Effects of He-Ion Radiation on Solid-State Uranyl Nitrate Compounds under Dry and Hydrated Atmospheric Conditions

Radioactive decay of uranium (U) and its related daughter/fission products emits ionizing radiation, including γ (γ) rays and α (α) particles, resulting in the formation of radical species and induced chemical reactivity in materials. While radioactivity is inherent to the chemistry of U, limited studies provide detailed changes at an atomistic level. This work describes the He-ion radiolysis of four solid-state U(VI) species: UO2(NO3)2·3(H2O) and MUO2(NO3)3 (M = K+, Rb+, Cs+). These materials were irradiated under different atmospheric conditions to further evaluate the impact of water radiolysis on the materials. Pre- and postirradiation analyses were conducted using electron paramagnetic resonance (EPR), Raman, and attenuated total reflectance infrared (ATR-IR) spectroscopy up to 25 MGy. The results indicated the presence of nitrate radical (NO3•) formation in all solid-state materials with similarities to those observed in γ-radiation studies. Irradiation of UO2(NO3)2·3(H2O) did not show evidence of reactive oxygen species bound to the U(VI) cation under inert conditions; however, surface reactivity was observed for samples irradiated in the H2O-saturated environment. Similar chemical changes were observed in the uranyl trinitrato compounds irradiated in the presence of H2O vapor, and there were observed differences in the reactivity depending on the identity of the alkali cation.