Experimental mixing enthalpy and thermodynamic modelling of UCl3-KCl system

Abstract

The molar enthalpies of mixing (ΔmixHm) in the liquid potassium chloride-uranium(III) chloride mixtures have been measured with a Calvet-type high temperature microcalorimeter over the entire composition range at 1113 K. All the melts are characterized by negative enthalpies of mixing with a minimum value of about −17.5 kJ·mol−1. The minimum of the enthalpy of mixing is shifted towards the potassium chloride-rich compositions and located in the vicinity of xUCl3 ~ 0.40. The obtained results are discussed in terms of complexes formation in the liquid mixtures under investigation.Experimental mixing enthalpy data were used together with existing literature data in re-examination of KCl-UCl3 phase diagram. Analysis of the literature data on KCl-UCl3 phase diagram has indicated that all of them are not correct because the existence of K3UCl6 compound was ignored in spite of experimental facts pointing its existence. Reinterpretation of all existing data by CALPHAD method showed that the system under investigation is characterized by existence of K2UCl5 and K3UCl6 congruently melting compounds, and three eutectics (KCl-K3UCl6, K3UCl6-K2UCl5 and K2UCl5-UCl3, respectively). K3UCl6 solid compound exists at relatively narrow temperature range from about 800 K to congruent melting at about 892 K.