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Current kinetic models for nuclear waste glasses (e.g. GM2001, GRAAL) are based on a set of mechanisms that have been generally agreed upon within the international waste glass community. These mechanisms are: hydration of the glass, ion exchange reactions (the two processes are referred as inter-diffusion), hydrolysis of the silicate network, and condensation/precipitation of partly or completely hydrolyzed species that produces a gel layer and crystalline phases on surface of the altered glass. Recently, a new idea with origins in the mineral dissolution community has been proposed that excludes inter-diffusion process as a potential rate-limiting mechanism. To understand how the so-called interfacial dissolution/precipitation model can change the current understanding of glass behavior, an in-depth review of the current knowledge with a special focus on inter-diffusion processes is considered. Also discussed is how experimental conditions change the predominant mechanisms and how one model may not be sufficient to explain the glass dissolution behavior under a wide range of geochemical conditions. In addition to the review of the above subjects, a key experiment used to account for the interfacial dissolution/precipitation model was replicated to further revisit the interpretation. It is concluded that the selected experiment design may lead to ambiguous conclusions and that, under the conditions investigated (dilute conditions, deionized water), evidence of inter-diffusion exists.

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Nuclear waste glasses dissolve at the forward dissolution rate (r f) in very dilute aqueous solutions, which can isolate the impact of the glass composition from solution feedback and alteration product effects. While it has long been known that pH and temperature (T) strongly impact r f , the impacts of glass composition have remained uncertain. In this work, r f data from 19 nuclear waste glasses were used with the aim of identifying the effect of glass composition on r f. The r f values were modeled as: r f = k 0 10-ηpH exp(−E a /RT), with k 0 , η, E a , and R, respectively, being the intrinsic rate constant, pH coefficient, apparent activation energy, and gas constant. However, no predictive correlation could be established between the individual model parameters (log[k 0 ], η, and E a) and glass composition for the glasses considered in this study, an outcome that was attributed to the strong positive correlation between the log[k 0 ] and E a parameters. Therefore, a model was fitted directly to the combined r f from all 19 glasses. This approach showed that 90% of the variation in r f data could be accounted for solely by T and pH effects. Therefore, any composition effects must be relatively small. After normalizing for differences in pH and T, the only notable differences in r f between the glasses were found to correlate with variations in the fraction of glass forming tetrahedra contributed by tetrahedral boron, f([4] B), with an abrupt threshold at a high value of f([4] B) (~0.22), where higher r f are predicted with no discernable composition effects below the threshold.

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Assessment of glass dissolution kinetics, under disposal relevant temperature and pH environments, is required to credibly estimate radionuclide release rates from vitrified radioactive waste. Leaching of the International Simple Glass (ISG) under acidic to hyperalkaline conditions was examined. Forward rate measurements have been obtained using the dynamic leaching SPFT protocol and rate parameters for B, Na and Si in the basic regime; errors in rates predicted using these parameters at high pH and temperature are significant because the fitting uses logarithmic data. Longer term behaviour under hyperalkaline conditions, representative of some disposal environments, was investigated using the PCT and MCC-1 static leaching protocols with Ca(OH)2 solutions for up to 120 days (PCT) and 720 days (MCC-1). In hyperalkaline conditions dissolution was incongruent for all elements and the presence of alternating zirconia-rich and zirconia-poor alteration layers was observed on all leached monoliths, indicating the occurrence of a self-organisation phenomenon during leaching.

11Many efforts have focused on the sequestration and immobilization of 99 Tc because the 12 radionuclide is highly mobile in oxidizing environments and presents serious health risks due to 13 its radiotoxicity and long half-life (t 1/2 = 213 000 a). One of the more common methods for Tc 14 removal from solution and immobilization in solids is based on reducing Tc from highly soluble 15 Tc (VII) to sparingly soluble Tc (IV) . In order to remove solution Tc through this reduction process, 16 the Tc-sequestering solid must contain a reducing agent and, ideally, the Tc-sequestering 17 material would function in a large range of chemical environments. For long-term stability, the 18 reduced Tc would preferentially be incorporated into the resulting mineral structure instead of 19 simply being sorbed onto the mineral surface. Here, we report results obtained from batch 20 sorption experiments performed in anoxic and oxic conditions with two sulfide-containing 21 2 potassium metal sulfide (KMS) materials, known as KMS-2 and KMS-2-SS. In deionized water 1 in anoxic conditions after 15 d of contact, KMS-2 is capable of removing ~45% of Tc and KMS-2 2-SS is capable of removing ~90% of Tc. The improved performance of KMS-2-SS compared to 3 KMS-2 in deionized water in anoxic conditions appears to be linked both to a higher pH 4 resulting from the batch sorption experiments performed with KMS-2-SS and a higher overall 5 purity of KMS-2-SS. Both materials perform even better in highly caustic (pH~13.5), high ionic 6 strength (8.0 M) simulated Hanford low-activity waste solutions, removing more than 90% Tc 7 after 15 d of contact in anoxic conditions. Post-reaction solids analysis indicate that Tc(VII) is 8 reduced to Tc(IV) and that Tc(IV) is bonded to S atoms in the resulting KMS-2 structure in a 9 Tc 2 S 7 form. In contrast to previous ion exchange experiments with other KMS materials, the 10 batch sorption experiments examining Tc removal cause the initially crystalline KMS materials 11 to lose much of their initial long-range order. 12 13