Electrochemistry of uranium in molten LiCl-LiF

Abstract

This work is focused on the electrochemical behaviour of uranium in molten eutectic LiCl–LiF (70–30 mol%) in the 550–650 °C temperature range. On tungsten electrode, U(III) ions were reduced in one step to U metal exchanging 3 electrons and oxidized also in one step to U(IV) exchanging one electron. Both systems were studied by cyclic and square wave voltammetries and chronopotentiometry. The reduction and oxidation mechanisms of U(III) ions were found to be diffusion controlled processes. The diffusion coefficient of U(III) was measured at different temperatures, and it followed an Arrhenius “type law”. Apparent standard potentials were measured in chloride-fluoride (LiCl-LiF eutectic) and pure chloride media (LiC-KCl eutectic). The addition of fluoride ions into a chloride salt leads to the formation of more stable complexes reduced at more negative potentials. This shift of reduction potentials is more pronounced for the U(IV)/U(III) transition (~500 mV) than for the U(III)/U system (~100 mV).