Chemical transformations of UF4 under controlled temperature and relative humidity

Abstract

• Chemical changes of UF 4 aged under controlled temperature and humidity were studied. • Aging at high relative humidity conditions caused hydration, forming UF 4 hydrates. • UF 4 exposed to high relative humidity formed UF 4 hydrates in as few as 90 days. • UF 4 exposed to moderate or low relative humidities was stable for up to 9 months. Uranium tetrafluoride (UF 4 ) is a critical compound in the nuclear fuel cycle , particularly as an intermediate in uranium hexafluoride (UF 6 ) and uranium metal production, and as a fuel for fluoride-based molten salt reactors . Changes to UF 4 in humid air were investigated by aging anhydrous UF 4 under various temperature and relative humidity (RH) conditions for up to 9 months. Powder X-ray diffraction (pXRD) and thermogravimetric analysis revealed UF 4 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 UF 4 is stable under ambient conditions. However, aging under high RH conditions (>90%) resulted in chemical speciation changes and formation of UF 4 and UF 4 hydrate mixtures. Specifically, UF 4 ·2.5H 2 O was formed within 30 days for UF 4 aged at 293 K and 95% RH. Uranium tetrafluoride hydrates with less than 2.5 waters of hydration were formed within 180 days for UF 4 aged at 308 K and 91% RH. These findings represent the first report of UF 4 hydrates forming from the reaction of UF 4 with atmospheric water vapor at ambient temperatures. Investigation of the surface composition of unaged and aged UF 4 samples with X-ray photoelectron spectroscopy revealed degradation of the UF 4 surface, consistent with the degradation observed for the bulk UF 4 from the pXRD measurements. From a nuclear forensics perspective, these short timeline aging studies suggest that the presence of UF 4 hydrate in UF 4 materials may indicate it was stored under high humidity conditions.