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The X-ray diffraction (XRD) phase analysis of different solidified uranium-based fluoride systems ((LiF–NaF)eut–UF4; (KF–LiF–NaF)eut–UF4; (LiF–NaF)eut–UF4–ZrF4 and (KF–LiF–NaF)eut–UF4–ZrF4) were examined in order to provide the basis for pyro-electrochemical extraction of uranium in molten fluorides. Several uranium-based species (Na2UF6, Na3UF7, K2UF6, K3UF7, UO2, K3UO2F5) were identified in the solidified melts. The role of oxygen in argon atmosphere was found to be critical in the formation of uranium species during the melting and solidification. In order to reduce the accumulated level of free oxygen traces in our experiments, zirconium (in the form of ZrF4) was used inside the melt as an oxygen buffer. It was found that ZrF4 can really stabilize the uranium species by complexation and protects them against the oxygenation. The results of this work highlight the importance of oxygen removal for obtaining pure deposit in the electrorefinning of uranium.

This article is focused on the electrochemical investigation (cyclic voltammetry and related studies) of the redox couple Sm(III)/Sm(II) in an eutectic LiF–CaF2 melt containing SmF3. The first step of reduction for Sm(III) ions involving one electron exchange in soluble/soluble Sm(III)/Sm(II) system was found on a tungsten electrode. The study of the Sm(II)/Sm(0) electrode reaction was not feasible, due to insufficient electrochemical stability of LiF–CaF2. The first step was found reversible at temperatures 1,075 and 1,125 K up to polarization rate 1 V/s and at temperature 1,175 K the process was reversible at all sweep rates applied in this study. The diffusion coefficients (D) of Sm(II) and Sm(III) ions were determined by cyclic voltammetry, showing that D decreases when oxidation state increase, while the activation energy of diffusion (E a) increases. The standard rate constants of charge transfer (k s) were calculated for the redox couple Sm(III)/Sm(II) at 1,075 and 1,125 K based on the data of cyclic voltammetry.

Electrochemistry of gadolinium and uranium in LiF–CaF2 (79–21 mol%) melt was studied using reactive Ni electrode and alloying reactions were observed. Deposits of gadolinium and uranium in the form of Gd–Ni and U–Ni intermetallic alloys were obtained after electrolysis by modulated current. Electrolysis of the same parameters was used also in the complex system of LiF–CaF2–UF4–GdF3 to demonstrate feasibility of selective deposition of uranium and therefore its separation from the system. Compact deposit of U–Ni alloy containing only traces of gadolinium was obtained.