Inhibition effect of ZrF(4) on UO(2) precipitation in the LiF–BeF(2) molten salt

The dissolution–precipitation behavior of zirconium dioxide (ZrO(2)) in molten lithium fluoride–beryllium fluoride (LiF–BeF(2), (2 : 1 mol, FLiBe)) eutectic salt at 873 K was studied. The results of the dissolution experiment showed that the saturated solubility of ZrO(2) 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) mol(3) kg(−3). The interaction between Zr(iv) and O(2−) was studied by titrating lithium oxide (Li(2)O) into the FLiBe melt containing zirconium tetrafluoride (ZrF(4)), and the concentration of residual Zr(iv) in the melt gradually decreased due to precipitate formation. The precipitate corresponded to ZrO(2), as confirmed by the stoichiometric ratio and X-ray diffraction analysis. The K′(sp) was 3.54 × 10(−5) mol(3) kg(−3), which was highly consistent with that from the dissolution experiment. The obtained K′(sp) of ZrO(2) was in the same order of magnitude as that of uranium dioxide (UO(2)), indicating that a considerable amount of ZrF(4) could inhibit the UO(2) formation when oxide contamination occurred in the melt containing ZrF(4) and uranium tetrafluoride (UF(4)). Further oxide titration in the LiF–BeF(2)–ZrF(4) (5 mol%)–UF(4) (1.2 mol%) system showed that ZrO(2) was formed first with O(2−) addition less than 1 mol kg(−1), and the precipitation of UO(2) began only after the O(2−) addition reached 1 mol kg(−1) and the precipitation of ZrO(2) decreased the ZrF(4) concentration to 0.72 mol kg(−1) (3 mol%). Lastly, UO(2) and ZrO(2) coprecipitated with further O(2−) addition of more than 1 mol kg(−1). The preferential formation of ZrO(2) effectively avoided the combination of UF(4) and O(2−). This study provides a solution for the control of UO(2) precipitation in molten salt reactors.