Characterisation of water penetration into polycrystalline UO2

In the event of exposure of spent nuclear fuel to groundwater in a final repository, the preferential dissolution of grain boundaries rather than matrix dissolution would cause a rapid increase of the surface area exposed to groundwater, with effects on the mobilisation of radionuclides and on the overall mechanical stability of the spent fuel pellet. In this respect, a research project has been launched at the Institute for Transuranium Elements (ITU, Karlsruhe, Germany), with the goal of gaining understanding of the mechanisms of the penetration of water into polycrystalline UO2 under conditions relevant for final disposal in a geological repository. As a first approach to this issue, the study has been initially focused on natural UO2 with well defined grain size and morphology. The experiments have then been extended to SIMFUEL, an inactive analogue of UO2 fuel containing elements simulating a spent fuel, to assess potential effects associated to the presence of fission products. The experiments consisted of static leaching tests on UO2 fuel pellets using 18O-labelled water. Unlike most of the diffusion studies on UO2, the experiments in this study were conducted at low temperature (≈ 25°C and 60°C), in order to reproduce the temperature range expected in the geological disposal scenario temperatures. To ensure sufficient contact time between the solid phase and the solution for penetration to be detectable experiments lasted 3-9 months. Experiments were carried out initially in oxidising conditions (in air or in N2 glovebox) and thereafter under 10 bar H2 atmosphere, in order to approach conditions more similar to the anoxic and reducing environment expected in the final repository. Pre- and post-corrosion surface characterisation was performed by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectrometry (XPS), while solution analysis was performed by inductively coupled plasma mass spectrometry (ICP-MS). The penetration of water was assessed by depth profiling of the tracer 18O using Secondary Ion Mass Spectrometry (SIMS) microscopy. Well-known analytical models were used to fit of the profiles and determine the relevant diffusion coefficients. For the first time in this study, the chemical diffusion coefficient of oxygen in different UO2+x phases was experimentally determined at temperatures below 100°C. The good agreement with extrapolated literature data obtained from high-temperature diffusion experiments verifies the viability of the SIMS depth profiling method for the determination of diffusion coefficients, even with a diffusion length of tens of nm. The study has also reported for the first time evidence of the presence of oxygen/water grain-boundary diffusivity in polycrystalline UO2 and an estimate of the diffusion coefficient at 25°C is here presented. The study has also shown that the possibility to observe grain boundary diffusion by SIMS depth profiling might depend to a large degree on the grade of surface oxidation of UO2. In general, all these experimental findings have given an important starting point for the study of water penetration in the spent fuel. Zusammenfassung ii Charakterisierung der Penetration von Wassermolekülen in polykristallines UO2 Stichwörter: Urandioxid; SIMFUEL; Sekundärionen-Massenspektrometrie; Tiefenprofil; Korrosion; Auslaugen; chemische Diffusion; Korngrenzendiffusion; Kristallgitter Diffusion; Sauerstoff-Diffusion; Endlagerung; abgebrannte Brennelement