Salts purification and voltammetric study of the electroreduction of U(IV) to U(III) in molten LiF–ThF 4

Abstract

Introduction of molten fluorides as reactor coolants and fuels (both as actinide burner and Th–U breeder) is one of the promising ways to sustainable development of these reactors. Special attention should be paid to the on-line control of the redox condition of the circulating molten fluorides mixtures, in particular, of the U(IV)/U(III) ratio by voltammetry. A “dry” technique for production and purification of metal fluorides, their mixtures, and fusion cakes without gaseous HF has been developed. The experimental studies confirmed that solid NH 4HF 2 can be efficiently used instead of the HF gas for conversion of UO 2 and ThO 2 to anhydrous tetrafluorides and for removal of oxygen-containing impurities from fluoride salts. The electrochemical behavior of UF 4 in a 77LiF–23ThF 4 (mole%) melt was studied by cyclic voltammetry at temperatures up to 800 °C. The studies have revealed that well reproducible voltammograms can only be obtained in partially reduced LiF–ThF 4–UF 4 melts containing UF 3, which are free from electropositive impurity ions. Processing of the experimental dependences shows that the basic calculated characteristics of the stage U(IV) electroreduction to U(III) considerably differ from the theoretical values corresponding to the one-electron process governed by a diffusion-controlled charge transfer. This points to quasi-reversibility of the electrode reaction studied. Since the Nernst equation can be used for processing voltammograms only reversible electroreduction U(IV), quasi-reversibility will lead to errors in voltammetric determination of the equilibrium relations U(IV)/U(III) in the melt studied. The results have to be considered as estimates or semiquantitative values.