Electrochemical reduction of $CeO_2$-$Nd_2O_3$ in molten LiCl-$CaCl_2$ eutectic salt

Abstract

Pyroprocessing has been developed for recycling spent nuclear fuel as non-proliferation way. Pyroprocessing consists of three major steps that are head-end process, electrochemical process, and waste treatment process. In electrochemical process, electrochemical reduction is an important step because it supplies a feed material for electrorefining process. In the electrochemical reduction process, oxide was reduced to metal. If unreduced oxides are delivered to the next electrorefining step, unreduced oxides react with uranium metal, and uranium oxide is formed. Uranium oxide is insoluble in LiCl-KCl. Therefore, additional process is needed to recover uranium. In the common electrochemical reduction process, the reduction yields of rare earth oxides are 30-40%. To increase the reduction yields of rare earth oxides, several researches have been conducted. However, rare earth oxides still cannot be reduced fully. In this study, electrochemical reduction of $CeO_2$ and $Nd_2O_3$ in molten LiCl-$CaCl_2$ eutectic salt, which are the major rare earth oxides, was investigated by constant voltage electrolysis. Compared to common LiCl molten salt, reduction yields of Ce and Nd are three times higher in LiCl-$CaCl_2$ eutectic molten salt. It is due to the decrease of reoxidation rate, indirect reduction by deposited calcium metal and reducing environment of molten LiCl-$CaCl_2$ eutectic salt on the basis of thermodynamic and electrochemical data.