Kinetic and thermodynamic approaches to the derivation of long-term release rates of species from defense waste glass are reviewed. It is concluded that at high flow rates kinetic factors are pre-dominant, while at low flow rates saturation of the aqueous medium with respect to major matrix elements, particularly with respect to silica present in the glass and in its alteration products, becomes a controlling factor. Quantitative calculations indicate that under likely repository conditions the release rates can be expected, in general, to fall below the NRC criterion of 10−5 yr−1.

A general rate equation was developed to describe the reaction of nuclear waste glasses with aqueous solutions as a function of pH, the ratio of sample surface area to solution volume, temperature, time, glass- and solution composition. Thermodynamic and kinetic models have been combined in a new model. Glass network dissolution, pH variation, precipitation of stable or metastable solid reaction products, silica saturation at the glass and solution interface (reaction zone) and a residual affinity for the long term reaction under near saturation conditions have been addressed. The model has been successfully used to interpret a great number of experimental data reported in literature.

Borosilicate waste glass durability in simulated repository environments can be assessed by use of static tests in leach vessels fabricated of the representative geomedia. Control of the oxidation potential during the test simulates a basalt repository environment. Under very anoxic conditions (i.e. at negative Eh values), the interactions between basalt and SRP waste glass in silica-saturated basaltic groundwaters are the same as those of basalt and groundwater when no waste glass is present. The lack of significant leaching of ions from the waste glass and the lack of any significant changes in either the leached surfaces of glass or basalt under anoxic conditions suggests that the components of this system are at equilibrium when oxygen is absent.

Interface reactions between two nuclear waste borosilicate glasses and repository components have been studied in an in-situ burial experiment in Stripa mine, Sweden. Exposures up to one year have been analyzed and are reported.

The results obtained up to date show an increased depth of attack ranging by: increased severity from the glass/granite interfaces, glass/glass interfaces to glass/bentonite interfaces. The rate and extent of surface attack is discussed in terms of ion exchange with species from the groundwater and leached from the repository components.

Environmental conditions in a repository are expected to be significantly different from those encountered by a glass tested under MCC-1 specifications. In addition to variations in flow rate and glass surface area to water volume ratio (SA/V), the water chemistry and presence of waste package components in the repository will most certainly affect the leaching behavior of the glass. The independent effects of each of these variables have been studied by numerous investigators. For example, it is well known that the leaching rate increases as the flow rate increases (1). Also, the rate of approach to saturation is increased as SA/V is increased under MCC-1 type testing (2). The use of silicate water generally decreases the rate of leaching while waste package components such as iron enhances the leaching rate of glass under MCC-1 type testing (3–4).

The corrosion behaviour in distilled water of five simulated European reference high level waste glasses has been investigated for times up to 8 months. The glasses were chosen in accordance with the Joint European Community research programme on HLW form characterization: two French glasses (SON58, SON64), one British glass (UK209) and two glasses designed to incorporate HLW stored at the Eurochemic plant in Belgium (SM58 and SAN60). In the reference condition (90°C, SA.V−1 = 1 cm−1) the leaching was characterized mainly by diffusion processes and by the onset of saturation in the leachate (these saturation effects were found to depend on glass composition). In more dilute conditions (SA.V−1 = 0.1 cm−1), matrix dissolution was the rate controlling mechanism. Increasing the leachant temperature to 150°C and higher lead to pronounced surface crystallization and, at least for one glass (SAN60), to an increased corrosion.

Surface analysis has been conducted on samples leached in a Soxhlet apparatus at 100°C in presence and in absence of air. The XPS and RBS techniques were applied to analyse the content of the silicon, iron and uranium while the nuclear reaction method was utilized to analyse the hydrogen content. The anoxic environment favours the release of iron while decreasing the dissolution of uranium. Hydrogen content is always higher in samples leached in presence of air.

Silicon depletion is evident in all cases. The diffusion process seems to regulate the growth of the layer on the glass surface. After long leaching time a detachment, at least partial, of this layer is observed.

