Study of extended defects created under irradiation in UO2 using the transmission electron microscopy

Lors de son irradiation en réacteur, le dioxyde d'uranium subit d'importantes modifications physico-chimiques (génération de bulles de gaz de fission, création de dislocations...). Le relâchement des gaz de fission est un critère important du point de vue de la sureté nucléaire, limitant le temps de vie du combustible en réacteur. Or, la croissance préférentielle des bulles localisées sur les défauts étendus a été mise en évidence expérimentalement. Le but de ce travail est donc d'étudier les dislocations induites par irradiation, afin d'améliorer la compréhension du comportement du combustible. Les objectifs sont de déterminer les caractéristiques des défauts étendus (vecteur de Burgers, plan d'habitat, nature interstitielle ou lacunaire), leurs mécanismes d'évolution (nucléation, grossissement), ainsi que l'influence de différents paramètres d'irradiation, tels que la fluence, la température et la présence d'atomes exogènes sur leur cinétique d'évolution. Pour ce faire, des études à effets séparés basées sur la réalisation d'irradiations aux ions (plateformes JANNuS d'Orsay et de Saclay) et de caractérisations in situ à différentes échelles comme des observations au Microscope Electronique en Transmission (CEMES, JANNuS Orsay), des mesures de Diffraction des Rayons X et de spectroscopie Raman, ont été mises en place. Enfin, la caractérisation du combustible irradié en réacteur réalisée à JRC-ITU, a révélé des défauts étendus très semblables à ceux induits par des irradiations aux ions, en termes de densité et de caractéristiques.During in-reactor irradiation, several phenomena take place in the uranium dioxide fuel: fission gas bubbles and extended defects (dislocation loops and lines) generation, doping by fission products, etc. Fission gas release is an important nuclear safety issue and represent, among others, a limiting factor for the fuel lifetime in reactors. It has been shown experimentally that the extended defects are preferential growth sites for fission gas bubbles. Hence, the study of extended defects created under irradiation is a significant step to better understand the fuel behavior. The aims of this study are to determine the extended defect characteristics (Burgers vectors, habit planes, interstitial or vacancy nature), their evolution mechanisms and the effect of the different irradiation parameters, such as fluence, temperature and exogenous atoms, on the evolution kinetics. To do that, separated-effects studies have been performed using ion irradiations/implantations (JANNuS facilities in Orsay and in Saclay) followed by in situ TEM characterizations (CEMES, JANNuS Orsay), XRD and Raman spectroscopy measurements. Finally, the characterization of fuel irradiated in reactor performed at JRC-ITU, revealed that extended defects are very much closed to those induced by ion irradiations, in terms of density and characteristics

Etude des défauts étendus induits par irradiation dans UO2 par microscopie électronique en transmission

During in-reactor irradiation, several phenomena take place in the uranium dioxide fuel: fission gas bubbles and extended defects (dislocation loops and lines) generation, doping by fission products, etc. Fission gas release is an important nuclear safety issue and represent, among others, a limiting factor for the fuel lifetime in reactors. It has been shown experimentally that the extended defects are preferential growth sites for fission gas bubbles. Hence, the study of extended defects created under irradiation is a significant step to better understand the fuel behavior. The aims of this study are to determine the extended defect characteristics (Burgers vectors, habit planes, interstitial or vacancy nature), their evolution mechanisms and the effect of the different irradiation parameters, such as fluence, temperature and exogenous atoms, on the evolution kinetics. To do that, separated-effects studies have been performed using ion irradiations/implantations (JANNuS facilities in Orsay and in Saclay) followed by in situ TEM characterizations (CEMES, JANNuS Orsay), XRD and Raman spectroscopy measurements. Finally, the characterization of fuel irradiated in reactor performed at JRC-ITU, revealed that extended defects are very much closed to those induced by ion irradiations, in terms of density and characteristics.Lors de son irradiation en réacteur, le dioxyde d'uranium subit d'importantes modifications physico-chimiques (génération de bulles de gaz de fission, création de dislocations...). Le relâchement des gaz de fission est un critère important du point de vue de la sureté nucléaire, limitant le temps de vie du combustible en réacteur. Or, la croissance préférentielle des bulles localisées sur les défauts étendus a été mise en évidence expérimentalement. Le but de ce travail est donc d'étudier les dislocations induites par irradiation, afin d'améliorer la compréhension du comportement du combustible. Les objectifs sont de déterminer les caractéristiques des défauts étendus (vecteur de Burgers, plan d'habitat, nature interstitielle ou lacunaire), leurs mécanismes d'évolution (nucléation, grossissement), ainsi que l'influence de différents paramètres d'irradiation, tels que la fluence, la température et la présence d'atomes exogènes sur leur cinétique d'évolution. Pour ce faire, des études à effets séparés basées sur la réalisation d'irradiations aux ions (plateformes JANNuS d'Orsay et de Saclay) et de caractérisations in situ à différentes échelles comme des observations au Microscope Electronique en Transmission (CEMES, JANNuS Orsay), des mesures de Diffraction des Rayons X et de spectroscopie Raman, ont été mises en place. Enfin, la caractérisation du combustible irradié en réacteur réalisée à JRC-ITU, a révélé des défauts étendus très semblables à ceux induits par des irradiations aux ions, en termes de densité et de caractéristiques

Atomic scale insights on the microstructure evolution of urania under irradiation

