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The implantation of He ions in UO2 polycrystals induces a strain in the implanted layer which can be characterized using Laue micro X-ray diffraction (µ-XRD). The strain tensor resulting from the ion implantation may not be reduced to a single out-of-plane strain component: it also has nonzero shear components. Their strong dependence upon crystal orientation is modeled using elasticity theory. This work demonstrates the potential of Laue µ-XRD for characterizing radiation effects in materials. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms involved in helium migration in α-SiC are investigated through the evolution of its microstructure and of the concentration profiles following annealing at 1300°C/30min for fluences of 1 and 5×1015 3Hecm−2 and a He implantation energy of 500keV. Helium profiling is performed using the 3He(d,α)1H NRA technique with an improved detection limit of 5atppm. The NRA and TEM techniques clearly show that depending on the initial fluence, a proportion of the helium is trapped within the grain and a part of the helium is released. Analysis of the helium profile changes after annealing enabled to determine a value of the volume diffusion coefficient close to (8±1)×10−17 m2 s−1 for both fluences studied. [Copyright &y& Elsevier]

• Void change during ion implantation in UO 2 was studied using in situ TEM. • The measured void size strongly depends on the TEM capabilities and settings. • When damage increases, small voids nucleate and saturate after 0.2–0.3 dpa. • Void size remains stable during the ion irradiations. • Temperature and xenon atom incorporation do not impact void change. Transmission electron microscopy experiments coupled with 4 MeV Au and 390 keV Xe implantations were conducted on polycrystalline UO 2 thin foils in order to study the void change with damage, temperature (ranging from −180 to 1100 °C) and exogenous xenon atoms incorporation. Void size is weakly dependent on the ion dose in the range of [0.01–10] × 1014 i/cm2, whereas void density increases strongly at the early beginning of irradiation and then saturates after 3–4 × 1013 i/cm2. Change in void size and density are shown as weak functions of temperature and xenon atoms incorporation in these experimental conditions. These results indicate a heterogeneous void nucleation and a change monitored by ballistic effects. Studying voids is very tricky. This work also highlights the strong dependence of the measured void size on the TEM settings and capabilities. Thus, it is important to discuss only data obtained in the same conditions to derive influence parameters. [ABSTRACT FROM AUTHOR]

Abstract: Transmission electron microscopy (TEM) experiments were carried out on the JANNUS platform (Joint Accelerators for Nano-science and NUclear Simulation) at CSNSM (Center of Nuclear Spectrometry and Mass Spectrometry) laboratory in Orsay. The experiment was devoted to the study of the evolution of the xenon aggregate population with increasing implantation fluence. A thin UO2 foil was implanted at fluences ranging from 3×1012 to 7×1014 atcm−2 with 390keV Xe3+ ions at an irradiation temperature of 873K. The TEM results indicate the presence of nanometer size bubbles above a fluence of 6×1012 Xecm−2 and an increase in the bubble number density was observed between 6×1012 Xecm−2 and 2×1014 Xecm−2. Above 2×1014 Xecm−2, the number density levels off at 4×1023 ±0.5×1023 m−3. [Copyright &y& Elsevier]

Abstract: This study deals with the behaviour of helium in a molybdenum liner dedicated to the retention of fission products. More precisely this work contributes to evaluate the release of implanted helium when the gas has precipitated into nanometric bubbles close to the free surface. A simple model dedicated to calculate the helium release in such a condition is presented. The specificity of this model lays on the assumption that the gas is in equilibrium with a simple distribution of growing bubbles. This effort is encouraging since the calculated helium release fits an experimental dataset with a set of parameters in good agreement with the literature. [Copyright &y& Elsevier]

A well-suited way to understand the behavior under irradiation of the UO 2 nuclear fuel is to use ion implantations and further analyze the microstructure. The present work is focused on the characteristics of extended defects created by Kr implantations in UO 2 polycrystals. The influence of many parameters on these extended defects has been studied separately: irradiation temperature (room temperature, 500 and 600 °C), ion fluence (5 × 10 15 and 1.26 × 10 17 i/cm 2 ), ion energy (4 and 27 MeV) and conditions of subsequent thermal annealing. The evolution of extended defects characteristics and density was determined using transmission electron microscopy and additional damage characterizations were performed by X-ray diffraction and Raman spectroscopy. Results obtained by the different analytical techniques are shown to be in excellent agreement and to complement literature data. In addition, this work presents the extended defect transformation induced by fluence (dislocation loops and lines → tangled dislocation network) and shows that temperature influences directly their evolution kinetics. Moreover we demonstrate that the temperature to allow extended defects mobility is different during irradiation and under thermal annealing conditions. Finally dislocation line density and swelling obtained from this study are found in the same order of magnitude as the ones obtained from in pile irradiations. [ABSTRACT FROM AUTHOR]

