Chemical alteration of UO₂ micro-particles in model lung systems

Uranium dioxide (UO₂) particles can be released from mines, nuclear fuel manufacturing, reactor accidents, and weapons use. They pose inhalation risks, yet their behavior in the human lung remains poorly understood. This study investigates the long-term chemical alteration and dissolution of µm-sized UO₂ particles in two model lung fluids: Simulated Lung Fluid (SLF) and Artificial Lysosomal Fluid (ALF), representing extracellular and intracellular lung environments, respectively. Particles were exposed to each fluid at 37°C for up to 180 days (SLF) and 900 days (ALF). In SLF, UO₂ showed low apparent solubility (<2 % U released to solution), but solid-phase analyses revealed significant oxidation of U(IV) (∼50 %) and formation of autunite-like sheets on the UO₂ surface. Secondary phase formation may lessen overall UO₂ dissolution, promoting long-term particle retention, whilst modifying particle chemical toxicity and cell uptake. In contrast, Monte Carlo simulations indicate that the SLF-induced surface alteration would reduce (>50 %) external radiation dose from the particles. In contrast, UO₂ readily dissolved in ALF (∼75 % U released to solution in 60 days, ∼100 % by 900 days). There was no evidence of secondary phase formation in ALF, but extensive particle matrix dissolution/disaggregation was observed by 30 days. Fragmentation of the UO₂ polycrystalline matrix may lead to release of smaller UO₂ crystallites, which could translocate more readily. Overall, this work provides new mechanistic insight into the fate of inhaled UO₂ under physiologically relevant conditions, highlighting a possible need to consider particle reactivity and alteration processes in health risk assessments.