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

Effect of oxide impurities on the dissolution behavior of Th4+, Be2+ and U4+ in fluoride salts

Oxides are one of the most important impurities in the fuel salt of molten salt reactors (MSRs), and excessive oxide impurities pose a risk to the safe operation of MSRs. This study focused on investigating the precipitation behavior between Th4+, U4+, and Be2+ with O2- in the 2LiF-BeF2 (FLiBe) eutectic salt system. The results showed that the solubility of UO2 was 5.52 × 10-3 mol kg-1, and the solubility product (Ksp) of UO2 was 6.14 × 10-7 mol3 kg-3 in FLiBe salt at 650 °C. It was also found that the O2- ion would firstly react with U4+ to form UO2, and then the excessive O2- would react with Be2+ to generate BeO in the FLiBe system. Despite conducting the solubility experiment of ThO2 and titration experiment of FLiBe-ThF4, the system failed to achieve the solubility and the Ksp of ThO2. The main reason for this was that O2- preferentially reacted with Be2+ over Th4+ to form precipitates, in other words, Be2+ exerted a protective effect against Th4+. Above all, this work experimentally demonstrated that in the FLiBe system, O2- preferentially combines with U4+ to form a precipitate, followed by Be2+, while Th4+ is relatively inert.

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.

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).