Probing the hydrolytic degradation of UF4 in humid air.

This manuscript describes the chemical transformations that occur during hydrolysis of uranium tetrafluoride (UF4) due to its storage in humid air (85% and 50% relative humidity) at ambient temperatures. This hydrolysis was previously reported to proceed slowly or not at all (depending on the percent relative humidity); however, previous reports relied primarily on X-ray diffraction methods to probe uranium speciation. In our report, we employ a battery of physiochemical probing techniques to explore potential hydrolysis, including Raman spectroscopy, powder X-ray diffraction, 19F nuclear magnetic resonance spectroscopy, scanning electron microscopy, and focused ion beam microscopy with energy-dispersive X-ray spectroscopy. Of these, only Raman spectroscopy proved to be particularly useful at observing chemical changes to UF4. It was found that anhydrous UF4 slightly oxidizes over the course of thirteen days to Schoepite-like uranium complexes and possibly UO3. In contrast, UF4 exposed to 50% relative humidity slightly decomposes into UO2F2, Schoepite-like uranium complexes, and possibly a high order uranium oxide that eluded chemical assignment (UxOy). Despite the rich chemical speciation observed in our Raman spectroscopy measurements, X-ray diffraction and 19F NMR measurements on the same material showed no changes. Microscopy measurements suggest that the observed reactions between UF4 and water occur primarily on the surface of UF4 particulates via a method that is visually similar to surface corrosion of metals. Therefore, we postulate that NMR spectroscopy and X-ray diffraction, which are well-suited for bulk analysis, are less suited than Raman spectroscopy to observe the surface-based reactions that occur to UF4 when exposed to humid air. Considering the importance of UF4 in the production of nuclear fuel and weapons, the results presented herein are widely applicable to numerous nuclear science fields where uranium detection and speciation in humid environments is of value, including nuclear nonproliferation and nuclear forensics.

Characterizing the solid hydrolysis product, UF4(H2O)2.5, generated from neat water reactions with UF4 at room temperature.

Uranium tetrafluoride (UF4) is an important intermediate in the production of UF6 and uranium metal. Room temperature hydrolysis of UF4 was investigated using a combination of Fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR), Raman and infrared spectroscopy, powder X-ray diffraction, and microscopy measurements. UF4(H2O)2.5 was identified as the primary solid hydrolysis product when anhydrous UF4 was stirred in deionized water. Static NMR and 19F magic angle spinning NMR measurements revealed that a small amount of uranyl fluoride can also form when anhydrous UF4 is left in water, although this species comprises less than 5% of the total sample with the remaining parts being UF4(H2O)2.5. Since UF4 is generally considered to be stable under ambient conditions, these findings mark the first time that a room temperature reaction between UF4 and water has been detected and analyzed without interference from additional chemical reagents. The Raman characterization of UF4(H2O)2.5 presented herein is the first on record. Since UF4 is one of the most used intermediates during chemical conversion of uranium ore to uranium metal for nuclear fuel and weapons, the results presented herein are applicable to numerous nuclear science fields where solid state detection of uranium is of value, including nuclear nonproliferation, nuclear forensics, and environmental remediation.