Evaluating the temperature dependence of Magnox waste glass dissolution

Before vitrified waste can be safely disposed of, a comprehensive understanding of the effects of temperature on wasteform aqueous durability must be acquired. In the experiments herein, a Magnox waste glass of 25 wt.% simulant waste loading was leached in static batch experiments at 40, 70, 80 and 90 °C to investigate the Arrhenius dependence of dissolution. Results from ICP-OES/MS, EDX, SEM and XRD analyses are discussed in terms of the roles of the glass species within Magnox waste glasses. Here we show that changing the dissolution temperature changed the rate of hydrolysis relative to interdiffusion. At higher temperatures, the initial release of sodium deviated from Arrhenius-type behaviour and instead displayed an almost flat Arrhenius plot; demonstrating changes in temperature affect sodium differently to other glass species. Whilst the activation energies of the lithium and boron releases were in the range of a mixed reaction, the higher activation energy of sodium at lower temperatures combined with its non-Arrhenius behaviour suggested the dissolution processes of lithium and sodium differed. These observations were attributed to the preference of sodium to charge compensate the boron network.This work was supported by EPSRC under an Industrial CASE award (Grant Ref: EP/M507350/1) with Radioactive Waste Management Ltd

Relating Magnox and international waste glasses

Magnox and international radioactive waste glasses observe significantly different aqueous dissolution behaviours. Previously, the effects of Li on the kinetics of dissolution and B network of analogues of a complex Magnox waste glass were investigated (Li-Mg-EM). Here, an “International Simple Glass” with Li substituted for Na at two ratios (Li-ISG) was fabricated to compare the aqueous durability of Magnox and international glasses. 11B MASNMR, 23Na MAS-NMR, 6Li1H CP-NMR and SEM studies are presented. The IIIB/IVB ratio and the role of Na in the pristine glasses varied insignificantly with the Li:Na ratio. Further, the B network of Li-ISG was shown to leach congruently. Li, Na and Mg-containing secondary phases were present for Li-Mg-EM, whilst no precipitates were observed for Li-ISG. Li in ISG was shown to be detrimental but further substitution of Li improved long term aqueous durability, which was attributed to a Li-Na mixed alkali effect for Li-ISG but not Li-Mg-EM

Assessing the effect of radioactive waste glass dissolution on early-stage radionuclide migration using simplified geological repository Monte Carlo transport models

AbstractThe vitrification of radioactive waste within glass and subsequent disposal within a geological disposal facility (GDF) requires a comprehensive understanding of the effect of glass dissolution on GDF performance. This paper aims to analyse the effect of both high-level and intermediate-level waste (HLW and ILW) glass dissolution source terms on radionuclide release into the geosphere just above the disposal vault (the ‘crown’). Radionuclide migration was simulated in GoldSim for HLW in either granite or clay host rocks with a bentonite buffer using carbon steel or copper canisters, whereas ILW simulations considered either granite or clay host rocks, in either bentonite buffer or cement backfill, using concrete or cast-iron canisters. Glass dissolution source terms were varied by coupling GoldSim and MATLAB to modify the initial, residual, and resumption dissolution rates of the glass or by applying the analytical GRAAL model to glass dissolution. HLW glass results indicate no preference of granite over clay host rocks for a given canister type but that a copper canister is preferable to steel. ILW results suggest that a granite–bentonite–cast-iron environment yields lowest crown activities with cast-iron preferable to concrete as the canister, bentonite preferable to cement as the buffer/backfill, and granite preferable to clay as the host rock. Varying glass dissolution source terms (initial, residual, and resumption dissolution rates) had an understood effect on radionuclide migration, although changes were arguably insignificant considering peak crown activity for both HLW and ILW.</jats:p

Diffusive processes in aqueous glass dissolution

AbstractHigh level nuclear waste is often immobilised in a borosilicate glass for disposal. However, this glass corrodes in contact with aqueous solutions. To predict radionuclide releases from wasteforms, their dissolution mechanisms must be understood. Understanding glass dissolution mechanisms presents a challenge across numerous other disciplines and many glass dissolution models still remain conflicted. Here we show that diffusion was a significant process during the later stages of dissolution of a simplified waste glass but was not evidenced during the initial stages of dissolution. The absence of measurable isotopic fractionation in solution initially supports models of congruent dissolution. However, the solution becoming isotopically lighter at later times evidences diffusive isotopic fractionation and opposes models that exclude diffusive transport as a significant mechanism. The periodically sampled isotopic methodologies outlined here provide an additional dimension with which to understand glass dissolution mechanisms beyond the usual measurement of solution concentrations and, post-process, nano-scale analysis of the altered glass.</jats:p

Impacts of lithium on Magnox waste glass dissolution

The safe storage of vitrified waste demands a thorough understanding of the effects of wasteform composition on chemical durability. One of the major compositional differences between UK glass and international glasses is the presence of Li. Based upon a 25 wt.% loading UK Magnox waste glass, two seven-component analogues of molar Li:Na 1.0 and 1.5 were leached to investigate the effects of Li on chemical durability. A comprehensive 11B nuclear magnetic resonance study then took place to investigate the B network before and after dissolution. Here we show that the proportion of IIIB to IVB units did not evolve as the Li:Na ratio was varied; suggesting that Na preferentially charge compensates the B network. However, the B network was shown to leach incongruently at 90 °C. Despite Li being shown to be detrimental to durability during the earlier dissolution regimes, the residual rates of alteration implied excess Li contents had no long-term effects on chemical durability. The observed incongruent dissolution of the B network and initially decreased chemical durability could be attributed to Li preferentially modifying the Si network, thereby promoting glass hydration and B network dissolution whilst the Na compensated IVB units were less affected than IIIB units

