Atomic level insight into irradiation effects in nuclear fuel materials

Microstructural evolution during in-pile irradiation, radiation damage effects and fission products behavior in UO2 nuclear fuel are key issues in understanding and for the modeling of the performance as well as safety characteristics of nuclear fuels in the reactor. In general, the overall performance of standard UO2 at high burnup is often limited by the physical phenomena of pellet-to-cladding mechanical interaction, gaseous swelling of a fuel pellet, and the release of stable fission gases. Therefore, alternative fuel materials with improved properties have been uncovered, such as chromia-doped UO2, capable to mitigate the above listed effects. This thesis work reports an experimental investigation on irradiation induced microstructure evolution in high burnup standard UO2 and chromia-doped UO2 spent fuels considering the role of dopant Cr and chemical effect of fission product elements, and a comparison of the results of doped fuel with those observed for the standard (undoped) UO2 material. To clarify the prevailing microstructural aspects associated with the development of the so-called high burnup structure, additional experiments have been performed for an evaluation of atomic-scale uranium and plutonium environment in intermediated burnup UO2. In order to investigate the chemical state(s) of the dopant Cr in UO2 crystals, micro-beam XANES experiments were conducted in fresh and irradiated doped fuels. Spent nuclear fuel is intensely radioactive and extremely difficult to investigate experimentally. Therefore, methods have been developed to prepare small sub-samples from targeted regions of the spent fuel pellets. Synchrotron based modern analytical techniques of scanning micro-beam X-ray fluorescence spectrometry, X-ray diffraction, and microprobe X-ray absorption spectroscopy have been used for structural analyses covering a wide range of local burnup from 50 to 100 MWd/kgU in the investigated materials. The obtained results are supplemented by electron probe micro analysis and transmission electron microscopy studies. The line profiles of characteristic X-ray diffraction peaks are analyzed in detail for a quantitative evaluation of UO2 lattice parameter with fuel burnup, microscale spatial distribution of residual strain within irradiated UO2 crystallites, and the remnant dislocation contents in spent fuels. The results provide a comprehensive and comparative overview of the irradiation induced microstructure evolution and structural changes in high burnup standard and doped UO2 fuels. A detail analysis of the of extended-range X-ray absorption spectra of irradiated samples provide novel insights into the local atomic arrangement of uranium and plutonium ions including their oxidation state assignment, and furnish a clear atomic scale picture of the uranium dioxide crystal structure modifications resulting from irradiation effects. The obtained experimental results of this study on the chemical and local structural specificity of uranium and plutonium in irradiated UO2 matrix shall be useful for fuel modelling studies that could account for the UO2 fuel lattice defects evolution as well as fission products and/or actinide products behavior under irradiation conditions.