I study the electronic structure and redox behavior of mixed-valence uranium oxides using first-principles calculations. My work investigates the nanoscale mechanisms of uranium reduction by magnetite, including the formation of uranium oxide nanoparticles, their self-assembly into nanowires, and the thermodynamic drivers of phase transformations in environmental systems. To address these processes from both thermodynamic and kinetic perspectives, I combine ab initio thermodynamics with first-principles electron-transfer theory, enabling a quantitative description of the stability, transformation pathways, and charge-transfer mechanisms governing uranium oxide formation under environmentally relevant conditions.