Phase Equilibria Studies of the LiCl-KCl-UCl₃ System

Regions of the LiCl-KCl-UCl₃ phase diagram used to represent the molten salt compositions generated during the electrorefining of used nuclear fuel were evaluated by studying the LiCl-UCl₃ and KCl-UCl₃ binary systems and several ternary mixtures. Phase transition temperatures of several binary and ternary mixtures made with LiCl, KCl, and UCl₃ were measured by using differential scanning calorimetry. Inductively coupled plasma-atomic emission spectroscopy was used to measure the gross compositions of the salt mixtures and X-ray diffraction (XRD) was used to identify the phases formed after they were thermally cycled and had cooled to room temperature. The LiCl-UCl₃ system has a eutectic transition at 763 ± 2 K for a mixture with 25 mol % UCl₃. The KCl-UCl₃ system has two eutectic transitions, one at 827 ± 3 K and another at 805 ± 4 K for mixtures with 19 mol % UCl₃ and 57 mol % UCl₃, respectively, and the congruently melting compound K₂UCl₅ was identified to have formed by XRD. The LiCl-UCl₃ and KCl-UCl₃ binary phase diagrams were developed and combined with the LiCl-KCl phase diagram to produce a portion of the LiCl-KCl-UCl₃ phase diagram. The LiCl-KCl-UCl₃ system includes two ternary eutectics, one occurring at 681 ± 6 K for the mixture with 33 mol % UCl₃, 42.0 mol % LiCl, and 25 mol % KCl, and the other at 619 ± 1 K for the mixture with 8 mol % UCl₃, 50.0 mol % LiCl, and 42 mol % KCl. The evaluation of these phase diagrams provides an improved understanding of the LiCl-KCl-UCl₃ systems generated during electrorefining.