Radiation Effects on Uranyl Tetrachloro Coordination Compounds: Impact of Lattice Water

Nuclear materials, such as uranium-bearing solids, are exposed to high levels of ionizing radiation throughout the nuclear fuel cycle; thus, it is important to develop a molecular-level understanding of how these materials behave and degrade in the presence of gamma (γ) irradiation. In the current study, three U(VI) tetrachloride complexes, M₂[UO₂Cl₄]·xH₂O (where M = K⁺, Rb⁺, or Cs⁺ and x = 0 or 2), and their respective chloride salts were exposed to 1-50 kGy of γ radiation using a ⁶⁰Co source. Irradiated materials were evaluated by using electron paramagnetic resonance (EPR) and Raman spectroscopy and were further explored by using density functional theory (DFT) methods. EPR spectra of the irradiated materials suggest the formation of a Cl-based radical for both the alkali salts and the uranyl tetrachloride compounds, and DFT calculations provide evidence that the Cl₂^-• radical is formed within these materials. The presence of water in the K⁺ and Rb⁺ compounds leads to additional spectroscopic signatures that could be traced back to water radiolysis and the formation of peroxide and superoxide species. DFT results support the formation of HO₂^• in the lattice and potentially the formation of a [UO₂Cl₃(O₂)]^3- species, highlighting the impact of water within the hydrated material to alter U(VI) speciation by radiolysis.