Thomas L. Goût
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An isotopic investigation into the aqueous dissolution processes of nuclear waste glass.
Thomas L. Goût*1, Sambuddha Misra2, Rui Guo1, Madeleine S. Bohlin1, Edward T. Tipper1,
Aleksey Y. Sadekov3, Ian Farnan1.

1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK.
2Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India.
3School of Earth Sciences, University of Western Australia, 35 Stirling Highway,
Crawley, WA 6009, Australia.

Reprocessing of spent nuclear fuel generates highly radioactive liquor as a waste product; an internationally accepted approach to immobilising the radionuclides in the waste liquor is vitrification in a borosilicate matrix. The vitrification process reduces the hazard associated with storing the waste and produces a product which is suitable for final disposal in a Geological Disposal Facility (GDF). In the UK vitrification process, molten glass waste product is poured into stainless steel containers and is then interim stored above ground until an underground GDF is available. The GDF will consist of multiple natural and constructed barriers but after many thousands of years groundwater will come into contact the vitrified waste, potentially leaching remaining long-lived radionuclides from the glass and into the geosphere. To accurately quantify the release of radionuclides from the glass and effectively assess the contribution of the wasteform to a GDF multi-barrier system, the aqueous dissolution processes of nuclear waste glasses must be well understood and modelled.

A number of mechanistic aqueous dissolution models currently exist, some of which conflict and many of these are significantly limited in their applications. Of note, the GRAAL model at high reaction progress (Fig. 1a) couples rate-limiting interdiffusion reactions through a highly passivating hydrated glass layer with dissolution of the porous “gel” layer, and subsequent precipitation from the saturated leachant as secondary phases1,2. Such a model therefore predicts the incongruent dissolution of mobile species, such as Li and B, ahead of a congruent Si hydrolysis front. Contradicting classical interdiffusion models such as the GRAAL model, in recent years a theory describing the dissolution of silicate minerals was extended to nuclear waste glasses3,4. This interfacial dissolution-reprecipitation model (Fig. 1b) rejects interdiffusion as a rate-limiting process and predicts that the glass dissolves congruently through contact with a super-saturated thin film of water; the dissolved glass species subsequently precipitate from this film to form an altered glass layer and secondary mineral phases. Owing to the difference in the residual dissolution rates predicted by these models, it is important to investigate which model, if either, is applicable to nuclear waste glass dissolution under predicted GDF conditions.

Figure 1: Illustrations of theoretical cross-sections of a nuclear waste glass leached in water at high reaction progress as described by a classical interdiffusion model1,2(a) and an interfacial dissolution-reprecipitation model3,4(b).

As a result of the significant difference between the dissolution behaviours of Li and B and the morphology of the altered layer as predicted by these two contrasting models, this study focussed on discerning whether diffusion was a rate-limiting step at high reaction progress. This was achieved through an accelerated dynamic dissolution experiment using a simplified alkali borosilicate analogue of UK Magnox nuclear waste glass. Isotope geochemical methodologies were applied to the sampled leachates from the experiment to measure with extreme precision how the Li and B isotope ratios in solution evolved with time. Additionally, after high reaction progress was achieved, the leachant in one set of reaction vessels was refreshed to attempt to elucidate the passivating nature of the altered layer. Not only were both isotope ratios highly suggestive of diffusion being a rate-limiting step at high reaction progress, but both isotope ratios were consistent with the dissolution mechanisms described by the GRAAL model and evinced the existence of a highly passivating hydrated glass layer. It is expected these results and methodologies may be extended to more complex glasses to better investigate their dissolution mechanisms under a variety of experimental conditions.

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2. Frugier, P., Gin, S., Minet, Y., Chave, T., Bonin, B., Godon, N., Lartigue, J. E., Jollivet, P., Ayral, A., De Windt, L. & Santarini, G. SON68 nuclear glass dissolution kinetics: Current state of knowledge and basis of the new GRAAL model. J. Nucl. Mater. 380, 8–21 (2008).
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