Electrochemical technique for detecting the formation of uranium-containing precipitates in molten fluorides

Abstract

The electrochemical behavior of U(IV) in molten LiF–NaF–KF (46.5:11.5:42mol) (FLINAK) eutectic salt was studied by cyclic voltammetry and square-wave voltammetry analyses. The reduction of U(IV) proceeded in two steps: reduction of initial U(IV) to U(III), followed by three-electron reduction of U(III) to U metal. Both reactions were reversible and controlled by ion diffusion at a scan rate of 0.02–0.3Vs−1. A linear relationship between the reduction current density of U(III)/U(0) and the concentration of U(IV) was also established within 0.01–0.084molkg−1. When different concentrations of Li2O were added to the FLINAK–UF4 system, the peak current density of U(III)/U(0) was depleted because of the formation of UO2 precipitates. The apparent solubility product (Ksp) of UO2 in FLINAK–UF4–Li2O melts was approximately 4.75×10−6 mol3/kg3, which can be used to evaluate the allowable amount of dissolved oxide ions in uranium-based fluorides.