Solubility of REM Fluorides and Oxides in a LiF–NaF–KF Melt

Abstract—The eutectic LiF–NaF–KF mixture is considered as a fuel salt for molten salt reactors for the transmutation of minor actinides; therefore, information on the solubility of fluorides and oxides in it as both fissile materials and fission products is necessary to substantiate reactor operating conditions and to develop methods for fuel salt refabrication. From this point of view, cerium and neodymium can be considered as both representatives of fission products and imitators of actinides. Cooling curves and isothermal holding are analyzed to determine the temperature dependence of the solubility of a mixture of cerium and neodymium fluorides and cerium and neodymium oxides in the LiF–NaF–KF melt. The total solubility of rare earth trifluorides is found to reach 51 wt % at 550°C and to be in good agreement with the data for individual cerium and neodymium fluorides. The solubility of REM oxides has significant kinetic difficulties, which is associated with the formation of intermediate compounds in the form of cerium and neodymium oxyfluorides and alkali metal oxides or peroxides. The solubility of cerium oxide is significantly lower than that of neodymium oxide, 0.049 and 0.221 wt % at 750°C, respectively.