A Low Temperature Solution Chemistry Process for Fluoride Salt (NaF) Waste Immobilization into Metal Halide Perovskite Structured Na2SnF6

Abstract

Complex salt waste streams generated from chemical reprocessing of spent nuclear fuels and advanced molten salt reactor technologies require innovative materials and processes for effective immobilization and management. In this study, we report a simple, low-temperature, solution-based approach for immobilizing fluoride salts, specifically alkali halides, into metal halide perovskite (MHP) waste forms. Using NaF as a representative system, systematic experiments were conducted to immobilize alkali-halides into Na2SnF6, achieving a high fluorine content of 40.9 wt% and a fluoride loading capacity of 57.4 wt%. By tailoring key parameters such as the use of surfactants, salt grain size, and reaction temperature, the low temperature solution chemistry process achieves consistently high immobilization efficiencies above 95%. X-ray diffraction (XRD) analysis coupled with Rietveld refinement confirmed that Na2SnF6 predominantly crystallizes in the tetragonal P4₂/mnm structure as the primary phase. A hexagonal metastable phase was also observed, attributed to rapid precipitation during synthesis. Microstructural characterization using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) confirm uniform elemental distribution and microchemical compositions consistent with the Na₂SnF₆ crystal structure. Thermogravimetric analysis (TGA) demonstrates thermal stability of the synthesized waste forms up to 400 °C. This work establishes a foundational, cost-effective pathway for immobilizing alkali fluoride salt waste into MHP-type structures with high waste loading using novel low-temperature solution chemistry for effective management of salt waste.