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

Abstract

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.

In Situ High-Temperature Raman Spectroscopy of UCl3: A Combined Experimental and Theoretical Study

“…It was found that the capillary sealing methodology successfully prevented degradation of the UCl 3 sample via oxidation and/or hydration (Figure S4). This was evident, as there are no U–O vibrational modes present in any of the spectra that were recorded between 300 and 1000 cm –1 ; in uranium oxychloride (UO 2 Cl 2 ), there is a strong temperature-dependent vibrational band located between 700 and 760 cm –1 . Additionally, there are no vibrational features near 450 cm –1 , which occurs in uranium dioxide [(U 4+ O 2 ) 0 ], and near 775 cm –1 , which occurs in uranyl [(U 6+ O 2 ) 2+ ] hydrates and oyxhydrates. − …”