International audienceUrania is commonly used as a fuel in nuclear industry. Urania is heavily irradiated during its in-reactor stay, and faces drastic microstructural modifications, including few percent swelling and increase of dislocation density. Dislocations loops nucleate first [1] and transform with increasing fluence into lines. However, the early stages of their nucleation are hardly attainable experimentally. One commonly infers that their nucleation is related to the aggregation of point defects or defects clusters into dislocations. In the present paper [2], we clarify the first steps of the effect of irradiation on urania by means of molecular dynamics simulations using empirical potentials. The irradiation dose is simulated by continuous accumulation of Frenkel pairs at 600DC, skipping the cpu-expensive displacement cascades.Starting from a defectless urania, we observe the nucleation and growth of dislocations under Frenkel pairs accumulation. Detailed analysis shows a four stages evolution (i) an increase of point defects (ii) then the nucleation of Frank loops 13 from the aggregation of point defects, (ii) the transformation of Frank loops into perfect loops 12 (iv) and finally their stabilization as lines. Our simulations also show a swelling up to 3.2% during the first stage in which point defects are present. This swelling suddenly decreases to 1.5 percent in the second stage, as soon as dislocations nucleate. Both stage (iv) and swelling agree with experimental data [1,3] and therefore strengthen the four stages scenario of the microstructure evolution of urania under irradiation

Effect of ballistic damage in UO2_2 samples under ion beam irradiations studied by in situ Raman spectroscopy

International audienceThe damage induced in uranium dioxide (UO$_2$) during ion irradiation at low energy was studied by micro-Raman spectroscopy. Polycrystalline UO$_2$ samples were irradiated by 0.9-MeV I, 2-MeV Au at 25DC and by 4-MeV Kr ions at -160DC in a wide range of fluence. In situ Raman measurements reveal similar spectra evolution no matter the ion beam used. The T$_{2g}$ band centred at 445 cm$^{-1}$ related to the fluorine structure reveals a broadening with the irradiation damage increase. In addition, several bands ranging from 500 to 700 cm$^{-1}$, which are attributed to sub- or sur-stoichiometric structural defects, are observed at the first time of irradiation. Their intensities rise up with the irradiation fluence increase to a similar asymptotic relative values for all the irradiation conditions. The obtained Raman kinetics are compared with data from the literature on the microstructure evolution observed by Transmission Electronic Microscopy (TEM) and on the fraction of displaced atoms determined by Rutherford Backscattering Spectroscopy in channelling mode (RBS-C)

Effect of coupled electronic and nuclear energy deposition on strain and stress levels in UO2_2

International audienceUO2 polycrystals were irradiated in the nuclear energy-loss regime, Sn (900 keV I and 2 Mev Au ions) and also with an additional electronic energy deposition, Se (900 keV I and 36 MeV W ions simultaneously, i.e. Sn&Se). The strain/stress state exhibited by the irradiated pellets was determined by x-ray diffraction measurements. Results show that both measured strain and estimated stress are lower in the dual-beam irradiated samples, indicating that there is an ionization-induced change in the ballistically-generated-defect spectrum. Furthermore, it is shown that the thin irradiated layers maintain the lattice parameter of the pristine material in the basal plane

Evolution of extended defects in polycrystalline Au-irradiated UO2 using in situ TEM: Temperature and fluence effects

International audienceIn situ Transmission Electron Microscopy irradiations were performed on polycrystalline UO2 thin foils with 4 MeV gold ions at three different temperatures: 600 °C, room and liquid nitrogen temperature. In order to study the dislocation evolution and to determine the growth mechanisms, the dislocation loop and line densities and the loop size repartition were monitored as a function of fluence, and irradiation temperature. We show that dislocation loops, with Burgers vectors along the directions, evolve into dislocation lines with increasing fluence by a loop overlapping mechanism. Furthermore, a fluence offset is highlighted between the irradiations performed at high and low temperature due to an increase of the defect mobility. Indeed, a growth by Oswald ripening is probably activated at room temperature and 600 °C and changes the kinetic evolution of loops into lines.After this transformation, and for all the irradiation temperatures, a steady state equilibrium is reached where both extended defects (dislocation lines and small dislocations loops -around 5 nm in size-) are observed simultaneously. A continuous nucleation of small dislocation loops and of nanometer-sized cavities formed directly by irradiation is also highlighted

TEM characterisation of helium platelets in implanted uranium dioxide

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Irradiation-induced microstructural transformations in UO2 accelerated upon electronic energy deposition

International audienceThe combination of electronic and nuclear energy deposition may have a significant effect on the defect production and evolution. In nuclear reactor, the nuclear fuel is exposed to the irradiation of several particles such as the fission fragments (FF). Along their path, the ratio of electronic to nuclear energy loss evolves. To understand the impact of this coupling on the fuel microstructure, single and dual-beam ion irradiations of uranium dioxide (UO2) were carried out. The damage evolution was investigated by Raman spectroscopy analysis correlated with Transmission Electronic Microscopy observations. A significant effect of electronic energy dissipation on defect formation and evolution was found, depending on the electronic energy loss level. With the increase of electronic energy loss, the microstructure evolution is even more pronounced

Early stages of irradiation induced dislocations in urania

International audienceThe early stages of nucleation and growth of dislocations by irradiation in urania is clarified based on the combination of experiments and atomistic calculations. It is established that irradiation induced dislocations follow a five stage process: (i) point defects are first created by irradiation, (ii) they aggregate into clusters, (iii) from which nucleate Frank loops, (iv) which transform into unfaulted loops via Shockley that in turn grow, and (v) finally reorganize into forest dislocations. Stages (i)–(iii) participate in the lattice expansion while the onset of lattice contraction starts with stage (iv), i.e., when unfaulted loops nucleate. Irradiation induced dislocations operate in the spontaneous recombination regime, to be opposed to the thermal diffusion regime. Body of arguments collaborates to this statement, the main one is the comparison between characteristic distances estimated from the dose rate (Vat/(K0×τ))1/3 and from the diffusion coefficient  (D×τ)1/2. Such a comparison identifies materials under irradiation as belonging either into the recombination regime or not