Elastic strains in polycrystalline UO 2 samples implanted with He: micro Laue diffraction measurements and elastic model
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Elastic strains in polycrystalline UO2 samples implanted with He: micro Laue diffraction measurements and elastic modeling Axel RICHARD1, Etienne CASTELIER1, Herve PALANCHER1, Jean-Sebastien MICHA2, Philippe GOUDEAU3 1
CEA, DEN, DEC/SESC, Centre de Cadarache, 13 108 St Paul lez Durance, France CEA, INAC, 38 054 Grenoble Cedex 9, France 3 Institut Pprime, CNRS-Universite de Poitiers-ENSMA, BP 30179 - 86 962 Futuroscope, France 2
Abstract : In the framework of the study of long-term storage of the spent nuclear fuel, polycrystalline UO2 samples have been implanted with He ions. The thin implanted layer, close to the free surface is subjected to elastic stresses which are studied by x-ray diffraction (micro Laue diffraction) and a mechanical modeling. A simple expression of the displacement gradient tensor has been evidenced; it concerns only three terms (ε3, ε4 and ε5) which strongly evolve with considered grain orientations. Finally, we show that results obtained with micro diffraction are in very good agreement with conventional x-ray diffraction measurements done in laboratory at macro scale. Keywords : Ion implantation, x-ray micro diffraction, strains measurements, modeling INTRODUCTION Majority of water reactors use uranium dioxide (UO2) as their fuel. Fission of 235U leads produces highly energetic fission products that move rapidly and deposit their kinetic energy throughout the UO2 fuel pellet. The high kinetic energy of the fission products is redistributed throughout an extended spatial region, resulting in a significant increase in the temperature of the fuel pellet. It is this heat deposited in the pellet that is used to heat the reactor coolant, water, and ultimately used to generate steam that drives the turbines to produce the electricity at a nuclear power station [1]. Complex physical phenomena take place in UO2 nuclear fuel in nuclear reactor and then in storage condition. One of the main important phenomena in the studies of the nuclear fuel spent is the structural swelling which is partially initiated by the alpha decay (helium production) of radioactive fission products created during irradiation. For studying this effect, asprepared UO2 samples are implanted with He ions which leads to the formation of a thin layer at the top surface of the sample. This approach allows easy handling of low radioactive samples while simulating storage conditions. X-ray diffraction is used to measure induced strains in the implanted layer: monochromatic macro-diffraction and polychromatic micro-diffraction in Laue mode. Measured strain tensor is validated thanks to a mechanical model. EXPERIMENTAL DETAILS Sample preparation In this study, four polycrystalline UO2 disks (8.2 mm in diameter, 1 mm thick) have been prepared following a standard process [2] which limits surface defects and maintains an O/U stoichiometric ratio close to 2. Average grain diameter is about 18 μm. These samples have been implanted with 60 keV He ions at different fluencies ranging between 1015 and 2x1016 ions/cm2.
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