Structural Stability and Photoluminescence Property of Cs₂UCl₆ Single Crystal Derived from Spent Nuclear Fuel

The recycling and reuse of trace uranium from spent nuclear fuel is of great significance for the safety management of the nuclear fuel cycle. However, stabilization of low-valent uranium has always been a challenge due to the ultraoxidizable nature of uranium ions, which remains relatively uncharted territory in spent fuel treatment. In the current study, U⁴⁺ was immobilized in Cs₂UCl₆ single crystal with a perovskite structure from uranyl under a strong acidic environment. A comprehensive and detailed understanding of Cs₂UCl₆ at the atomic scale has been achieved by combining density functional theory (DFT) with high-resolution integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) imaging, which was captured by utilizing Cs-corrected TEM for the first time. Furthermore, the results obtained from X-ray excitation and the photoexcitation effects produced by PL at 280, 330, and 360 nm provide compelling evidence for the ability of U⁴⁺ to form excitable bands around the Fermi level. The as-synthesized Cs₂UCl₆ demonstrates excellent thermal stability above 275 °C, as evidenced by in situ Raman spectroscopy and thermogravimetric analysis, while a degradation pathway initiated by CsCl upon exposure to water vapor was revealed by synchrotron X-ray diffraction. Thermal and chemical stability can be further elevated by consolidating it into a metal-organic framework (MOF) via hot pressing. The current study provides a promising strategy to reuse and functionalize the spent nuclear fuel.