Electrochemical and thermodynamic behavior of La, Ce, Nd, Dy in High temperature molten LiCl-KCl eutectic

Abstract

There is a growing demand for development of an advanced nuclear power system from several points of view: less amount and toxicity of nuclear waste; higher non-proliferative characteristic; more advanced safety features; lower cost for generation; and more sustainable energy source not limited by resources. Pyroprocessing is considered as the appropriate spent nuclear fuel reprocessing technology which guarantees non-proliferation in the Korean back-end fuel cycle planning. The spent fuel is separated electrochemically into uranium and other elements group in electrorefining and electrowinning process after it undergoes preconditioning. Molten LiCl-KCl eutectic is used as the solvent medium and the temperature is set as 773K. The reduction/oxidation of uranium, transuranic elements, rare earth elements, and other fission products and the current determined by electron transfer Butler-Volmer equation depend on their standard equilibrium potential and electrode potential. Diffusion coefficient can be used to resolve the mass transfer equation composed of diffusion, migration, and convection. Furthermore, the solubility of elements in LiCl-KCl is basic thermodynamic information to investigate the material balance in the whole process. In this work, cyclic voltammetry (CV) and chronopotentiometry (CP), which are electrochemical analysis techniques, were used to investigate the redox reaction mechanism and to calculate the standard potential and diffusion coefficient of lanthanum, cerium, neodymium, and dysprosium in 773 K LiCl-KCl eutectic system. The selected elements were the three most abundant lanthanide elements in spent fuel and one of heavy lanthanide elements. La, Ce, and Dy revealed one step electron transfer, while Nd exhibited two step electron transfer of Nd(III)/Nd(II) and Nd(II)/Nd(0). The solubility of chloride form of those elements in 773 K LiCl-KCl was investigated using ICP-AES in sample depth from the top to bottom. Solubility limits of Ce and...