Uranium Electrorefining in 3LiCl-2KCl Based Melts

Abstract

Molten halides can be effectively employed for electrorefining of refractory and radioactive metals with relatively negative reduction potentials and high melting temperatures. In the present study electrorefining of uranium was studied for developing pyrochemical reprocessing technology of metallized spent nuclear fuels. 3LiCl-2KCl molten salt was chosen as the working electrolyte. Uranium containing electrolytes were prepared by dissolving UCl3 in the solvent salt melt in an argon filled glove box to obtain various desired concentrations of uranium in the electrolyte. The working temperature in the refining experiments varied between 450 and 750 °C. The experiments on uranium electrorefining were carried out in a stainless steel semi-industrial water-cooled electrolyser. Crude uranium metal was loaded in a specially designed molybdenum anodic basket. Glassy-carbon crucible was used to hold the melt. Molybdenum rod was employed as a cathode. The main goal of the electrolytic refining process is obtaining high-purity uranium metal. Specific energy consumption is also very important. The results of the electrolytic refining depend on various parameters. In the present study current density, temperature, uranium concentration in the electrolyte, and specific quantity of electricity passed through the cell were selected as variables, and the current efficiently was chosen as a response factor. Dense and compact uranium deposits were obtained under low current densities, while application of relatively high current densities led to the formation of uranium dendrites. Intermediate values of current density allowed producing more coherent deposits that could be easily scraped off the cathode. Increasing temperature led to a sharp change of the cathodic deposits morphology. The reason for this is the formation of tetragonal beta-uranium instead of the low-temperature orthorhombic alpha modification. The values of cathodic current efficiency in the most of experiments confirmed the three-electron scheme of uranium reduction. Cathodic current density under certain conditions exceeded 95 %. The amount of salt retained in the cathodic deposit varied from 7 to 25 %, values typical for dendrite electrolytic metals.