Exchange Current Density and Diffusion Layer Thickness in Molten LiCl-KCl Eutectic: A Modeling Perspective for Pyroprocessing of Metal Fuels

摘要

The DIFAC (DIFfusion of Actinides in EleCtrorefiner) computer code for pyroprocessing, developed earlier by the authors, is modified in the present work to model electrorefining at the liquid cadmium electrode. The modeling of electrorefining of metal fuels requires accurate knowledge of two important kinetic parameters: exchange current density i_o and diffusion layer thickness δ. These are estimated in the present work by polarization methods and employing Tafel and Allen-Hickling analysis for Gd~(3+)/Gd, U~(3+)/U, and Zr~(2+)/Zr couples in LiCl-KCl eutectic at 773 K for an inert cathode and compared with literature data, wherever possible. The equilibrium potentials for these couples at an inert electrode are found to be -1.94, -1.52, and -1.22 V, respectively, at 773 K. Electrochemical studies are also carried out in LiCl-KCl eutectic to estimate i_o and δ for the anodic dissolution of Na-bonded U-Zr and Gd-U-Zr alloy and are compared with the anodic dissolution of U-Pu-Zr alloy. The equilibrium potential of Na-bonded U-Zr alloy in LiCl-KCl-UCl_3 was found to be -1.46 V, and those for Gd-U-Zr alloy in blank LiCl-KCl and LiCl-KCl-UCl_3 were -1.56 and -1.34 V, respectively, at 773 K. The exchange current densities of Na-bonded U-Zr and Gd-U-Zr alloy were found to be in the range of 40.1 to 46.5 mA • cm~(-2) and 16.8 to 27.3 mA • cm~(-2) at 773 K, respectively. A preliminary design of the liquid cadmium electrode suitable for laboratory-scale experiments on uranium- and plutonium-based systems is also reported in the present work. The i_3 and δ of gadolinium, uranium, and zirconium are subsequently estimated at the liquid cadmium electrode at 773 K. The equilibrium potentials for Gd~(3+)/Cd_6Gd, U~(3+)/[U]~(Cd), and Zr~(2+)/Cd_3Zr couples in LiCl-KCl eutectic at 773 K for the liquid cadmium electrode are found to be -1.35, -1.13, and -1.12 V, respectively. Finally, a few algorithms are proposed for modeling electrorefining data at the liquid cadmium electrode for multicom-ponent systems.