Solubility and precipitation investigations of UO2 in LiF–BeF2 molten salt

Abstract

The solubility of UO2 in molten LiF–BeF2 (2:1 mol) (FLiBe) eutectic salt at 600 °C was studied by chemical and electrochemical methods. The results of dissolution experiment showed that the saturated solubility of UO2 in this melt was 2.37 × 10−3 mol/kg and its corresponding apparent solubility product (Ksp’) was approximately 1.67 × 10−5 mol3/kg3. When more Li2O were added to the FLiBe–UF4 system, the cathodic peak current of U(IV) in the electrochemical cyclic voltammetry (CV) curve accordingly decreased because of precipitate formation. The precipitate corresponded to UO2 as determined by stoichiometric ratio (concentration variation of U4+ and O2−) and X-ray diffraction (XRD) analysis. Compared to the chemical analysis method, the CV technique was confirmed to be more feasible for accurate determination of concentration of U(IV). Meanwhile, the Ksp’ value was also obtained to be 1.33 × 10−5 mol3/kg3 during the whole oxide titration procedure, which was highly consistent with that from the dissolution experiment. With the value of Ksp’, the allowable amount of dissolved oxide ions (oxide tolerance) can be theoretically estimated in the FLiBe–UF4 system.