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.

四氟化铀 (UF ₄ ) 是核燃料循环中的关键化合物，特别是作为六氟化铀 (UF ₆ ) 和金属铀生产的中间体，以及作为氟化物基熔盐反应堆的燃料。更改UF ₄由老化无水UF在潮湿的空气中进行了研究₄的各种温度和相对湿度（RH）条件下长达9个月。粉末 X 射线衍射 (pXRD) 和热重分析表明，在大多数研究条件（293 和 308 K，以及 ≤75% RH）下，UF ₄在老化研究期间基本保持不变，这与 UF ₄的常规考虑一致在环境条件下是稳定的。然而，在高 RH 条件 (>90%) 下老化导致化学形态变化和 UF ₄和 UF ₄水合物混合物的形成。具体而言，UF ₄ ·2.5H ₂ O在 293 K 和 95% RH 下老化的UF ₄在 30 天内形成。对于在 308 K 和 91% RH 下老化的UF _4，在 180 天内形成了水合水少于 2.5 的四氟化铀水合物。这些结果表示UF的第一报告₄水合物从UF的反应形成₄在环境温度下大气中的水蒸气。未老化和老化 UF ₄表面成分的研究具有 X 射线光电子能谱的样品揭示了 UF ₄表面的降解，这与从 pXRD 测量中观察到的块状 UF ₄的降解一致。从核法证学的角度来看，这些短时间老化研究表明，UF ₄材料中存在 UF ₄水合物可能表明它是在高湿度条件下储存的。