Electrochemical and Thermodynamic Properties of CeCl₃on Liquid Cadmium Cathode (LCC) in LiCl-KCl Eutectic Salt

Abstract

Electrochemical and thermodynamic properties of Ce 3+ /Ce on liquid cadmium cathode (LCC) were investigated in LiCl-KCl salt at various temperatures. Cyclic voltammograms only attributing to Ce 3+ /Ce reaction were obtained by background subtraction method and diffusion coefficients of Ce on the LCC were determined at temperatures ranging from 723 K to 798 K. Furthermore, thermodynamic properties of the Ce-Cd intermetallic compound formations were measured via an open circuit chronopotentiometry technique at temperature from 698 K to 823 K. The linear dependence of the Gibbs free energy of the formation was observed as a function of temperature, from which the enthalpy and entropy of the formation were calculated. Here, activity and partial molar Gibbs free energies of Ce in two phase coexisting states were calculated through the measurement of electromotive force. Tafel and linear polarization were conducted with varying Ce concentration in Cd. The exchange current densities (i 0 ) of Ce on the LCC were calculated from both techniques in LiCl-KCl-1 wt% CeCl 3 at 773 K. The resulting data sets of i 0 from both methods show a near linear trend of i 0 within the solubility limit of Ce in Cd. However, i 0 of Ce remains unchanged with increasing Ce concentration beyond the solubility limit. Treatment of the long term radioactive wastes from nuclear fuel cycle have become an essential matter in nuclear industries and public concern.1 One of the most promising option is known as Pyrochemical or Pyroprocessing technology, whose main method is electrorefining in chloride form of salt. This technology has a potential to treat used oxide/metal fuels and produce plutonium (Pu) in company with uranium (U), and minor actinides (MAs), resulting strong resistance for nuclear proliferation.1-3 The main step within this system is an electrorefining process, in which most of U is selectively recovered using a solid cathode in molten lithium-potassium chloride (LiCl-KCl) salt at high temperature (typically at 773 K). Then, the residual U, Pu and MAs are simultaneously collected with small amount of lanthanides into a liquid cadmium cathode (LCC) due to their small activity in liquid metals. 4 Since the operation of the LCC plays an important role in the feasibility of electrochemical processes toward material detections and accountability, 5 extensive studies have been done to understand electrochemical and thermodynamic features of U, Pu, and MAs on the LCC. Shirai et al. 6,7 investigated the reaction of U 3+ /U and Pu 3+ /Pu couples on the LCC. The redox reaction for both couples were almost reversible, and the reduction potential on the LCC showed more positive values than that at the molybdenum electrode. Murakami and co-workers 8 measured diffusion coefficients of actinides and rare earth elements with LCC by performing chronopotentiometry (CP) in LiCl-KCl at temperature ranging from 723 K to 823 K. Castrillejo et al. 5,9,10 investigated the activities and Gibbs energy of rare earth materials in both Cd and Bi liquid electro...

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