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X-ray Absorption Spectroscopy (XAS) was used to study the behavior of krypton as a function of its concentration in UO 2 samples implanted with Kr ions. For a 0.5 at.% krypton local concentration, by combining XAS results and DFT + U calculations, we show that without any thermal treatment Kr atoms are mainly incorporated in the UO 2 lattice as single atoms inside a neutral bound Schottky defect with O vacancies aligned along the (100) direction (BSD1). A thermal treatment at 1273 K induces the precipitation of dense Kr nano-aggregates, most probably solid at room temperature. In addition, 26 ± 2% of the Kr atoms remain inside BSD1 showing that Kr-BSD1 complex is stable up to this temperature. Consequently, the (in-)solubility of krypton in UO 2 has to be re-evaluated. For high Kr concentration (8 at.%), XAS signals show that Kr atoms have precipitated in nanometer-sized aggregates with internal densities ranging between 4.15(7) g cm −3 and 3.98(5) g cm −3 even after annealing at 873 K. By neglecting the effect due to the UO 2 matrix, the corresponding krypton pressures at 300 K were equal to 2.6(3) GPa and 2.0(2) GPa, respectively. After annealing at 1673 K, regardless of the initial Kr concentration, a bi-modal distribution is observed with solid nano-aggregates even at room temperature and larger cavities only partially filled with Kr. These results are very close to those observed in UO 2 fuel irradiated in reactor. In this study we show that a rare gas can be used as a probe to investigate the defect creation and their stability in UO 2 . [ABSTRACT FROM AUTHOR]

Abstract: In this study, the effects of radiation on helium migration are investigated through the analysis of polycrystalline uranium dioxide samples irradiated at fluences up to 5×1015 at.cm−2 with 8MeV iodine ions. Following irradiation, samples are implanted with 500keV 3He+ ions at fluences in the range of 1016 at.cm−2. Three nuclear reaction analysis (NRA) techniques are subsequently implemented using the 3He(2H,1H)4He reaction. The influence of temperature using NRA was first studied based upon 3He depth profile changes and the on-line monitoring of helium release. The effect of the sample microstructure was also investigated at the grain scale by performing analyses of the helium spatial distribution with a nuclear microprobe. Neither substantial helium release nor depth profile changes are observed at temperatures below 900°C in irradiated samples. Following annealing at temperatures above 1000°C, a substantial proportion of the implanted helium is released from the samples. From this temperature upwards, the two dimensional He cartographies reveal that the gas has been preferentially released in the vicinity of grain boundaries. These results can be interpreted in the light of previous studies in terms of gas precipitation and re-solution. Helium precipitation is enhanced in irradiated samples up to 900°C because of the presence of irradiation induced defects. At temperatures in excess of 1000°C, the precipitated helium is partly returned to the matrix hence it is preferentially released in regions adjacent to grain boundaries, which appear to act as defect sinks. [Copyright &y& Elsevier]

Within the frame of the long-term evolution of spent nuclear fuel in dry disposal, the behavior of He in UO 2 polycrystals has to be studied. Here, strain relaxation in He implanted samples has been characterized using in situ X-ray diffraction during thermal annealing. The influence of a wide range of experimental parameters (annealing atmosphere, He ion energy, orientation of the UO 2 grains probed by X-rays) has been evaluated. If each of them contributes to the strain relaxation kinetics in the implanted layer, strain relaxation is not completed for temperatures below 900 °C which is equivalent to what has been found on He implanted UO 2 single crystals, or aged UO 2 pellets doped with α-emitters. In the case of implantation with 500 keV He ions, we clearly show that strain relaxation and He release are not correlated for temperatures below 750 °C. [ABSTRACT FROM AUTHOR]

Helium behaviour in irradiated uranium dioxide may play an important role in the mechanical stability of nuclear fuels during and after its use in nuclear power plants. Helium migration mechanisms in bulk UO 2 have already been the subject of theoretical studies but there is a lack of experimental data relating to the most stable location in the crystal. To this end, we have studied uranium dioxide samples implanted with helium ions at low fluence before and after thermal annealing in the range 600 and 800 °C. UO 2 single crystals were implanted with 50 keV −3 He ions at the fluence of 1 × 10 15 at cm −2 and the location in the lattice of helium atoms was investigated using NRA (Nuclear Reaction Analysis) based on the reaction of 3 He with deuterons ( 3 He (d,p) 4 He) in a channelling mode, recording angular scans across axes and planes. Furthermore, the uranium sub-lattice was analysed by the classical RBS method. After implantation, the experimental angular scans recorded across the main crystallographic axes and along major planes show that the helium atoms in their large majority occupy octahedral interstitial sites. No modification of the occupied crystallographic site was found after annealing at 600 °C. Conversely, no crystallographic relationship between matrix and helium signals was revealed following annealing at 800 °C. The latter feature is likely related to the clustering of implanted helium atoms into gas-filled bubbles. These experimental results have been quantified and interpreted using Monte Carlo simulations with the McChasy code. [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: 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]

Abstract: To understand the mechanisms related to helium migration in nuclear oxide fuels, 3He was implanted at 2×1016 at.cm−2 in UO2 sintered disks. The samples were then annealed at temperatures ranging between 800°C and 1300°C. Changes in helium concentrations were investigated using two nuclear reaction analysis techniques based on the 3He(d,α)1H reaction. The first technique uses a macrobeam in order to measure helium depth profiles averaged over a 500×500μm2 area. The second technique uses a microbeam and provides a quantitative two-dimensional helium concentration cartography, for which each pixel corresponds to a depth integrated helium signal. The analysed zones were optically characterised which enabled a detailed analysis of the impact of the material microstructure on helium release from the samples, with an emphasis on the study of grain boundary effects. [Copyright &y& Elsevier]

Abstract: A two-dimensional analysis of helium in UO2 sintered disks has been achieved on a micrometric scale by using the NRA technique and upgrading the detection arrangement. The transmission mode detection procedures, localization of the region scanned and normalization of the helium lateral distributions are now well established. For sintered UO2 pellets implanted at a dose of 2×1016 3He/cm2, the results show that the lateral helium distribution is correlated to the sample microstructure after annealing for 1h at 1300°C. The enhanced desorption of helium in the vicinity of grain boundaries points to them playing an essential part in helium diffusion in UO2. [Copyright &y& Elsevier]