Glass - Back to the Future!

Presenting Author:
Dr. Ian L. Pegg

article posted 4th May 2016

Dr Ian Pegg is Professor of Physics and Director of the Vitreous State Laboratory (VSL) at The Catholic University of America where he manages and directs a staff that has reached 110 scientists, engineers, and technicians working in a variety of basic and applied research and development areas. His research has spanned various areas of materials science including the optimisation of processes and glass compositions for use in nuclear waste disposal, geopolymers, nano-materials, and thermoelectrics. Over the past 30 years, he has led numerous vitrification R&D programmes involving the development and characterisation of glass formulations and the demonstration and scale-up of Joule-heated melting processes, including those employed at successfully at West Valley and Savannah River. He is the Principal Investigator for the Hanford research and technology programme at VSL, which supports the design, construction, and operation of what will be the world's largest nuclear waste vitrification facility, as well as for similar vitrification support projects for the Defence Waste Processing facility at the Savannah River Site, the Rokkasho vitrification facility, and the Sellafield site in the UK. Dr. Pegg has served as a technical team member on several successful multi billion-dollar treatment facility proposals. He previously held positions at the National Institute for Standards and Technology and in the Department of Chemistry and Biochemistry at UCLA. He holds a PhD in physical chemistry from The University of Sheffield, UK, as well as MBA and BSc degrees.

Glass Formulation Optimisation for Treatment of Nuclear Wastes

Ian L. Pegg
Vitreous State Laboratory, The Catholic University of America, Washington, DC, USA

Glass formulations for the immobilisation of nuclear wastes must meet a variety of requirements that depend on the composition and nature of the waste, the selected processing technology, and the disposal scenario. These requirements can be translated into a set of property constraints that prospective glass formulations must meet. Additionally, economic considerations favour glass formulations that have high waste loadings and high melting rates. Increased waste loadings decrease the amount of glass that must be made in order to treat a given amount of waste, which reduces both treatment and disposal costs. Similarly, higher melting rates decrease the time required to process the waste, and therefore operational costs. Consequently, for any given waste vitrification problem it is of considerable practical importance to understand the factors that limit waste loadings and melting rates and the potential for glass formulation modifications that can improve those limits. These factors can vary considerably but examples include the leach resistance of the product, melt properties such as viscosity and electrical conductivity, secondary phase formation in the melt, refractory corrosion, and melt foaming. Secondary phase formation can include crystalline phases such as spinels and noble metals or salt phases such as sulphates, molybdates, chromates, and halides, all of which can cause operational issues. Molten sulphate salts, for example, are highly corrosive, and more electrically conductive, lower melting, and less viscous than the glass melt. In addition, these salts have high aqueous solubility and key radionuclides such as technetium, cesium, and strontium, are preferentially partitioned into them, which can compromise product quality. Waste loadings in sulphate-containing wastes can therefore be limited by molten salt formation, which can be mitigated by using glass formulations with improved sulphate solubility and reduced tendency for molten salt segregation in the reacting feed material. These issues will be illustrated by examples of high waste loading glass formulations developed for high level wastes (HLW) and low activity waste (LAW) for the Hanford site in the USA; HLW at the Rokkasho site in Japan; and intermediate level wastes at the Sellafield site in the UK.