This dissertation covers three related research projects investigating the nanoscale interactions of uranyl peroxide nanoclusters with monovalent and divalent cations. Each project utilizes complementary experimental techniques, such as ultra-small angle X-ray scattering (USAXS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), single crystal X-ray diffraction (SC-XRD), and ultraviolet-visible near-infrared (UV-vis-NIR) spectroscopy, to study the behavior of the nanoclusters in aqueous systems. The first project investigated the nanoscale interactions between U60Ox30, [((UO2)(O2))60(C2O4)30]60-, and neptunium(V) as a function of neptunium concentration. Our findings showed that neptunium induces aggregation of U60Ox30 when the concentration was ≤ 10 mM Np, while (NpO2)2C2O46H2O(cr) and studtite (((UO2)(O2)(H2O)22H2O(s)) formed at 15–25 mM Np. These results suggest that neptunium coordinates with the bridging oxalate ligands in U60Ox30, leaving metastable uranyl peroxide species in solution.The second project explored the nanoscale interaction between U60 [(UO2)(O2)(OH)]60 and U60Ox30 with plutonium, which was added as Pu(VI). Our results showed that Pu(VI) was reduced to Pu(V) in the presence of U60 and a mix of Pu(IV) and Pu(V) in the presence of U60Ox30 over a two-week period. Pu(V) subsequently promoted the aggregation of U60 in the form of blackberries and U60 macro-aggregation in the form of U60 blackberry brambles. The latter represents a new structure not previously identified in the literature. In the U60Ox30 system, Pu(IV/V) promoted aggregation in the form of large mass fractals. All aggregates became more compact with increasing plutonium concentration, suggesting possible encapsulation of plutonium within U60 and U60Ox30. The third project investigated the aggregation of U60 nanoclusters as a function of alkali and alkaline earth metal concentrations. Our results showed that counterion-mediated attraction was the primary driver for U60 aggregation. Other factors, such as cation concentration, charge, and hydration radii, also influence the size and type of aggregates. We observed a distinct trend in aggregate size triggered by the addition of alkali and alkaline earth metal cations: Na+ > K+ > Rb+ > Cs+ and Mg2+ > Ca2+ > Sr2+ > Ba2+. Tertiary structures were most prevalent among alkali metal cations, specifically for systems containing 9.5 mM Rb+ or 9.5 mM K+, but were also observed in the system containing 2.5 mM Ba2+. The compactness of the aggregates played an important role in overall size, with cations that had larger atomic radii and smaller hydration spheres producing the most compact aggregates. Further research is necessary to deepen our understanding of the mechanisms underlying aggregate formation and the behavior of other uranyl peroxide nanoclusters.These studies provide insights into the nanoscale interactions of uranyl peroxide nanoclusters with actinides elements, alkali metals, and alkaline earth metals. Our findings provide the basis for the application of uranyl peroxide nanoclusters throughout the nuclear fuel cycle, perhaps most specifically in the separation of used nuclear fuels.