Structural Characterization of Novel Tin Fluorophosphate Nuclear Salt Waste Forms

Recently, phosphate glass, specifically tin fluorophosphate glass/glass-ceramics (TFPG/TFPGC) or SnF₂-SnO-P₂O₅, has gained attention as a potential vitrification medium for chloride-based nuclear salt waste. Greater chemical durability and density along with lower melting temperatures imparts TFPG/TFPGC distinct advantages in stability, waste volume, and synthesis ease/cost relative to industry-standard borosilicate waste forms. The tendency of phosphate glass/glass-ceramic systems to succumb to hygroscopicity in ambient conditions, however, impedes further implementation of TFP compounds as waste forms unless their structural matrices can be improved to combat this. The discovery in this work of crystalline phase formations of stronadelphite (Sr₅F(PO₄)₃), cerium phosphate (CePO₄), and neodymium ultraphosphate (NdP₅O₁₄) within novel 30 mass-% chloride salt waste-loaded TFPG samples, which were observed to resist hygroscopicity, was used as a basis to modify TFP chloride salt waste forms which did not naturally form these structures through artificial phase incorporation. Initial XRD and SEM-EDS analysis has revealed that the inclusion of 3 mass-% single-phase fluorapatite [2(Ca₅F(PO₄)₃)], of which stronadelphite is a structural analog, into specific 30% waste-loaded TFPGs resulted in higher chlorine retention rates and structural phase-stability than both non-included and other phase-included TFPGCs. Further structural testing of Nd-Ultraphosphate phase-inclusion and CaO substitution of TFP waste forms, along with additional chemical and thermal analyses, is ongoing.