Abstract: In this work we focus on experiments involving implantation of 500keV 3He ions in sintered polycrystalline material. Samples are implanted at low fluences (∼2×1013 ions/cm2) and subsequently isothermally annealed in a highly sensitive thermal desorption spectrometry (TDS) device PIAGARA (Plateforme Interdisciplinaire pour l’Analyse des GAz Rares en Aquitaine). The helium fluencies studied are two to three orders of magnitude lower than previous Nuclear Reaction Analysis (NRA) experiments carried out on identical samples implanted at identical energies. The fractional release of helium obtained in the TDS experiments is interpreted using a three-dimensional axisymmetric diffusion model which enables results to be quantitatively compared to previous NRA data. The analysis shows that helium behaviour is qualitatively independent of ion fluency over three orders of magnitude: helium diffusion appears to be strongly inhibited below 1273K within the centre of the grains presumably as a result of helium bubble precipitation. The scenario involving diffusion at grain boundaries and in regions adjacent to them observed at higher fluencies is quantitatively confirmed at much lower doses. The main difference lies in the average width of the region in which uninhibited diffusion occurs. [Copyright &y& Elsevier]

Abstract: We have studied the microstructure dependence of He bubble formation in silicon carbide. Helium accumulation in SiC was performed by 500keV 3He implantation at room temperature with a fluence of 5×1015 cm−2. Depth concentration profiles have been investigated in 6H-SiC single crystals and α-SiC polycrystals by NRA spectrometry. Cross-sectional TEM samples have been imaged to study bubble formation. After annealing at 1300°C, results clearly demonstrate an influence of grain boundaries on He retention yield in α-SiC polycrystals while helium is totally released from single crystals. Polycrystals also display the formation of intragranular overpressurized bubbles while no bubbles are observed in single crystals. Interpretations are proposed on the basis of the nature of He traps. [Copyright &y& Elsevier]

Abstract: Nuclear reaction analyses (NRA) based on the 3He(2H,4He)1H reaction were previously performed to follow the evolution of implanted 3He in polycrystalline UO2 samples. Experimental results pointed to an enhancement above 800°C of the diffusion coefficient of helium over several microns in the vicinity of the grain boundaries, with respect to the diffusion coefficient within the grain. This was ascribed to the fact that grain boundaries are probably defect sinks which locally modify the defect concentrations. This study aims at demonstrating the particular effect of grain boundaries on helium migration. To this end, 3He implanted polycrystalline UO2 samples were cracked then annealed at 900°C. Helium migration in the vicinity of the grain boundaries and near the crack was investigated by means of NRA microanalyses. Helium depletion extends over far larger distances in the vicinity of the grain boundaries than near the crack. Experimental evidence has been collected of the particular effect of grain boundaries on helium migration, which do not act as free surfaces at which helium atoms are simply released. [Copyright &y& Elsevier]

• Diffusion of Xe and Kr implanted UO2 was studied at 1300°C. • A diffusion model based on Fick's second law was developed. • Xe and Kr diffusion coefficient at 1300°C was found to be (1.73 ± 0.15)x10−20 m2/s. • Xe and Kr migration is subjected to two trapping mechanisms due to radiation induced defects. The fission of uranium dioxide produces gaseous elements degrading nuclear fuel properties. A thorough understanding of the transport and release of gaseous products is thus essential. The present work focuses on xenon and krypton migration mechanism in uranium dioxide. Desorption experiments on ion implanted UO 2 were performed at 1300°C. Xe and Kr releases were simulated using a mesoscale model that was developed taking into account single gas atom diffusion and defect traps. We showed that the defects have a high influence on Xe and Kr migration mechanisms and therefore have to be considered to accurately determine diffusion coefficients. We evaluated the diffusion coefficient of Xe and Kr at (1.73 ± 0.15)x10−20 m2/s at 1300°C and we showed that the diffusion of rare gases is subjected to two trapping mechanisms. The first occurs during the ion implantation and the second during high-temperature annealings. The nature of the trapping sites is discussed in the light of the literature on radiation induced defects. This study also consolidates the use of non activated UO 2 implanted with heavy ions as a less-hazardeous substitute for irradiated UO 2. [ABSTRACT FROM AUTHOR]