Dissolution of UK High‐Level Waste Glass Under Simulated Hyperalkaline Conditions of a Colocated Geological Disposal Facility

Abstract

We report analysis of chemical durability of UK HLW MW+25% simulant glass under model hyperalkaline conditions of a colocated geological disposal facility. Glass powders and monoliths were dissolved for 168 days in saturated Ca(OH) 2 . Dissolution in the presence of high concentrations of Ca (>200 mg/L) was an order of magnitude lower than dissolution in water. Dissolution of Si did not occur until a Ca:Si ratio of <2 was achieved. The mechanism of dissolution involved the incorporation of Ca into the hydrated surface (initial, incubation regime), the precipitation of C-S-H phases, including a range of compositions in the C-(N)-(A)-S-H and M-S-H systems (intermediate regime), and the precipitation of C-S-H phases (the residual regime). Thermodynamic analysis and consideration of the CaO-SiO 2 -H 2 O phase diagram suggest that the rate-limiting step of glass dissolution in Ca-rich solutions is Ca-Si equilibrium, involving the precipitation of C-S-H phases, which change in chemical composition as a function of solution chemistry. In low SA/V ratio experiments, the dissolution progressed only to the initial incubation regime, resulting from fewer surface sites for Ca incorporation. Overall, these results suggest that Ca and Si in solution play an important role in the long-term durability of UK HLW in Ca-rich solutions.

Corrosion of the International Simple Glass under acidic to hyperalkaline conditions

“…Our previous work on the performance of a model ILW glass in saturated Ca(OH) 2 solutions at 50°C and 90°C (pH(RT) = 12.5), representing a cementitious environment, demonstrated that the presence of Ca leads to the formation of relatively protective alteration products on the glass surface. [8][9][10][11] These results together with those of Andriambololona et al 12 and Chave et al 13 demonstrate the importance of Ca in determining the behaviour of the glass studied under such pH conditions. Meanwhile Gin et al 14 have shown that at pH(90°) = 11.5 in a KOH solution the international simple glass (ISG) dissolves congruently.…”

“…The forward dissolution rate (R i , where i is Si, B or Na) of the ISG at pH(RT) 9 and 40°C was obtained from the mean value of the dissolution rates measured at log 10 )] value of -7.0 was chosen to conduct rate law determination experiments, with confidence that dilute conditions, enabling the accurate measurement of forward dissolution rates, would be maintained over all temperatures studied; the data of Neeway et al 16 indicates that a log 10 [(q/S)/(m s -1 )] value of -7.0 is sufficient to achieve forward rate conditions at 90°C which is expected to be the most aggressive condition studied here. Forward rates (R B , R Si and R Na ) …”

“…This suggests that there is a reduced level of Ca in solution that must be achieved before Si is observed in solution, in agreement with previous observations on a model ILW glass 8 and a sample of MW-25% simulant waste glass altered in Ca(OH) 2 . 10 The Ca/Si values (Table 3) measured on the MCC-1 samples suggest that tobermorite-type (Ca/Si = 0.83) alteration products might form at earlier times, with other phases, such as afwillite (Ca/Si = 1.5) possibly becoming prevalent later on. However, PHREEQC geochemical modelling suggests that afwillite is in equilibrium (saturation indices of 0.36, 0.57, 1.00, 0.49 and 0.70 for 90, 180, 360, 540 and 720 days respectively) and that tobermorite should be supersaturated (saturation indices of 11.61, 12.18, 13.44, 11.97 and 12.63 for tobermorite-11Å for 90, 180, 360, 540 and 720 days respectively) with respect to the leachate solution from 90 days onwards.…”

“…The cluster sizes ranged from 4 to 13 μm and always appeared on the outside of the sample, rather than within the alteration layer, or between the alteration layer and the glass surface, indicating that they form by precipitation from solution, rather than a re-ordering of the alteration layer. Geochemical modelling with PHREEQC indicates that the formation of crystalline C-S-H precipitates, particularly tobermorite-9Å, tobermorite-11Å and tobermorite-14Å, is thermodynamically favourable in Ca-rich systems with only a relatively small concentration of Si (<10 ppm) in solution (see also Jennings 32 and Corkhill et al 10 ); for example the calculations give saturation indices of 2.98, 3.92 and 0.14 for 9 Å, 11 Å and 14Å-tobermorite respectively after 30 days with a Si concentration of 1 ppm. The PHREEQC calculations also suggest that zeolitic phases might not be expected to be saturated during the dissolution of ISG in Ca(OH) 2 (negative saturation index values for all zeolites at all timepoints), although this could be due to insufficient thermodynamic data.…”

“…(7)- (8); b Compiled forward dissolution rates using data from this study and from Neeway et al 16 showing log 10 …”