Chemical Interactions between Thorium(IV) and O2- in the FLiBe and FLiNaK Molten Salt Systems

Abstract

The chemical interactions between thorium(IV) and O2- in the LiF-BeF2 (2 : 1 mol%) (FLiBe) and LiF-NaF-KF (46.5 : 11.5 : 42 mol%) (FLiNaK) molten salt systems were studied at 873 K. The precipitation-dissolution behavior of Th(IV) oxide/oxyfluoride species, as well as the speciation law of these species and their relationship with oxide concentration in fluoride melts, were clarified. The results showed that the interactions between Th(IV) and O2- in the FLiBe melt would only produce ThO2. The saturated solubility of ThO2 in the FLiBe melt was 0.057 mol/kg, and its corresponding apparent solubility product ( $K_{sp}^{\prime }$ ) was $\left(8.8\pm 0.1\right)\times {10}^{-4}$  mol3/kg3 and $\left(8.6\pm 0.7\right)\times {10}^{-4}$  mol3/kg3 by dissolution and oxide titration methods, respectively. However, in the FLiNaK melt, the interactions between Th(IV) and O2- were more complex than that in FLiBe, and Th(IV) oxyfluoride species (i.e., Th2OF104- and ThOF2) were produced. Th2OF104- was initially formed with the ratio of $r\le 0.5$ ( $r={\left[{\text{O}}^{2-}\right]}_{\text{added}}/{\left[\text{Th}\left(\text{IV}\right)\right]}_{\text{initial}}$ ), and the Th2OF104- was converted to ThOF2 precipitate when $r>0.5$ . Therefore, the soluble Th2OF104- is the important intermediate between Th(IV), O2-, and ThOF2 precipitation. The difference in results of the FLiBe and FLiNaK melts was attributed to the difference of free F- content in the two systems. The present work provided important reference and fundamental data for understanding the nature of thorium fuel precipitation and how to control it in the molten fluoride media, which is of significant importance for the safe operation of molten salt reactor (MSR).