Abstract: [Background] The separation and cyclic utilization of thorium is an important part of thorium-uranium fuel cycle for molten salt reactor. Therefore, separation of thorium from fission products, especially lanthanides, is the key step to electrochemical separation of thorium in molten salt because of their similar chemical properties.[Purpose] The aim of this work is to explore the influence of fluorine ion (F^-) on electrochemical properties of 5 lanthanides in LnF₃-LiCl-KCl molten salt and to confirm the feasibility of electrochemical separation of thorium from lanthanides in chloride molten salt.[Methods] Transient electrochemical techniques such as cyclic voltammetry and square wave voltammetry were employed to investigate the reduction mechanism and diffusion coefficient of Ln ions on a tungsten inert electrode.[Results] In LnF₃-LiCl-KCl molten salt, Ce³⁺ and Gd³⁺ were reduced into metal Ce and Gd according to a one-step process by exchanging 3 electrons; Nd³⁺ was reduced into metal Nd through two steps; Sm³⁺ and Eu³⁺ couldn't be reduced into metal in tungsten electrode, and both of them were transformed into divalent state. The existence of F^-would not change the electrode reaction process of Ln cations. The differences of deposition potential between Th and the 5 Lns on inert electrode were larger than 0.19 V, which demonstrated theoretically that it was feasible to separate Th from Lns in LiCl-KCl molten salt. Compared with pure chloride molten salt, the existence of F^-decreased the activity of Ln³⁺, resulting in the decrease of diffusion efficiency of all Ln ions.[Conclusion] The study provided experimental foundation for feasibility investigation of the separation of thorium fluoride from lanthanides fluoride in LiCl-KCl molten salt.

Key words: LnF₃-LiCl-KCl, Cyclic voltammetry, Square wave voltammetry, Electrode process, Diffusion efficient