Microscopy and spectroscopy of plutonium dioxide aging under ambient and near-ambient conditions

Characterization of nuclear materials under controlled conditions promotes a better understanding of their chemistry and provides insight into their stability. Of particular importance is identifying how environmental and storage conditions affect both bulk and sub-bulk scale chemical and physical properties. Such data may provide insight for nuclear forensic applications and can help address fundamental safety and storage concerns. In this work, plutonium dioxide (PuO₂) and thorium dioxide (ThO₂) aging was studied as a function of temperature and relative humidity (RH) while simultaneously observing the effects of fluoropolymer containment. Morphological changes were monitored using scanning electron microscopy and interpreted utilizing a previously-published lexicon of descriptive imaging terminology. Changes in phase were observed using X-ray diffraction measured on a single-crystal X-ray diffractometer to permit sub-bulk scale ( < 10 mg) analysis. Ancillary techniques, including energy-dispersive X-ray, infrared, Raman, and X-ray photoelectron spectroscopy, were used to further probe and interpret the observed aging effects. ThO₂ materials exhibited no change in macroscopic appearance and microstructure, whereas PuO₂ underwent transitions to mixed plutonium dioxide/plutonium fluoride hydrate and mixed plutonium dioxide/ammonium plutonium fluoride phases under ambient temperature at 81% RH and 45 °C at 81% RH, respectively. These results highlight important stability differences between actinide oxides and underpin the belief that further research is needed to elucidate the fundamental chemical and physical properties of many actinide compounds, like PuO₂ and ThO₂, near ambient temperatures and humidities.