Investigation of the Quasi-Binary Phase Diagram FLiNaK-NdF₃

The NdF₃ solubility in molten eutectic FLiNaK, which is a conceivable medium for a molten salt reactor (MSR), was determined by the quasi-binary phase diagram FLiNaK-NdF₃. The eutectic mixture FLiNaK was prepared by direct melting of components LiF, NaF and KF·HF. The acidic anhydrous salt (KF·HF) was used instead of the hygroscopic KF. The NdF₃ was sintered by hydrofluorination of Nd₂O₃. The oxygen impurity in the prepared eutectic FLiNaK, determined by an oxygen analyzer LECO OH836, was 0.036 wt.%, whereas the NdF₃ contained 0.04 wt.% of oxygen. A part of the FLiNaK-NdF₃ quasi-binary phase diagram was obtained using two thermal analysis techniques: differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The FLiNaK-NdF₃ phase diagram in the region of 0-30 mol.% NdF₃ contains one eutectic at 2 mol.% NdF₃ and 450 °C and two peritectic points: 8 mol.% NdF₃ at 500 °C and 22 mol.% NdF₃ at 575 °C. The region of the FLiNaK-NdF₃ phase diagram below the liquidus line is rather complicated due to the complex structure of the multicomponent system in its molten state, as in its solid state. The NdF₃ solubility in FLiNaK is about 5 mol.% at 490 °C and 29 mol.% at 700 °C; this means that the process of the MA transmutation in the MSR can be carried out in molten FLiNaK with a content of actinides as high as 15-20 mol.% in the temperature range of 550-650 °C.