Electrolytic reduction of CrF3 and Cr2O3 in molten fluoride salt

The electrochemical behavior of Cr³⁺ in molten LiF-NaF-KF (46.5 : 11.5 : 42 mol%) (FLiNaK) was studied by cyclic voltammetry (CV) at 600 °C. With an acceptable solubility and a relatively positive reduction potential of solute Cr³⁺, the electrolytic reduction of chromium in FLiNaK-CrF₃ melt was performed on a tungsten electrode by potentiostatic electrolysis. After electrolysis for 21.5 h, the Cr³⁺ in the melt was effectively removed as confirmed by ICP-OES and CV. Then, the solubility of Cr₂O₃ in FLiNaK with ZrF₄ additive was analyzed by CV. The results showed that the solubility of Cr₂O₃ was greatly promoted by ZrF₄ and the reduction potential of zirconium is far more negative than that of chromium, which makes the electrolysis of chromium from Cr₂O₃ material possible. Thus, the electrolytic reduction of Cr in a FLiNaK-Cr₂O₃-ZrF₄ system was further performed by potentiostatic electrolysis on a nickel electrode. After electrolysis for 5 h, a thin layer of chromium metal (with a thickness of c.a. 20 μm) was deposited on the electrode, as confirmed by SEM-EDS and XRD techniques. This study verified the feasibility of electroextraction of Cr from the FLiNaK-CrF₃ and FLiNaK-Cr₂O₃-ZrF₄ molten salt systems.

Raman and density functional theory studies of lutecium fluoride and oxyfluoride structures in molten FLiNaK

Combined with Raman spectroscopy and density functional theory (DFT) calculations, the micro-structures in molten FLiNaK-LuF₃ and FLiNaK-LuF₃-Li₂O systems were studied. Both LuF₅^2- (D_3h) and LuF₆^3- (O_h) anions were identified in molten FLiNaK, and their relative content varied with the concentration of LuF₃. For regarding the affection of oxygen anion, Li₂O was added into the molten FLiNaK-LuF₃ (20 mol%) sample. The lutecium oxyfluoride anion Lu₂OF₈^4- was firstly formed which possesses a linear LuOLu geometry with two LuF₄ moieties bridged by one single oxygen atom. Further increasing the Li₂O content to 10 mol% resulted in the formation of two species, which belonged to the Lu₂O₂F₄^2- and Lu₂O₂F₆^4- anions. When increased the concentration of Li₂O to 20 mol%, a new species appeared which was approximate to the oxide structure.

On the Structures of Thorium Fluoride and Oxyfluoride Anions in Molten FLiBe and FLiNaK

The structures of thorium fluoride and oxyfluoride ions in molten FLiBe-ThF₄ and FLiNaK-ThF₄ were investigated by Raman spectroscopy and density functional theory calculations. Thorium fluorides are present in the form of ThF₆^2- (O_h) and ThF₇^3- (C_2v) in molten FLiNaK-ThF₄. Similar speciation was identified in FLiBe-ThF₄, and the thorium fluoride anions are in equilibrium with free F^- ions and beryllium fluoride anions, which are responsible for the red shift of the beryllium fluoride bands in the Raman spectra. With the addition of Li₂O into the FLiNaK-ThF₄ and FLiBe-ThF₄ melts, the Th₂OF₁₀^4- anion with a linear Th-O-Th geometry was formed at the expense of thorium fluoride anions. The beryllium fluoride bands in the Raman spectra of FLiBe-ThF₄ exhibit a blue shift upon Th₂OF₁₀^4- formation, which results from the release of free F^- ions that can further react with beryllium fluoride. Insoluble thorium oxides were found in the FLiNaK and FLiBe melts at a Li₂O concentration of 15 mol %, and the Th₂OF₁₀^4- anion is, therefore, a bridge connecting the soluble thorium fluorides and insoluble thorium oxides in molten fluorides.

Inhibition effect of ZrF4 on UO2 precipitation in the LiF–BeF2 molten salt

This study provides an effective solution for controlling and monitoring the nuclear fuel precipitation (UO₂) in molten fluorides, which is of great importance for the safe operation and fuel salt design of molten salt reactor (MSR).