Claire L Corkhill
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Dr. Claire Corkhill is a Reader and an EPSRC Early Career Research Fellow in the Department of Materials Science and Engineering at the University of Sheffield. With a research background in mineralogy and geochemistry, her research focuses on understanding the mechanisms and kinetics of nuclear waste dissolution under conditions relevant to the geological disposal of these materials, which includes an understanding of coupled geochemical processes within the engineered barrier. One of the key research topics of her research team involves the state-of-the-art determination of nuclear waste glass durability, using a range of high resolution analytical techniques, such as transmission electron microscopy, vertical scanning interferometry, µ-focus x-ray absorption spectroscopy and µ-focus x-ray diffraction

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Cementing our understanding of nuclear waste glass durability in high pH disposal environments
Colleen Mann1, Daniel J. Backhouse1, Adam J. Fisher1, Jeremy Eskelsen2, Karine Ferrand3, Karel Lemmens3, Eric Pierce2, Neil C. Hyatt1, Russell J. Hand1 and Claire L. Corkhill1
1NucleUS Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, UK
2Oak Ridge National Laboratory, Tennessee, USA
3SCK.CEN, Mol, Belgium

Under the generic scenario envisaged for the geological disposal of vitrified UK high and intermediate level waste glass (HLW and ILW, respectively), high pH environments, formed through corrosion of the engineered barrier system, are expected to dominate the ground water chemistry thousands of years into the future. Of particular interest to the development of predictive glass corrosion models for the UK’s future geological disposal facility is the co-location of vitrified ILW with cementitious ILW materials (Fig. 1), which may lead to the interaction of vitrified products with a high pH (pH 10 – 13) groundwater derived from interaction with cementitious waste and backfill in the ILW repository in the long-term. Furthermore, there is potential for leachate from cement vault structures (~ pH 13) to interact with HLW. It is therefore necessary to build a complete kinetic and mechanistic understanding of UK HLW and ILW glass dissolution in high pH cement leachates.

Figure 1. A conceptual scenario (centre) for the co-disposal of the UK’s vitrified and cementitious Intermediate Level Waste (ILW) (left) with vitrified High Level Waste (HLW) and spent nuclear fuel (right) in a hard rock environment.

Having previously studied extensively the influence of saturated solutions of Ca(OH)2, NaOH and KOH on the dissolution of nuclear waste glass analogues, and found them to significantly influence the glass durability when compared to dissolution in pure water, our recent studies have focused on elucidating the response of glass dissolution mechanisms and kinetics to more complex cement leachate solutions. Such solutions have a high pH (ranging from pH 11 – 13) and contain a mixture of K, Na, Ca and Mg, in addition to Al, Si, Fe and carbonate. The composition of “synthetic cement leachates” was modelled based on the interaction of groundwater with cement over a 100,000 year period (Fig. 2).

Figure 2. Schematic showing the main pH-controlling phases within cement, as a function of geological disposal time. Firstly, KOH and NaOH are the dominant phases in solution, giving a high pH (known as young cement water, YCW); following this, Portlandite (Ca(OH)2) begins to dissolve, giving rise to a solution rich in Na, K and Ca (“young cement water with added Ca, YCWCa”). Evolved Cement Water (ECW) is rich in Ca, with a lower pH than in the “young” cement waters, and Old Cement Water (OCW) is buffered to a pH of ~10, owing to the dissolution of calcium silicate hydrate (CSH) phases.

This presentation highlights the results of batch experiments, where sodium aluminoborosilicate nuclear waste glass simulants, including the compositions MW25 (representative of UK HLW glass), LBS (representative of a well characterised, conceptual ILW glass) and the International Simple Glass (a six-component simplified HLW glass composition in use by glass scientists around the world), were exposed to “young”, “evolved” and “old” cement leachates.

Utilising a combination of solution geochemical analysis (e.g. ICP-OES) and high resolution surface analytical techniques (e.g. TEM, SAED, µ-focus XRD) we elucidate the mechanisms responsible for the formation of complex and intricate alteration layers resulting from the interaction of the glass with the cement leachates (Fig. 3). We find that pH is the main driver of alteration layer porosity – the highest pore diameter size was <15 nm – and this, in turn, significantly influences the dissolution rate. The formation of alkali- / alkali-earth silica gels through an alkali-silica reaction in the altered surface of the glass is promoted by the high pH and also charge compensation in the silica gel; hydrated ionic radius and ΔG of hydration arguments can be made to describe the selection of alkali / alkali-earth elements within the silica alteration layer. Layers composed of K-rich alkali silica gels exhibited the least protection to the altered glass surface, which those composed of Ca- or Mg-rich alkali silica gels are the most protective. However, the surface of glasses that exhibit the formation of Ca/Mg-alkali silica gels are also found to comprise Mg-bearing clay precipitates, such as smectite, which contribute to ongoing dissolution of glasses, through consumption of silica from the glass, the solution, and ultimately from the alkali-silica alteration layers.

These data are being compiled to provide a comprehensive insight to how high pH cement materials used in the construction, or disposed in the near vicinity of, a geological disposal facility may significantly influence the long-term durability of vitrified nuclear waste.

Figure 3. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Selective Area Diffraction (SAED) and TEM/Energy Dispersive Spectroscopy (EDS) analysis of alteration layers formed on the surface of the International Simple Glass in contact with “young cement water”.