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

The electrochemical behavior of U(IV) in molten LiF-NaF-KF (46.5:11.5: 42 mol) (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 0.02-0.3 V s(-1). A linear relationship between the reduction current density of U(III)/U(0) and the concentration of U(IV) was also established within 0.01-0.084 mol kg(-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 x 10(-6) mol(3)/kg(3), which can be used to evaluate the allowable amount of dissolved oxide ions in uranium-based fluorides. (C) 2016 Elsevier Ltd. All rights reserved.