This research thesis progresses along two pathways, alteration of spent nuclear fuel and identification of said alteration. The relative abundance of uranium as an energy resource, coupled with the high costs of spent nuclear fuel reprocessing and the associated risks of nuclear proliferation make a strong case for direct disposal of SNF in deep underground geological disposal facilities. The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater with potential high alpha radioactivity even after hundreds of years. Considering the lack of oxygen at repository depths of 500 m to few km below the Earth, anoxic dissolution experiments with uranium dioxide in various solid forms was conducted to investigate secondary phases formation, the escape of radioactivity in the form of dissolved uranium and electrochemistry evolution to understand the redox changes happening in the surface and solution of our experiments due to the interaction of water and spent nuclear fuel.

The other research thrust in this thesis is the analysis of potential secondary phases via non-destructive scientific techniques such as scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), electron- backscattered diffraction (EBSD), X Ray Diffraction (XRD), Raman spectroscopy with a chapter dedicated on 17O NMR. For the latter, given the long alteration timeline for uranium dioxide, it is challenging to achieve sufficient alteration products for analysis within a short 4-year PhD program. Some known uranium compounds were synthesized and enriched with 17O, an NMR active isotope with spin 5/2 for investigation of their properties. The many advantages of NMR over conventional x- ray diffraction technique render this an important chapter, especially when alteration products may not be single- phase and crystalline.