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.