 1000°C and 1300°C at oxygen potentials at which the material exhibits extrinsic behaviour. We show how a careful use of both measurements in conjunction can be an indication of the operative migration mechanism.

Abstract: The growth of a U3O7 oxide layer during the anionic oxidation of UO2 pellets induced very important mechanical stresses due to the crystallographic lattice parameters differences between UO2 and its oxide. These stresses, combined with the chemical processes of oxidation, can lead to the cracking of the system, called chemical fragmentation. We study the crystallographic orientation of the oxide lattice growing at the surface of UO2, pointing to the fact that epitaxy relations at interface govern the coexistence of UO2 and U3O7. In this work, several results are given: - Determination of the epitaxy relations between the substrate and its oxide thanks to the Bollmann’s method; epitaxy strains are deduced. - Study of the coexistence of different domains in the U3O7 (crystallographic compatibility conditions at the interface between two phases: Hadamard conditions). - FEM simulations of a multi-domain U3O7 connected to a UO2 substrate explain the existence of a critical thickness of U3O7 layer.

Abstract: The effect of a tetravalent dopant, Th4+, on the oxidation of UO2 was investigated using a thermogravimetry and X-ray diffraction analysis. Th-doped UO2’s with various dopant contents were prepared and their oxidation kinetic curves were obtained from the weight gains during air-oxidation. For the first oxidation step from (U1-yThy)O2 to (U1-yThy)4O9, the oxidation kinetic curves showed the same gradient regardless of the Th content. The inhibition of the oxidation reaction occurred dominantly in the second step, from (U1-yThy)4O9 to (U1-yThy)3O8. At the plateau of the second stage, the calculated O/M values and the X-ray diffraction patterns revealed that the O/M ratio was decreased with an increase of Th content. The relationship between the mean formal charge and the composition showed that the oxidation to (U1-yThy)3O8 proceeds within a certain limit. The lattice parameter of the initial material seems to affect the oxidation rate of the first step. And the oxidation reaction stopped when the average formal charge of the U atoms reached a value of 5.3.