The reaction between aqueous solutions and borosilicate glasses designed for commercial or defense waste immobilization produces a hydrated layer on the surface of the glass which can be characterized by infrared reflectance spectroscopy. Specular reflectance curves, obtained by Fourier transform infrared spectroscopy, can be deconvoluted by Kramers-Kronig analysis to obtain true absorption spectra. The pattern of Si-O stretching modes changes for alkali silicate glass, indicating changes in the network polymerization. The characteristic intense band of the borosilicate glasses simply changes intensity in a way that scales with degree of hydration. The progressive hydration of the glass surface also appears as a broad OH band which can be extracted from the reflectance curve by the deconvolution process.

The early stages of glass dissolution are marked by rapid changes in pH for leach solutions with low buffer capacity. Consideration of both dissolution reactions and the electrical neutrality requirement allow the calculation of exchanged cation equivalents from the depletion of the H+-ion reservoir. Comparison was made between sodium trisilicate glass, a commercial waste glass (PNL 76−68), and the defense waste reference glass. The released Na+ calculated from the pH curve of sodium trisilicate glass has the expected parabolic dependence on time. Thenuclear waste glasses react with H+ more slowly and the pH - time plots do not have the shape of a titration curve. The equivalents of cations released follow a power law function, the exponent of which depends on initial pH. There are two distinct rate regimes for all glasses with initial fast rates related to consumption of H+ and later, slower rates nearly independent of H+ activity.

The French LWR reference glass was leached under different experimental conditions. Analysis of the leachate together with electron microscopic examination of the leached surfaces led us to the following conclusions. The corrosion of the nuclear glasses is congruent at the hydration front, but only a fraction of the elements enters the solution ; the other elements contribute to the formation of the surface layer resulting in incongruent dissolution from the standpoint of the entire sample. The leaching rate in static experiments was observed to decrease sharply in time. Specific experiments were performed to assess the protective role of the hydrolyzed layer with respect to the silica solubility limit, and the predominant effect of the solubility limit was clearly demonstrated.

During the past ten years extensive data have been determined for the corrosion of nuclear waste forms in short-term laboratory experiments (usually less than one year). The long-term behavior of glass has been inferred by: (1) the acceleration of corrosion rates at high temperatures [1]; (2) the use of high surface areas of the glass to small volumes of solution [1]; and the analysis of natural glasses altered over long periods of geologic time [2, 3]. The most recent efforts have concentrated on understanding the mechanisms of corrosion [1, 4, 5]. The corrosion mechanism may be used to make long-term extrapolations of the “stability” of the waste form. In this paper, we consider a linear time dependence for the corrosion under near saturation conditions and use a rate equation in the QTERM code [6, 7, 8] to model the long-term behavior of the German glass, C-31−3EC [9], JSS A [10, 11] and SRL TDS 131 [1]. The data base for C-31−3EC has been published elsewhere [9, 12, 13, 14], and we include experimental work completed by Rabe for boron and silica, at 200°C.

Nuclear reaction analysis is discussed as a method for determining the distribution of deuterium beneath the surfaces of glasses exposed to heavy water solutions for various times and temperatures. Limited examples of both conventional depth profiling for deuterium and a new technique for deuterium mapping are presented. The possible use of deuterium mapping for investigating the kinetics and mechanisms of the leaching of glasses is discussed.

The leaching experiments of Soxhlet type have been carried out[l, 2] and on the basis of the results we developed the mathematical leaching model which used one dimensional diffusion and could treat the growth of surface layers. The model adopted the following assumptions: 1) The velocity of the bulk glass-surfacé layers boundary depends on time alone. 2) Some of the diffusing substances are immobilized in the surface layers by an irreversible first-order reaction. 3) A fictitious film exists at the solution-surface layers interface. The fundamental equations were established based on these assumptions and the numerical solutions were obtained by the Crank-Nicholson implicit method. The values of the diffusion coefficient, the reaction rate and the film mass transfer coefficient were obtained by a trial-and-error method. The applicability of the model was confirmed by the fact that the leaching mechanisms deduced from the calculated results were consistent with those mechanisms deduced from the experimental results. The present study showed the proposed model was valid for calculation of the leach rates of waste glasses when surface layers grew during leaching, and the study also indicated which parameters should be measured experimentally to predict the leach rates.

