Thermodynamic modeling of the KCl-LiCl-NaCl-UCl3 system for molten salt electrolysis and reprocessing of spent nuclear fuel

Sodium-cooled fast reactors utilize metallic fuels that include bond-sodium within the fuel element. Molten salt electrolysis at 773 K with eutectic KCl-LiCl mixed with UCl₃ as an electrolyte can recover the actinides from spent fuel. The critical factor affecting the useful life of the electrolyte is the increase in liquidus temperature from the accumulation of lanthanides, actinides, and sodium. Therefore, thermodynamic modeling of the KCl-LiCl-NaCl-UCl₃ system was carried out by considering experimental data from the present work and literature as input. The liquidus and solidus temperatures for the two ternary systems, KCl-NaCl-UCl₃ and LiCl-NaCl-UCl_3, were determined using differential scanning calorimetry. The thermodynamic parameters for pure UCl₃ were optimized for liquid and solid states over a wide temperature range. Several constituent binary (AkCl-UCl₃; Ak: K, Li, Na) and ternary (KCl-LiCl-UCl₃, KCl-NaCl-UCl₃ and LiCl-NaCl-UCl₃) systems were assessed or reassessed in this work. A new intermediate phase (K₃UCl₆) was included in the reassessment for the KCl-UCl₃ system. There is good agreement between the experimental and calculated thermochemical and phase diagram data for all the systems optimized in the present work. This work is beneficial to determine the effect of NaCl on the liquidus temperature and other thermodynamic properties of KCl-LiCl electrolyte mixed with UCl₃ for improving the efficiency of molten salt electrolysis.