Dissolution mechanisms of simplified analogues of complex UK radioactive waste glasses
Lead Research Organisation:
University of Cambridge
Department Name: Earth Sciences
Abstract
To investigate the aqueous durability of UK nuclear waste glasses. The project involves making the link between the behaviour of complex glasses and their simplified analogues from which mechanistic information on dissolution and passivation can be determined. Simplified borosilicate glasses will be formulated to mimic the behaviour of the complex glasses presenting at the UK waste vitrification plant at Sellafield. A combination of ICP-MS, electron microscopy and nuclear magnetic resonance measurements of glasses will be made to determine the nature of the alteration layer on the glass surface and its influence on the overall level of glass dissolution.
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/R513180/1 | 01/10/2018 | 30/09/2023 | |||
| 1733710 | Studentship | EP/R513180/1 | 01/04/2016 | 31/03/2020 | Thomas Goût |
| Description | Isotope-specific quantitative analysis complemented dissolution experiments to show that lithium in simplified analogues of UK radioactive waste glasses caused incongruent dissolution of the boron network but had no long-term effect on aqueous durability; thereby showing that vitrified nuclear waste packages containing excess lithium contents could be disposed of in the same manner as those of standard lithium contents, without the need for specialised disposal strategies. Conversely, it was shown that the addition of lithium to a simplified international radioactive waste glass composition improved aqueous durability. This significant difference in dissolution behaviour of the primary glass phase and differences in the nature and quantities of secondary phase precipitates formed between UK and international simplified analogues demonstrated a relationship between the analogues could not be simply formed through comparisons of leachate compositions and leached glass surfaces. These results contributed to our understanding of how glasses of variable lithium contents might behave in a geological waste disposal facility and how boron is released from waste glasses. Subsequent studies took place to develop an understanding of the effects of dissolution temperature on the kinetics of UK waste glass alteration. Initial analyses of the leached glass surfaces and leachates suggested that the dissolution mechanisms were consistent across the range of temperatures probed with the exception of the dissolution processes of sodium. However, stronger evidence and a new method to probe radioactive waste glass dissolution mechanisms were required to confirm these results. It was shown that lithium isotope fingerprinting techniques could be applied to the leachates of a simplified analogue of a UK radioactive waste glass to investigate whether diffusive processes were taking place during dissolution. The results from this initial proof-of-concept experiment demonstrated that dissolution of the simplified analogue was initially congruent, but the isotopic signatures of the leachates at longer leaching durations showed evidence for diffusive processes. Subsequently, these techniques were applied to probe the dissolution mechanisms of a complex simulant UK waste glass composition and the temperature dependence of these mechanisms. It was shown through the initially isotopically lower leachates that diffusive processes took place during the initial moments of dissolution such that dissolution was initially incongruent. Using an isotopic mass balance model these diffusive processes were shown to persist throughout dissolution, but dissolution at the longest duration was predominantly congruent. This result which contrasts with the simplified analogue results and the postulated dissolution mechanisms of international waste glasses. This therefore highlighted notable differences in the mechanisms controlling the dissolution of simplified analogues and the complex glasses they aim to represent, likely arising from the absence of elements which are beneficial to aqueous durability from the simplified glass network. Further, dissolution mechanisms were shown to be consistent at accelerated laboratory temperatures and expected geological disposal facility temperatures. This work therefore validates the use of high-temperature datasets in predicting dissolution behaviour in otherwise identical lower temperature disposal facility environments. |
| Exploitation Route | It is firstly envisioned this work will impact academics who aim to investigate the mechanisms of nuclear waste glass dissolution. The lithium isotopic techniques employed in this work provide a novel, much-needed additional dimension with which to directly probe and understand the dissolution processes of nuclear waste glasses. It is expected that this isotopic information could also be incorporated as an additional term into models of glass dissolution to better predict glass corrosion behaviour over geological timescales. As such, combined with the understanding of the effects of lithium contents and dissolution temperature on UK waste glass dissolution this work presents, it is also expected this work will contribute to the safety case for a UK high level waste geological disposal facility. |
| Sectors | Energy,Environment |