Chemical transformations of UF4 under controlled temperature and relative humidity

•Chemical changes of UF4 aged under controlled temperature and humidity were studied.•Aging at high relative humidity conditions caused hydration, forming UF4 hydrates.•UF4 exposed to high relative humidity formed UF4 hydrates in as few as 90 days.•UF4 exposed to moderate or low relative humidities was stable for up to 9 months. Uranium tetrafluoride (UF4) is a critical compound in the nuclear fuel cycle, particularly as an intermediate in uranium hexafluoride (UF6) and uranium metal production, and as a fuel for fluoride-based molten salt reactors. Changes to UF4 in humid air were investigated by aging anhydrous UF4 under various temperature and relative humidity (RH) conditions for up to 9 months. Powder X-ray diffraction (pXRD) and thermogravimetric analysis revealed UF4 remained largely unchanged during the aging study for most of the conditions investigated (293 and 308 K, and ≤75% RH), consistent with the conventional consideration that UF4 is stable under ambient conditions. However, aging under high RH conditions (>90%) resulted in chemical speciation changes and formation of UF4 and UF4 hydrate mixtures. Specifically, UF4·2.5H2O was formed within 30 days for UF4 aged at 293 K and 95% RH. Uranium tetrafluoride hydrates with less than 2.5 waters of hydration were formed within 180 days for UF4 aged at 308 K and 91% RH. These findings represent the first report of UF4 hydrates forming from the reaction of UF4 with atmospheric water vapor at ambient temperatures. Investigation of the surface composition of unaged and aged UF4 samples with X-ray photoelectron spectroscopy revealed degradation of the UF4 surface, consistent with the degradation observed for the bulk UF4 from the pXRD measurements. From a nuclear forensics perspective, these short timeline aging studies suggest that the presence of UF4 hydrate in UF4 materials may indicate it was stored under high humidity conditions.