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Complementary X-ray absorption fine structure (XAFS) and Raman spectroscopy studies were conducted on various UCl 3 concentrations in alkali chloride salt compositions. The samples were 5 mol % UCl 3 in LiCl (S1), 5 mol % UCl 3 in KCl (S2), 5 mol % UCl 3 in LiCl-KCl eutectic (S4), 50 mol % UCl 3 in KCl (S5), and 20 mol % UCl 3 in KCl (S6) molar concentrations. Samples were heated to 800 °C and allowed to cool to room temperature with measurements performed at selected temperatures; the highest temperatures showed the most stability and will be primarily referenced for conclusions. The processing and interpretation of the Raman and extended X-ray absorption fine structure (EXAFS) peaks revealed several uranium-oxygen bond lengths and symmetries in the samples before, during, and after heating. Based on published thermodynamic data of similar systems, X-ray absorption fine structure spectroscopy, and identification of Raman peaks, a β variation of α-U 3 O 8 , typical at room temperature, is the suspected dominant phase of all samples at high temperatures (800 °C). In the existing literature, this β structure of U 3 O 8 was synthesized by slow cooling of uranium oxides from 1350 °C. This paper suggests the rapid formation of the compound due to the decomposition of the uranium chlorides or oxychlorides at increasing temperatures and O 2 reaction kinetics.

Complementary X-ray absorption fine structure (XAFS) spectroscopy and Raman spectroscopy studies were conducted on several UCl 3 concentrations in several chloride salt compositions. The samples were 5% UCl 3 in LiCl (S1), 5% UCl 3 in KCl (S2), 5% UCl 3 in LiCl-KCl eutectic (S3), 5% UCl 3 in LiCl-KCl eutectic (S4), 50% UCl 3 in KCl (S5), and 20% UCl 3 in KCl (S6) molar concentrations. Sample S3 had UCl 3 sourced from Idaho National Laboratory (INL), and all other samples were UCl 3 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 UCl 3 . XAFS and later Raman spectra measured, however, did not correctly match the literature and computational spectra for the prepared UCl 3 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 UCl 3 salts. The oxychlorides present may be both a function of the unknown O 2 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.