Radiation endurance in Al2O3nanoceramics

The lack of suitable materials solutions stands as a major challenge for the development of advanced nuclear systems. Most issues are related to the simultaneous action of high temperatures, corrosive environments and radiation damage. Oxide nanoceramics are a promising class of materials which may benefit from the radiation tolerance of nanomaterials and the chemical compatibility of ceramics with many highly corrosive environments. Here, using thin films as a model system, we provide new insights into the radiation tolerance of oxide nanoceramics exposed to increasing damage levels at 600 °C-namely 20, 40 and 150 displacements per atom. Specifically, we investigate the evolution of the structural features, the mechanical properties, and the response to impact loading of Al2O3 thin films. Initially, the thin films contain a homogeneous dispersion of nanocrystals in an amorphous matrix. Irradiation induces crystallization of the amorphous phase, followed by grain growth. Crystallization brings along an enhancement of hardness, while grain growth induces softening according to the Hall-Petch effect. During grain growth, the excess mechanical energy is dissipated by twinning. The main energy dissipation mechanisms available upon impact loading are lattice plasticity and localized amorphization. These mechanisms are available in the irradiated material, but not in the as-deposited films

Activities on SRF Multilayers at Orsay / Saclay

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Multilayers Activities at Saclay / Orsay

In the investigations on the high gradient SRF cavities, the superconducting multilayer is a promising alternative. The predictions show that SIS nanocomposite (Superconductor/Isolator/Superconductor) could improve the efficiency limited by the bulk Nb itself used today for accelerating cavities. We started, at the IPNO lab incollaboration with the CSNSM lab (CNRS) and Irfu lab (CEA), an experimental study to test the screening effect on multilayer assemblies. Based on 3rd harmonic magnetometer and a TE011 SRF cavity, measurements of first critical magnetic field HC1 and surface resistance of samples have been performed. These promising results are the starting point of the MBE deposition developments. This setup is devoted to optimize the best organization of the multilayer to produce the model sample, and to find, in a close future, a realistic solution to apply this technique on an accelerating SRF cavity

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

Synergy of electronic and nuclear energy depositions on the kinetics of extended defects formation in UO2, based on in situ TEM observations of ion-irradiation-induced microstructure evolution

International audienceDuring in reactor operations, the slowing down of fission fragments generates most of the radiation damage in uranium dioxide. Fission fragments deposit their energy to both atomic and electronic subsystems through nuclear and electronic interactions. To study the possible synergistic effects of nuclear and electronic energy deposition on the microstructure, UO2 thin foils have been irradiated with 0.39 MeV Xe and/or 6 MeV Si ions at 298 K using single or dual beam ion irradiations. The evolution of extended defects was characterized by in situ Transmission Electron Microscopy. Results show a similar evolution with fluence increase, irrespective of the ion beam: a nucleation of small dislocation loop that increase in density up to a saturation value, followed by an increase of the average loop size. However, the kinetics differs according to the irradiation conditions. In the case of the dual beam irradiation, all the phenomena occur at a lower value of damage level. The local increase of the temperature along the 6 MeV Si ion path seems to lead to an enhanced defect mobility, as in the case of irradiations performed at higher temperatures

Full characterization of dislocations in ion-irradiated polycrystalline UO2_2

International audienceIn order to fully characterize the dislocation loops and lines features (Burgers vectors, habit/slip planes, interstitial or vacancy type) induced by irradiation in UO2, polycrystalline thin foils were irradiated with 4 MeV Au or 390 keV Xe ions at different temperatures (25, 600 and 800 °C) and fluences (0.5 and 1 × 1015 ions/cm2), and further analyzed using TEM. In all the cases, this study, performed on a large number of dislocation loops (diameter ranging from 10 to 80 nm) and for the first time on several dislocation lines, reveals unfaulted prismatic dislocation loops with an interstitial nature and Burgers vectors only along the -type directions. Almost 60% of the studied loops are purely prismatic type and lie on {110} habit planes perpendicular to the Burgers vector directions. The others lie on the {110} or {111} planes, which are neither perpendicular to the Burgers vectors, nor contain them. About 87% of the dislocation lines, formed by loop overlapping as fluence increases, are edge or mixed type in the {100} slip systems, as those induced under mechanical load

Influence of exogenous xenon atoms on the evolution kinetics of extended defects in polycrystalline UO2_2 using in situ TEM

International audienceThis work aims at improving our understanding of the UO2 nuclear fuel behavior under irradiation in general and of the microstructural evolutions during damage build-up at the nanometer scale in particular. Within this frame, we studied the influence of exogenous xenon atoms on the extended defect change in polycrystalline UO2 thin foils through in situ TEM characterizations. The analysis of the TEM results and their comparison to our previous work (Onofri et al. JNM 482, 2016, 105) clearly show an impact of the implanted xenon atoms on the evolution kinetics of extended defects at 25 and 600 C unlike lower temperatures (180 C in our case)

Extended defect change in UO2 during in situ TEM annealing

International audiencePredicting the nuclear fuel microstructure at each moment of its irradiation cycle (nominal, power transient, accidental conditions) is a significant nuclear safety issue. For that, it is necessary to understand the impact of irradiation parameters on the microstructure. This study provides insight about the temperature effect on dislocations. In situ thermal annealing up to 1400°C on pre-irradiated polycrystalline UO2 thin foils was performed inside a TEM for the first time. The aim of the current study is to establish the kinetic and the mechanisms of thermal recovery of extended defects induced by irradiation. Whatever the initial irradiation conditions, extended defect recovery was observed around 1000-1100°C, in good agreement with literature data. Dislocation line disappear mainly by climb and dislocation loops move by pencil glide along the Burger vector directions. Defect growth by coalescence of dislocation loops is also observed

Changes in voids induced by ion irradiations in UO2: In situ TEM studies

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