Nepheline Formation in High Level Waste Glass
EMSL Project ID
39968
Abstract
Fabrication, storage, and disposal of high-level nuclear waste (HLW) in borosilicate glass is a costly but necessary process to promote the safety and security of legacy nuclear waste. Since the cost is directly related to the amount of vitrified HLW, it is desirable to maximize waste loading in HLW glass to decrease the overall volume for storage and disposal. One significant limitation to waste loading in glass is the aluminum concentration. The source of Hanford tank waste aluminum was from fuel cladding, which was dissolved in nitric acid then neutralized with NaOH, creating wastes high in both Al and Na. Sodium- and aluminum-rich HLW glasses are prone to nepheline (NaAlSiO4) crystallization. Since nepheline nominally removes three moles of glass-forming oxides (Al2O3 and SiO2) per mole of Na2O, the formation of this phase can result in severe deterioration of the chemical durability in a given glass, providing a challenge for long-term repository stability of the waste form. It has been demonstrated that nepheline is unlikely to form outside of its primary phase field in the ternary Na2O-Al2O3-SiO2 submixture phase diagram, and the “nepheline discriminator” (ND) was introduced to describe this constraint. There is much interest in pursuing ways to extend acceptable glass formulation space beyond this criterion and into composition regions with higher alumina concentrations. One recent attempt that extended the range of compositions allowable for formulation, while still avoiding nepheline formation, appealed to optical basicity. Optical basicity (OB) has been shown to correlate to transport properties, including viscosity, electrical and thermal conductivity, diffusion, thermochemical properties, and even waste glass chemical durability. Additionally, OB is closely related to other structural and chemical descriptors of glasses that represent an overall average state of oxygen in the melt or glass. Unlike ND, which is determined solely on the relative composition of SiO2, Al2O3, and Na2O, OB is determined by the entire glass composition, thus containing contributions from all the oxides, notably B2O3, which has been shown to have a strong inhibitory effect on nepheline formation.
Many studies using Raman spectroscopy and nuclear magnetic resonance (NMR) have looked at local coordination of Al, B, Si, and O in alumino-boro-silicate glasses, though no definitive understanding is yet available. In the process of refining current models for nepheline formation, it is necessary to study a set of glasses to refine understanding of the glass structure and precursors for nepheline crystal formation. The approach to couple the modeling task with understanding of the local glass structure is to choose glass compositions with similar ND and/or OB values which exhibit different behaviors with respect to nepheline crystallization. These glasses are then subject to Raman spectroscopy and NMR to understand the glass structure of the quenched and crystallized glasses. We hope, then, to demonstrate correlations between quenched glass structure and propensity for nepheline formation, and correlate glass compositional space with local glass structure that favors nepheline formation. Specifically, we anticipate that particular distributions and conformations of Si, Al, B, and Na will lead to conditions favorable for nepheline formation.
Project Details
Start Date
2013-02-11
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Marcial J., J. Kabel, M. Saleh, N.M. Washton, Y. Shaharyar, A. Goel, and J.S. McCloy. 2018. "Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) - eucryptite (LiAlSiO4) join." American Ceramic Society Bulletin 101, no. 7:2840-2855. PNNL-SA-132244. doi:10.1111/jace.15439
Mccloy JS, NM Washton, PL Gassman, J Marcial, JL Weaver, and RK Kukkadapu. 2015. "Nepheline crystallization in boron-rich alumino-silicate glasses as investigated by multi-nuclear NMR, Raman, & Mössbauer spectroscopies." Journal of Non-crystalline Solids 409:149-165. doi:10.1016/j.jnoncrysol.2014.11.013