Vitrified nuclear waste durability in complex natural environments

Lead Research Organisation: University of Sheffield
Department Name: Materials Science and Engineering

Abstract

Understanding the long-term durability of nuclear waste glass in the subsurface is important in the UK and internationally as many countries intend to dispose of vitrified radioactive waste in underground geological disposal facilities. In order to ensure safe disposal, we need to be confident that radioactive elements will remain isolated and immobilised for sufficient time to allow radioactivity to decay to safe levels. There will be multiple barriers in place (e.g. a metal container and engineered backfill) to delay groundwater from reaching the nuclear waste glass but eventually contact with water is expected. Although there are a number of laboratory tests currently used to determine the rate of glass dissolution in water all accelerate corrosion by increasing the temperature, surface area, or both and give very different predictions depending on the test conditions. Laboratory tests are also performed under simplified, stable, sterile conditions and using deionised water taking no account of how changing geochemical conditions will affect glass corrosion rates. This fellowship will combine materials science, geochemistry and geomicrobiology to study how glass corrodes in real-time in dynamic complex natural environments. I will improve understanding of key factors affecting corrosion (temperature, groundwater geochemistry, saturation, and microbiology) using the Ballidon long duration experiment, where glass samples have been buried for nearly 50 years. To predict the durability of nuclear waste glass thousands of years into the future I will study simulant nuclear waste glass's in conditions relevant to UK and US disposal concepts. The result of this novel investigation will be to critically evaluate, and improve, upon durability tests for glass, to build an improved model of glass corrosion and to establish further long duration experiments to inform the safety case for geological disposal in the UK and abroad.

Planned Impact

My research into glass durability in complex natural environments will directly impact upon beneficiaries in the UK nuclear industry including governmental policy makers 'Radioactive Waste Management' (RWM) responsible for carrying out the government's policy to dispose of high level vitrified waste in a Geological Disposal Facility (GDF). The fellowship directly addresses three key areas within RWM's "Geodisposal Science and Technology Plan 2016" and advances understanding of vitrified waste dissolution under UK relevant burial conditions. Furthermore, through improvements to current dissolution tests, this research will benefit companies developing glass compositions under consideration for the immobilisation of UK legacy intermediate level waste streams (e.g. Geomelt Ltd, National Nuclear Laboratories and the University of Sheffield). Improved low temperature dissolution tests will be investigated as part of this fellowship and could be applied to non-heat generating vitrified intermediate level wastes.
This science is also of interest to policy makers abroad particularly the US Department of Energy, responsible for disposal of US vitrified radioactive wastes. The study of glass dissolution under variable saturation, low temperatures, and changing groundwater composition (work packages 2-5) is pertinent to the safety case for a newly constructed Integrated Disposal Facility for low activity waste at the Hanford Site, Washington State, US. In addition, a number of US simulant glasses have been buried at the Ballidon site (the main natural analogue site studied) and the advanced site characterisation proposed during this fellowship will assist in interpretation of corrosion on those samples. Beneficiaries include Pacific North-West National Laboratories and Savannah River Laboratories who are both involved in ongoing research into US glass durability and collaborative work with the UK. In Europe, my findings and method development will be of interest to other countries working towards geological disposal of vitrified radioactive wastes and at a similar or more advanced stage of GDF implementation including France, Belgium, Sweden and Finland.

This research advances the underpinning science for the proposed UK GDF and, therefore, may help to answer questions posed by environmental groups and members of the public concerned with the safe disposal of vitrified radioactive waste. This research is timely as the RWM will begin another round of consultations in 2018 to allow potential volunteer communities to come forward to discuss hosting the UK GDF. This body of research will contribute to the underpinning science available for members of the public to access during this consultation phase.

Despite the applied nature of this fellowship experiments will advance the fundamental understanding of glass corrosion that may also interest other communities concerned with glass durability. For example work package 3, investigating the role of microorganisms in glass corrosion, may be of interest to archaeologists interesting in ancient glass sample preservation. Furthermore, improved prediction of glass durability under a range of geochemical conditions may interest commercial glass manufacturers (e.g. for the construction industry, semiconductors, scientific instruments).

Publications

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