Identification and Decomposition of Uranium Oxychloride Phases in Oxygen-Exposed UCl₃ Salt Compositions

Complementary X-ray absorption fine structure (XAFS) spectroscopy and Raman spectroscopy studies were conducted on several UCl₃ concentrations in several chloride salt compositions. The samples were 5% UCl₃ in LiCl (S1), 5% UCl₃ in KCl (S2), 5% UCl₃ in LiCl-KCl eutectic (S3), 5% UCl₃ in LiCl-KCl eutectic (S4), 50% UCl₃ in KCl (S5), and 20% UCl₃ in KCl (S6) molar concentrations. Sample S3 had UCl₃ sourced from Idaho National Laboratory (INL), and all other samples were UCl₃ sourced from TerraPower. The initial compositions were prepared in an inert and oxygen-free atmosphere. XAFS measurements were performed in the atmosphere at a beamline, and Raman spectroscopy was conducted inside a glovebox. Raman spectra were able to confirm initial UCl₃. XAFS and later Raman spectra measured, however, did not correctly match the literature and computational spectra for the prepared UCl₃ salt. Rather, the data shows some complex uranium oxychloride phases at room temperature that transition into uranium oxides upon heating. Oxygen pollution due to failure of the sealing mechanism can result in oxidation of the UCl₃ salts. The oxychlorides present may be both a function of the unknown O₂ exposure concentration, depending on the source of the leak and the salt composition. Evidence of this oxychloride claim and its subsequent decomposition is justified in this work.