Morphological Characterization of Uranyl Fluoride Particles via Atomic Force Microscopy

Uranium hexafluoride (UF₆) undergoes a rapid hydrolysis reaction when exposed to atmospheric water. In addition to producing hazardous HF gas, the hydrolysis reaction produces uranyl fluoride (UO₂F₂), a radioactive solid phase particulate material. Because of the technological utility of UF₆ in the nuclear fuel cycle, understanding the transport properties of UO₂F₂ aerosol produced via UF₆ hydrolysis is important for accident scenarios. Moreover, the fundamental chemical and physical properties of the UF₆ hydrolysis reaction are not completely understood. Recently, several experiments on the aerosol phase properties of UO₂F₂ produced in this way have shown that under most relevant conditions, the particle size distribution (PSD) of UO₂F₂ can be extremely small, approximately 3 to 5 nm, which is well below the threshold that can be routinely observed via scanning electron microscopy (SEM). Although readily observable in the aerosol phase, observation of nanometer-sized particles in the condensed phase (i.e. deposited on surfaces) remains a challenge. Here, we have used atomic force microscopy (AFM) to study the PSD and morphological characteristics of UO₂F₂ deposited at low and high concentrations under different humidity conditions, a primary variable in the hydrolysis reaction. We find strong agreement between PSD measured in the aerosol phase via scanning mobility particle sizing and PSD measured via AFM, with particle sizes peaked below 4 nm for low-humidity conditions. At higher humidity, the distribution is centered around 5 to 10 nm but extends up to 20 nm. These results are in stark contrast to previous measurements using SEM that show PSD on the order of 300- to 1000-nm particle sizes; moreover, these are the first direct measurements of individual particles of UO₂F₂ having been produced via UF₆ hydrolysis deposited on surfaces. These measurements, therefore, open a new avenue for collecting and detecting UO₂F₂ in the condensed phase and further refine the PSD, which is critical for environmental transport determinations.