Interactions between Oxide and LiF-BeF2-ZrF4-UF4 System through Electrochemical Techniques

Abstract

The electrochemical reduction of UF4 in LiF-BeF2-UF4 (FLiBeU) and LiF-BeF2-ZrF4-UF4 (FLiBeZrU) molten salt was investigated by cyclic voltammetry (CV) and squarewave voltammetry (SWV). Two cathodic reduction peaks related to U4+/U3+ and U3+/U were detected in FLiBeU. Only the reduction peak of U4+/U3+ was confirmed in FLiBeZrU, and the linear relation between peak current density and U content within 1.1 mol% was established through SWV. Results of SWV, inductively coupled plasma optical emission spectroscopy and X-ray diffraction analyses demonstrated that only ZrO2 precipitated with the addition of Li2O to FLiBeZrU at a Zr-to-U molar ratio (n Zr /n U) greater than 4. Conversely, ZrO2 and UO2 co-precipitated at n Zr /n U less than 4. Raman spectroscopy analyses of FLiBeZrU with various n Zr /n U values indicated that [ZrF6]2– was primarily converted to[ZrF7]3– at n Zr /n U > 4 and further to [ZrF8]4– at n Zr /n U < 4. Due to the large steric hindrance and high negative-charge repulsion of [ZrF8]4–, its reaction with O2– was inhibited, so the excess O2– reacted with U4+ to produce UO2 precipitates.