Designing More Versatile Nuclear-Waste Glasses Based on Structural Insights from Nuclear Magnetic Resonance Spectroscopy

Scott Kroeker is a Professor of Chemistry at the University of Manitoba. His NMR-centred research program focuses on the characterization of disordered inorganic solids. He gained his first exposure to magnetic resonance during his undergraduate degree at the University of Winnipeg, where he did medical research using in vivo MR imaging and spectroscopy. His master’s degree was carried out under the tutelage of Ted Schaefer, where he used ultrahighresolution NMR spectroscopy and MO calculations to study the conformational behaviour of benzene derivatives in solution. His Ph.D. in Physical and Theoretical Chemistry was obtained from Dalhousie University in 1999 in the solid-state NMR research group of Roderick Wasylishen. After an NSERC PDF at Stanford University in the Department of Geological and Environmental Sciences studying glasses and minerals with Jonathan Stebbins, he took up an Inorganic Chemistry faculty position at the University of Manitoba in 2001. Since then he has developed a research program focusing principally on the structure of oxide glasses for applications including nuclear waste disposal, solid-state electrolytes, and soft-tissue wound healing. He is a founding member of the Manitoba Institute of Materials and serves on the steering committee of the National Ultrahigh-Field NMR Facility for Solids. As an EPSRC visiting professor at the University of Cambridge (2007-08), he became interested in high-temperature NMR spectroscopy, and more recently spent a research fellowship at Le Studium Institute of Advanced Studies in Orléans (2014-15) doing in situ NMR at CEMHTI (Conditions Extrême et Materiaux: Haute Température et Irradiation).

Scott Kroeker

Department of Chemistry, University of Manitoba, Winnipeg, Canada, R3T 0C8

Resumo

Crystallization control is a key aspect of many industrial applications of glasses and glassceramics. In glasses used for nuclear waste disposal, preventing crystallization is important to retain radioactive species within a durable glass matrix for long-term immobilization. When the complexity of such materials makes it impossible to prevent crystallization, strategies for selective crystallization have been proposed to ensure that the devitrification products are not water-soluble and do not contain radioactive ions. Despite significant research efforts, limiting waste loading remains necessary to ensure that radioactive species do not leach out of the glasses over long periods of time. We have been exploring the role of cation field strength in mixed network-forming glasses including boron, silicon, aluminum and phosphorus to compete

effectively for oxygen against poorly soluble ions, forcing them into high-coordinate disordered environments which disfavour nucleation. A similar principle can be applied with network modifiers to improve glass homogeneity. Such compositional tuning is capable of increasing the retention of molybdenum and sulfur in the glassy phase by up to a factor of four. Central to this approach is the structural understanding obtained by nuclear magnetic resonance (NMR) spectroscopy, which is used both to quantify the devitrification products and to characterize the structural chemistry of these complex glasses, identifying specific bonding sites in the base glasses which can accommodate the problem ions, thereby facilitating the design of multicomponent materials best suited for their incorporation. In favourable cases, NMR can also be used to spectroscopically observe crystallization and infer structural changes at the elevated temperatures found in radioactive glass canisters, providing results more relevant to geologic repositories. While these studies focus on preventing nucleation in complex glasses, the principles may also prove valuable in understanding crystallization in glass ceramics.