The leaching of nuclear waste glasses in aqueous solution is still considered to be the most important process when discussing a potential release of radionuclides. However, in the last few years it has become clear that the release mechanism under static leaching conditions is very complex involving several processes [1]. The kinetics gave no evidence for pure leaching (i.e. the alkali-ion exchange) which should result in a square root time law, or for pure corrosion (i.e. congruent dissolution of the glass) which should result in a linear time law. Moreover, fits using a combination of these two processes gave unsatisfactory results. This is certainly due to additional processes which are not considered by ion exchange and congruent dissolution. Some explanations may be the build up of alteration layers, saturation effects of some elements with phase formation due to solubility limits and disappearance of metastable phases. To describe such a complicated system thermodynamic data are needed and measurements of mass losses as well as analyses of leachates and surfaces layers have to be performed.

In the absence of any identified solid phase host (other than the original glass), boron has been assumed to accumulate in the fluid during the reaction of borosilicate glass waste forms with aqueous fluids. Using this assumption, it is possible to define a boron index which can be used to monitor the amount of glass that has been dissolved and to provide a worst-case measure of the degradation of the primary glass waste form. Several boron-containing silicate phases have been identified thus invalidating the assumption that boron does not precipitate. The effect is apparently small and the assumption that boron release is a direct measure of degree of alteration of borosilicate glass is still probably a good one.

Time-temperature-transformation curves have been determined for SRL-165 and SRL-131 waste glasses for all potential waste compositions. SRL-165 glasses, which contain less alkali, exhibit less complex devitrification and higher overall durability than SRL-131 waste glasses. Devitrification was found to have less effect on the durability of waste glasses than changes in composition. Non-uniform dissolution at the glass leached layer interface is observed. However, leached layers formed after 28 days shield the crystal interfaces from further dissolution.

The morphology and chemical structure of surface layers formed during the aqueous leaching of the SRP-type borosilicate waste glass in simulated ground waters have been studied by SEM/EDXS, XRD, and surface spectroscopy. Comparison of layers formed in deionized, silicate, and brine waters shows the processes of both corrosion and precipitation to be highly localized. The corrosion process proceeds on the glass surface preferentially at points of stress and inhomogeneity. Crystalline and noncrystalline precipitates form in well-defined regions within the surface layers indicating the layer development process to be dominated by species migration and local saturation effects.

An evaluation of a broad set of HLWg lass corrosion data implies that adequate corrosion models must also take into account kinetics slower than √t. Solubility limits may not give a sufficient explanation for these cases. Before all, the long term leach rates are not infinitely small. A proposal is made how “slow” kinetics can be included in a model.

A simple open-system model is used to evaluate the effect of groundwater flow on borosilicate glass dissolution. With appropriate assumptions, the mass balance equation is: dc/dt=R-kfc;R=k(ceq-c)

where c is the concentration of dissolved species i, t is time, kf is the flushing frequency (i.e., volumetric flow rate divided by fixed pore volume), and R is the normalized rate of dissolution. A first-order dependence of R on departure of c from the equilibrium saturation concentration, Ceq, is also assumed. Results from steady-state (dc/dt = 0; c = css = constant) static and dynamic flow tests on a borosilicate glass at 90°C were used to calculate R and the dissolution rate constant, k, as a function of flow rate. The calculated results of the model are in good agreement with independent measurements.

There are three important conclusions. First, even the slowest flow rate used (0.1 ml/hr) corresponds to a high flow rate with respect to the intrinsic dissolution rate of the glass and expected repository flow conditions. Second, previous release models that scale radionuclide release rate directly to solubility concentration may overestimate release at high flow rates. This is because previous models fail to account for the pronounced decrease in steady-state solution concentration at the solid interface with increasing flow rate. Third, increased flow rate accelerates the glass dissolution rate without changing the reaction mechanism. This effect is attributable to constant replenishment of undersaturated solution at the glass surface and an apparent increase in the dissolution rate constant with increasing flow rate.

The International Waste Glass Study found important similarities, based on correlations of composition and leaching performance, among many different defense and commercial waste glass compositions being studied worldwide. In addition, the beneficial effect of waste on glass performance was confirmed for the systems evaluated.