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.