The immobilization of technetium-99 ((99)Tc) in a suitable host matrix has proven to be a challenging task for researchers in the nuclear waste community around the world. In this context, the present work reports on the solubility and retention of rhenium, a nonradioactive surrogate for (99)Tc, in a sodium borosilicate glass. Glasses containing target Re concentrations from 0 to 10,000 ppm [by mass, added as KReO(4) (Re(7+))] were synthesized in vacuum-sealed quartz ampules to minimize the loss of Re from volatilization during melting at 1000 °C. The rhenium was found as Re(7+) in all of the glasses as observed by X-ray absorption near-edge structure. The solubility of Re in borosilicate glasses was determined to be ~3000 ppm (by mass) using inductively coupled plasma optical emission spectroscopy. At higher rhenium concentrations, additional rhenium was retained in the glasses as crystalline inclusions of alkali perrhenates detected with X-ray diffraction. Since (99)Tc concentrations in a glass waste form are predicted to be <10 ppm (by mass), these Re results implied that the solubility should not be a limiting factor in processing radioactive wastes, assuming Tc as Tc(7+) and similarities between Re(7+) and Tc(7+) behavior in this glass system.

SummaryThe purpose of this study was to find compositions that increase waste loading of high-alumina wastes beyond what is currently acceptable while avoiding crystallization of nepheline (NaAlSiO 4 ) on slow cooling. Nepheline crystallization has been shown to have a large impact on the chemical durability of high-level waste glasses. It was hypothesized that there would be some composition regions where high-alumina would not result in nepheline crystal production, compositions not currently allowed by the nepheline discriminator.Optical basicity (OB) and the nepheline discriminator (ND) are two ways of describing a given complex glass composition. This report presents the theoretical and experimental basis for these models. They are being studied together in a quadrant system as metrics to explore nepheline crystallization and chemical durability as a function of waste glass composition. These metrics were calculated for glasses with existing data and also for theoretical glasses to explore nepheline formation in Quadrant IV (passes OB metric but fails ND metric), where glasses are presumed to have good chemical durability. Several of these compositions were chosen, and glasses were made to fill poorly represented regions in Quadrant IV.To evaluate nepheline formation and chemical durability of these glasses, quantitative X-ray diffraction (XRD) analysis and the Product Consistency Test were conducted. A large amount of quantitative XRD data is collected here, both from new glasses and from glasses of previous studies that had not previously performed quantitative XRD on the phase assemblage.Appendix A critically discusses a large dataset to be considered for future quantitative studies on nepheline formation in glass. Appendix B provides a theoretical justification for choice of the oxide coefficients used to compute the OB criterion for nepheline formation.v

Historical data have been critically compiled and analyzed for investigating the quantity of nepheline (NaAlSiO4) precipitated as a function of composition in simulated nuclear waste glasses. To understand compositional effects two primary methods were used: (1) investigating the Al2O3–SiO2–Na2O ternary while filtering for different B2O3 levels and (2) creating a quadrant system consisting of compositions reduced to two representations: (i) the nepheline discriminator (ND) which depends only on the SiO2 content by weight normalized to the total weight of the Al2O3–SiO2–Na2O submixture and (ii) the optical basicity (OB) which contains contributions from all constituents in the glass. Nepheline precipitation is expected to be suppressed at high SiO2 levels (ND > 0.62) or at low basicities (OB < 0.55–0.57). Changes in sodium aluminosilicate glass OB values due to additions of CaO and B2O3 correlate with the observed effects on nepheline formation. It is proposed that additional composition space is available for formulating high waste loading, high‐Al2O3 nuclear waste glasses when consideration is given to location on the Al2O3–SiO2–Na2O submixture as well as OB.

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