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

The dissolution-precipitation behavior of zirconium dioxide (ZrO₂) in molten lithium fluoride-beryllium fluoride (LiF-BeF₂, (2 : 1 mol, FLiBe)) eutectic salt at 873 K was studied. The results of the dissolution experiment showed that the saturated solubility of ZrO₂ in the FLiBe melt was 3.84 × 10^-3 mol kg^-1 with equilibrium time of 6 h, and its corresponding apparent solubility product (K'_sp) was 3.40 × 10^-5 mol³ kg^-3. The interaction between Zr(iv) and O^2- was studied by titrating lithium oxide (Li₂O) into the FLiBe melt containing zirconium tetrafluoride (ZrF₄), and the concentration of residual Zr(iv) in the melt gradually decreased due to precipitate formation. The precipitate corresponded to ZrO₂, as confirmed by the stoichiometric ratio and X-ray diffraction analysis. The K'_sp was 3.54 × 10^-5 mol³ kg^-3, which was highly consistent with that from the dissolution experiment. The obtained K'_sp of ZrO₂ was in the same order of magnitude as that of uranium dioxide (UO₂), indicating that a considerable amount of ZrF₄ could inhibit the UO₂ formation when oxide contamination occurred in the melt containing ZrF₄ and uranium tetrafluoride (UF₄). Further oxide titration in the LiF-BeF₂-ZrF₄ (5 mol%)-UF₄ (1.2 mol%) system showed that ZrO₂ was formed first with O^2- addition less than 1 mol kg^-1, and the precipitation of UO₂ began only after the O^2- addition reached 1 mol kg^-1 and the precipitation of ZrO₂ decreased the ZrF₄ concentration to 0.72 mol kg^-1 (3 mol%). Lastly, UO₂ and ZrO₂ coprecipitated with further O^2- addition of more than 1 mol kg^-1. The preferential formation of ZrO₂ effectively avoided the combination of UF₄ and O^2-. This study provides a solution for the control of UO₂ precipitation in molten salt reactors.