Electrochemical Behavior of Lu3+ and Ultrasonication-enhanced Density-driven Separation of LuBi in Liquid Bi for Pyrochemical Applications

Abstract

Used nuclear fuel partitioning by group separation is a key strategy for a sustainable nuclear fuel cycle. Bi has been proposed as a dense liquid medium for the group separation of intermetallic compounds (IMCs). However, its applicability to elements whose IMC densities are close to that of liquid Bi remains uncertain. In this study, Lu was selected as a representative element to evaluate the density-based vertical distribution of LuBi in liquid Bi. The electrochemical behavior of Lu 3+ in LiCl-KCl at 773 K was investigated by cyclic voltammetry using inert and Bi-coated electrodes. From these measurements, the apparent standard potentials, diffusion coefficients, and exchange current densities of the Lu 3+ /Lu and Lu 3+ /LuBi couples were derived, indicating that both redox systems exhibit reversible and diffusion-controlled behavior. Bi-Lu alloys were then prepared by galvanostatic electrodeposition in LiCl-KCl-LuCl 3 using a liquid Bi cathode and by direct incorporation of Lu metal into liquid Bi. Without ultrasonication, Lu incorporation into Bi was strongly hindered by the high surface tension, causing Lu accumulation near the cathode surface and incomplete Bi-Lu alloy formation. Moreover, the formed LuBi did not follow the density-based vertical distribution. In contrast, ultrasonication promoted Lu incorporation, LuBi IMC formation, and density-driven migration of LuBi toward the bottom of the Bi pool, even though the density difference between LuBi and liquid Bi is only ~6%. These results demonstrate the feasibility of density-based separation of IMCs in liquid Bi despite a small density difference.