A Study of the Dissolution of Nuclear Waste Glasses in Highly-Alkaline Conditions

Long-term disposal of nuclear waste is a problem for the world’s nuclear energy industry. The preferred option for the UK for Intermediate- and High-Level Waste (ILW & HLW) is for long-term emplacement in a Geological Disposal Facility (GDF), which requires a robust safety case based on the long-term behaviour of the waste. This work investigates one aspect of the long-term behaviour: the dissolution of the waste in highly-alkaline conditions, in the case of the formation of a highly-alkaline plume within the GDF by the interaction of groundwater with cementitious materials.
Dissolution experiments were performed on a range of glass compositions with varying CaO and MgO contents and B/Al ratios to analyse their effects on glass dissolution at high-pH. Ca and Mg are expected to be present in the GDF and in UK HLW, and are known to significantly affect glass dissolution. The effect of varying B/Al ratio is relevant to the comparison of natural glasses (B/Al = 0), with nuclear waste glasses (B/Al ~ 1 – 10). Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy was performed to identify the effects of composition on glass structure. Dissolution experiments were also performed on existing glasses pertinent to UK nuclear waste disposal to determine their performance.
The addition of CaO and MgO, in replacement for Na2O, was found to lead to a decrease in IVB units, due to the inability of the divalent Ca2+ and Mg2+ cations to charge-compensate for two IVB tetrahedra each. The increased strength of this effect in the Mg-containing glasses suggests that Mg may be behaving as an intermediate oxide. Increasing the ratio of B/Al in aluminoborosilicate glasses was found to lead to an increase in the proportion of IVB units.
Glass dissolution resistance was found to correlate with replacement of Na2O for CaO and MgO. Ca-containing glasses displayed a higher dissolution resistance than those containing Mg, due to a combination of structural and solution factors. Increasing the B/Al ratio of the glasses led to a decrease in dissolution resistance, believed to be due to the greater resistance of Si – O – Al bonds to hydrolysis compared to Si – O – B bonds. The leachant cation (Ca or K) was found to have a significant effect on dissolution behaviour.
The International Simple Glass (ISG) was found to behave differently to MW-25% (UK simulant HLW glass) in Ca-rich, high-pH solutions, suggesting that it is not helpful as an analogue for the dissolution of UK nuclear waste glasses. The dissolution of a laboratory-made basaltic glass was found to be partially comparable to that of natural basaltic glasses, indicating that care must be taken when comparing the dissolution resistance of natural basaltic glasses with nuclear waste glasses. Significant localisation of elements in alteration layers during dissolution, e.g. Zr for ISG, suggests that the mechanism of dissolution in these experiments was coupled dissolution-reprecipitation.