Marie Kirkegaard1 2 Andrew Miskowiec1 Michael Ambrogio1 John Langford1 Brian Anderson1

1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
2, University of Tennessee, Knoxville, Knoxville, Tennessee, United States

Uranyl fluoride (UO2F2), the hydrolysis product of uranium hexafluoride (UF6), is a byproduct of the nuclear fuel cycle. Understanding the chemical behavior of uranyl fluoride in various environmental conditions is important for nuclear forensics applications. Uranyl fluoride exists in two known crystal structures. The hexagonally-coordinated anhydrous crystal can be converted to a pentagonally-coordinated hydrate of the form ([(UO2F2)(H2O)]7●(H2O)4) in the presence of gas-phase water. We have probed this phase transition using in-situ Raman spectroscopy with temperature and relative humidity control.

At elevated water vapor pressure, the uranyl fluoride hydrate undergoes further transformations to uranyl hydroxide and peroxide species. These transitions were studied using micro-Raman spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy to analyze particles equilibrated at different water vapor pressures. The formation of uranyl peroxide species is especially noteworthy because of the absence of hydrogen peroxide in the system, which has generally been assumed to be necessary for the formation of these species.