Synthesis and characterization of oxyapatite [Ca2Nd8(SiO4)6O2] and mixed-alkaline-earth powellite [(Ca,Sr,Ba)MoO4] for a glass-ceramic waste form

This paper discusses the synthesis, characterization, and chemical durability assessment of oxyapatite [Ca2Nd8(SiO4)6O2] and mixed-alkaline-earth powellite [(Ca,Sr,Ba)MoO4]. These are the major crystalline phases that precipitate from the melt during cooling of a glass-ceramic waste form currently being evaluated for immobilizing wastes produced during the aqueous reprocessing of used nuclear fuel. The oxyapatite was made at 99.7% purity using a solution-based process followed by heat treatments. The powellite, made by melting carbonates and slow-cooling the melt, formed two different phases, one rich in Ca (74.7%) and Sr (19.4%) (balance is Ba) and the other rich in Sr (25.6%) and Ba (65.6%) (balance is Ca). Following static dissolution tests after 5 days, the oxyapatite phase had a maximum normalized loss of 0.024 g m−2 for Ca and the powellite phases showed a higher normalized loss from the Ba-powellite (0.22 g m−2) compared with the Sr-powellite (0.06 g m−2) and Ca-powellite (0.02 g m−2). Additionally, crystal structure data were measured using X-ray diffraction and are compared in detail with literature data of powellites and oxyapatites of similar chemistries.