Impacts of lithium on Magnox waste glass dissolution

The safe storage of vitrified waste demands a thorough understanding of the effects of wasteform composition on chemical durability. One of the major compositional differences between UK glass and international glasses is the presence of Li. Based upon a 25 wt.% loading UK Magnox waste glass, two seven-component analogues of molar Li:Na 1.0 and 1.5 were leached to investigate the effects of Li on chemical durability. A comprehensive 11B nuclear magnetic resonance study then took place to investigate the B network before and after dissolution. Here we show that the proportion of IIIB to IVB units did not evolve as the Li:Na ratio was varied; suggesting that Na preferentially charge compensates the B network. However, the B network was shown to leach incongruently at 90 °C. Despite Li being shown to be detrimental to durability during the earlier dissolution regimes, the residual rates of alteration implied excess Li contents had no long-term effects on chemical durability. The observed incongruent dissolution of the B network and initially decreased chemical durability could be attributed to Li preferentially modifying the Si network, thereby promoting glass hydration and B network dissolution whilst the Na compensated IVB units were less affected than IIIB units

Predicting radioactive waste glass dissolution with machine learning

The vitrification of high-level nuclear waste within borosilicate glass and its disposition within a multi-barrier repository deep underground is accepted as the best form of disposal. Here, the ability of machine learning to predict both static and dynamic glass leaching behavior is analysed using large-scale unstructured multi-source data, covering a diverse range of experimental conditions and glass compositions. Machine learning can accurately predict leaching behavior, predict missing data, and time forecast. Accuracy depends upon the type of learning algorithm, model input variables, and diversity or size of the underlying dataset. For static leaching, the bagged random forest method predicts well, even when either pH or glass composition are neglected as input variables, additionally showing potential in predicting independent glass dissolution data. For dynamic leaching, accuracy improves if replacing final pH with a species dissolution rate as an input variable, although results show no preferred output species (Si, Na, or Al)

Assessing static glass leaching predictions from large datasets using machine learning

© 2020 Elsevier B.V. Radioactive waste vitrified within glass is planned to be ultimately disposed of within a geological disposal facility. This study has applied machine learning to predict static glass leaching using an international experimental database of approximately 450 glasses to train/test various algorithms. Machine learning can accurately predict B, Li, Na, and Si releases for this complex database with Tree-based algorithms (notably ‘BaggingRegressor’ and ‘RandomForestRegressor’ in Python). This is provided that leaching experiment results, including elemental releases, are incorporated within the algorithm training variables, given that this study finds inaccurate prediction solely using initial test parameters as features. The trained algorithms underwent additional testing using an external database with prediction showing worse performance, likely due to substantial MgO and Na2O pristine glass oxide compositional variations across databases, with B releases generally being overestimated and Na underestimated. The use of molar oxide content performed significantly better than weight-fraction oxide for learning

Temperature dependent lithium isotope fractionation during glass dissolution

Understanding the mechanisms by which borosilicate glasses corrode in contact with aqueous solutions remains a challenge to the safety case for the geological disposal of vitrified high-level nuclear waste. Here, lithium isotope fingerprinting techniques were applied to the leachates of a simulant Magnox waste glass to probe the mechanisms of aqueous corrosion at both short and long timescales (6 hours to 464 days). Experiments took place at 40 and 90 °C to assess the consistency of the dissolution mechanisms across a range of commonly employed temperatures and the legitimacy of applying higher temperature experimental datasets to understand glass corrosion within a disposal facility at lower temperatures. Two competing release mechanisms were observed for lithium (diffusion and hydrolysis), and the relative proportions of these mechanisms changed through time. Leachates initially had lower δ⁷Li values than the pristine glass (-2.7‰ at 40 °C and -1.1‰ at 90 °C relative to the pristine glass) at both temperatures due to lithium leaching incongruently through diffusive processes. The greater offset between solution and solid at lower temperatures indicates a larger rate of diffusion (incongruent dissolution) relative to the rate of hydrolysis (congruent dissolution) at lower temperatures. The fraction of lithium released through diffusion relative to the fraction of lithium released through hydrolysis then increased at both temperatures with time up to 126 days, increasing from 0.47 and 0.22 at 6 hours to 0.66 and 0.41 at 126 days at 40 and 90 °C respectively. Subsequently, the fractions of lithium released through diffusion sharply decreased to 0.36 at 40 °C and 0.22 at 90 °C after 464 days, consistent with network hydrolysis coupled with secondary phase precipitation later controlling the long-term release of Li at both temperatures. Throughout the duration of the experiments (464 days) the δ⁷Li values in solution increased to 9.0‰ at 40 °C and 10.0‰ at 90 °C due to the formation of talc and montmorillonite phases at 40 °C and additional smectite phases at 90 °C. Further, no evidence for the formation of a diffusive barrier to the transport of lithium within the alteration layers became apparent during the later stages of dissolution at either temperature. However, the fraction of lithium leached through diffusion was still significant throughout all stages of dissolution. Lithium isotope ratios in solution were correlated with the transition from a system which was increasingly dominated by lithium diffusion as the dissolution rate slowed to one which was controlled by hydrolysis coupled with secondary phase precipitation at long durations. Alongside elemental ratios in solution, these results were consistent with the same set of mechanisms governing dissolution across the temperature range studied