Abstract
Active fission products such as lanthanide metals, alkaline earth metals and some actinides from the used nuclear fuel (UNF) get accumulated in LiCl-KCl eutectic salt electrolyte during the electrorefining operation of pyroprocessing technology. The accumulation of fission products results in the formation of molten multicomponent salt system and could lead to the deviation in the physical properties such as phase transition temperatures and thermodynamic properties such as Gibbs free energy. The change in these properties could affect the efficiency of a salt electrolyte and it can lead to the decrease in the deposition of pure uranium on the cathode. Hence, it is necessary to study the deviation in the properties as a function of temperature and composition. Lanthanides are concerning as they tend to form soluble chlorides and behave chemically and electrochemically the same as uranium and plutonium. Hence, they may get co-deposited and act as a neutron poison in the recycled fuel. Solubility, phase transition temperature, and thermodynamic data are crucial for the operation of the electrorefining process. However, such data is rare particularly for higher order of salt mixtures.In this work, the physical properties such as solubility and the phase transition temperatures were investigated along with thermodynamic analysis. The solubility of one or more lanthanide chlorides in LiCl-KCl eutectic salt was measured by creating a solid-liquid equilibrated system, sampling the liquid phase and analyzing the liquid composition using the inductively coupled plasma mass spectrometry (ICPMS) technique. In addition, a novel thermal ramping up technique coupled with the first derivative analysis was used for LiCl-KCl, NdCl3 + LiCl-KCl eutectic salt, LaCl3 + CeCl3 + LiCl-KCl eutectic salt, LaCl3 + NdCl3 + LiCl-KCl eutectic salt systems to determine the phase transition temperatures. Furthermore, the physical states of salt systems were observed and confirmed by the images captured using an endoscope. Moreover, the surrounded ion model was used to determine the Gibbs free energy of mixing for different salt systems around 727.0°C (1000 K) to study the effect of the concentration of lanthanide elements on the thermodynamic properties.
In this work, the manual feeling using the thermocouple to confirm the equilibrium condition of salt systems combined with the ICPMS technique for the compositional analysis was found to be not a feasible approach to determine the real solubilities of lanthanide elements in LiCl-KCl eutectic salt. Furthermore, it was found that the first derivative approach works well to determine the phase transition events for single as well as multicomponent salt systems. In addition, multiple transition temperatures were observed as the concentration of lanthanide elements increased in LiCl-Kcl eutectic salt. In addition, the negative Gibbs free energy of mixing and enthalpy of mixing indicates that the mixing process is thermodynamically favorable, exothermic, and the mixture is moving towards a more stable state where it is unlikely for the system to revert to its original state.