Lanthanide Silicide Formation on Si Electrode in LiCl–KCl Melt for Immobilization of Chloride Waste in Pyroprocessing of Nuclear Fuels

During the electrorefining of spent nuclear metallic fuels for recycling actinides, fission products of rare-earths, alkaline-earths and alkalis accumulate in LiCl-KCl melt in the form of their chlorides.For the purpose of periodically removing the accumulated fission product chlorides from the melt and stabilizing them in a waste form, a novel usedsalt treatment process was proposed wherein electrochemical rare-earth silicide formation on a Si cathode was used as one of the main reactions.In this study, electrochemical measurements using a Si plate electrode were performed in LiCl-KCl melt containing lanthanide chlorides at 723 K to obtain basic knowledge of designing the Si cathode configuration and evaluating the composition of the recovered fission products in the proposed used-salt treatment process.A linear relationship between the thickness of the dense silicide layer and the square root of the electrolysis time in potentiostatic electrolysis at -1.5 V vs. Ag/AgCl was seen, which suggested the dense silicide layer growth governed by a diffusion in the layer.We also found a dependence of the distribution of lanthanides between the silicides and the melt on the applied cathodic current density during galvanostatic electrolysis for their recovery as silicides from the melt containing multi-lanthanide chlorides.