Pulsed Neutron Diffraction Study of Molten CsCl-NaCl-YCl₃: Approaches from Fundamentals to Pyrochemical Reprocessing

Octahedral ionic species are known to exist in molten rare earth chlorides as well as molten alkali metal chlorides containing UO₂2+ ions. This phenomenon can be utilized in the pyrochemical reprocessing of nuclear spent fuels (MOX co-deposition) in which candidate molten salt solvent systems to stabilize UO₂2+ and/or PuO₂2+ ions were proposed for a molten CsCl-NaCl system. Lanthanum trichloride reported in MS8 was replaced by yttrium trichloride since both compounds are known to have different crystal structures but form the same types of octahedral complex anions and their clusters on melting. In the present work, not only the ionic species existing in the molten CsCl-NaCl-YCl3 system but also the short-range structure of the molten CsCl-NaCl solvent system was studied in some detail by pulsed neutron diffraction. No complexation occurred in any solvent melt, but the formation of octahedral complex ions such as YCl63- and the clustering of these were detected by the addition of YCl3.