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 Xray, 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 degrees 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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Characterization of plutonium dioxide (PuO2) and thorium dioxide (ThO2) aging under different temperature and relative humidity conditions revealed significant stability differences between actinide oxides, providing important insights for nuclear forensic applications and fundamental safety and storage concerns. The study used a variety of techniques, including scanning electron microscopy and X-ray diffraction, to analyze the morphological and phase changes in the materials, highlighting the need for further research on the chemical and physical properties of actinide compounds near ambient temperatures and humidities.