Thermal conversion in air of rare-earth fluorides to rare-earth oxyfluorides and rare-earth oxides

•Thermal conversion of REF3 to REOx (RE = La, Ce, Pr, Nd, Tm, Yb, Lu).•Immobilization of Nd2O3 in lanthanide borosilicate glass waste form.•Documented differences in phase-temperature relationships for transitions.•Expanding applications towards RE radionuclide disposal pathways. The goal of this study was to evaluate a potential method to convert rare-earth (RE) fluorides to oxides through thermal conversion in air. The RE elements, which are common neutron poisons, could potentially be removed from fluoride-based molten salt reactors via precipitation through fluoride-to-oxyfluoride or fluoride-to-oxide conversion mechanisms. In this study, the phase transformations of seven different REF3 compounds (RE = La, Ce, Pr, Nd, Tm, Yb, and Lu) at temperatures ranging from 400 to 1400 °C in air were investigated with X-ray diffraction. The LaF3, PrF3, NdF3, TmF3, YbF3, and LuF3 compounds transformed to oxyfluorides first and then to oxides, whereas CeF3 transformed directly to an oxide. A waste form option for the resulting REOx products is a lanthanide aluminoborosilicate (LABS) glass, which was demonstrated in this paper by converting NdF3 to Nd2O3 and immobilizing the Nd2O3 at 60 mass% in a LABS glass.