Neutron Diffraction Study of the Structural Changes Occurring During the Low Temperature Oxidation of UO 2
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1215-V10-05
Neutron Diffraction Study of the Structural Changes Occurring During the Low Temperature Oxidation of UO2 Gianguido Baldinozzi1, 2, Lionel Desgranges3, Gurvan Rousseau3,1,2 1 SPMS MFE, CNRS Ecole Centrale Paris, Chatenay-Malabry, France. 2 DEN DMN SRMA LA2M MFE, CEA Saclay, Gif-sur-Yvette, France. 3 DEN DEC SESC LLCC, CEA Cadarache, St. Paul-lez-Durance, France. ABSTRACT The oxidation of uranium dioxide has been studied for more than 50 years. It was first studied for fuel fabrication purposes and then later on for safety reasons to design a dry storage facility for spent nuclear fuel that could last several hundred years. Therefore, understanding the changes occurring during the oxidation process is essential, and a sound prediction of the behavior of uranium oxides requires the accurate description of the elementary mechanisms on an atomic scale. Only the models based on elementary mechanisms should provide a reliable extrapolation of laboratory results over timeframes spanning several centuries. The oxidation mechanism of uranium oxides requires understanding the structural parameters of all the phases observed during the process. Uranium dioxide crystal structure undergoes several modifications during the low temperature oxidation that transforms UO2 into U3O8. The symmetries and the structural parameters of UO2, β-U4O9, β-U3O7 and U3O8 were determined by refining neutron diffraction patterns on pure single-phase samples. Neutron diffraction patterns, collected during the in situ oxidation of powder samples at 483 K were also analyzed performing Rietveld refinements. The lattice parameters and relative ratios of the four pure phases were measured during the progression of the isothermal oxidation. The transformation of UO2 into U3O8 involves a complex modification of the oxygen sublattice and the onset of complex superstructures for U4O9 and U3O7, associated with regular stacks of complex defects known as cuboctahedra which consist of 13 oxygen interstitial atoms. The structural modifications during the oxidation process are discussed. INTRODUCTION The development of advanced nuclear energy systems requires scientific data in order to ensure their safe behaviour throughout the fuel life cycle from fabrication to end of life storage. Neutron diffraction is a very valuable tool for the characterization of UO2 ceramic nuclear fuel, because neutrons can probe bulk samples and provide reliable information relative to the O sublattice, overcoming the severe limitations of X-ray diffraction experiments. In this in-situ neutron diffraction experiment we have demonstrated the influence of complex oxygen defects [1] and clusters on the oxidation mechanism in UO2. The engineering of future advanced nuclear energy systems will require new materials and chemical processing techniques that provide structural integrity, phase stability and process efficiency under extreme conditions of radiation, temperature and corrosive environments for timeframes that in some cases span several thousands of years. Fuel developm
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