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

Abstract

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 O²⁻ 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 O²⁻ addition less than 1 mol kg⁻¹, and the precipitation of UO₂ began only after the O²⁻ 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 O²⁻ addition of more than 1 mol kg⁻¹. The preferential formation of ZrO₂ effectively avoided the combination of UF₄ and O²⁻. This study provides a solution for the control of UO₂ precipitation in molten salt reactors.