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To understand the mechanism of fuel oxidation, in the event of exposure of spent nuclear fuel to groundwater in a final repository, the key parameters are the diffusion coefficients for oxygen and water in UO 2 . In the temperature range relevant for geological disposal (20-100 C), we conducted static corrosion experiments, for 3-6 months, with 18 O-labelled water on different UO 2 matrices (single-crystal, polycrystalline UO 2 , SIMFUEL) so that we could evaluate the penetration of the 18 O-tracer by means of secondary ion mass spectrometry depth profiling, measuring the 18 O/ 16 O ratio. Fick's second law was applied to fit the diffusion profiles in the single crystal and to determine the chemical diffusion coefficient of oxygen, which was found to be in good agreement with the Arrhenius plot extrapolation from high-temperature literature data. Profiles of polycrystalline UO 2 could be fitted with the Levine-MacCallum model, showing that grain boundaries behave like high-diffusivity paths. The presence of secondary phases precipitated along the grain boundaries, as in the case of SIMFUEL, showed a hindering effect for this grain-boundary diffusion, reducing the measured penetration depth.

A radioactive particle embedded in a sediment collected from the Irish Sea was examined with spectrometric methods to understand the potential bioavailability of its constituents. Images of the particle surface were acquired in the backscattering mode by scanning electron microscopy. The elemental composition of the particle surface layer was measured using energy dispersive~SEM-EDX! and wavelength dispersivẽ SEM-WDX! X-ray spectrometers. The investigation showed that the sample consists of a calcite matrix in which uranium is present in the form of separate inclusions. The diameter of U inclusions was less than 10 mm. Synchrotron radiation based X-ray fluorescence in confocal geometry~confocal m-XRF! was used to determine the spatial distribution of elements in the particle. Three-dimensional reconstructions of the Ca, Cr, Mn, Fe, Zn, Sr, Ba, Pb, and U distributions were performed. The oxidation state of uranium in the different inclusions was determined by synchrotron radiation based X-ray absorption in confocal geometry~confocal m-XANES!. The isotopic composition of uranium was measured by secondary ion mass spectrometry. The results revealed that uranium was depleted in 235 U. Pu and other actinides were not detected.