in-situ X-ray Absorption Spectroscopy of Oxygen and Moisture Exposed Uranium Trichloride Molten Fuel Salts

In recent years the concept of a molten salt reactor (MSR) has been identified as one of
the leading generation IV nuclear reactor concepts . The fuel studied in this experiment
is the non-fissile isotope, Uranium-238. In applications the fuel mixture will be a
proportion of uranium-238 and uranium-235. This work builds on the state of literature
and understanding of interactions and molecular structure and dynamics of uranium
chloride in molten chloride salts. X-ray absorption spectroscopy (XAS) data were
collected for between 5 and 50% molar UCl3 in LiCl, KCl, and LiCl-KCl eutectic salts from
between 20 and 800°C. The initial goal of this work was to better understand UCl3
structure in molten salts and identify potential ternary formation, specifically K2UCl5.
Results showed that even within single containment and reliable preparation methods,
samples experienced unintended intense oxidation and hydration. Complex
oxychlorides formed at low temperatures that quickly decomposed under heating and
formed a uniform β, high temperature, phase of U3O8 in all samples. These results
strengthen understanding, specifically of in molten salt environments, of oxychloride
formation and hydration, oxychloride decomposition range, and introduce a mechanism
of the β-U3O8 formation at lower temperatures than previously reported.