Durability Of Nuclear Waste Glasses Research Articles

Effects of alteration product precipitation on glass dissolution

Understanding the mechanisms that control the durability of nuclear waste glass is paramount if reliable models are to be constructed so that the glass dissolution rate in a given geological repository can be calculated. Presently, it is agreed that (boro)silicate glasses dissolve in water at a rate dependent on the solution concentration of orthosilicic acid (H4SiO4) with higher [H4SiO4] leading to lower dissolution rates. Once the reaction has slowed as a result of the buildup of H4SiO4, another increase in the rate has been observed that corresponds to the precipitation of certain silica-bearing alteration products. However, it has also been observed that the concentration of silica-bearing solution species does not significantly decrease, indicating saturation, while other glass tracer elements concentrations continue to increase, indicating that the glass is still dissolving. In this study, we have used the Geochemist’s Workbench code to investigate the relationship between glass dissolution rates and the precipitation rate of a representative zeolitic silica-bearing alteration product, analcime [Na(AlSi2O6)⋅H2O]. To simplify the calculations, we suppressed all alteration products except analcime, gibbsite (Al(OH)3), and amorphous silica. The pseudo-equilibrium-constant matrix for amorphous silica was substituted for the glass pseudo-equilibrium-constant matrix because it has been shown that silicate glasses act as a silica-only solid with respect to kinetic considerations. In this article, we present the results of our calculations of the glass dissolution rate at different values for the analcime precipitation rate constant and the effects of varying the glass dissolution rate constant at a constant analcime precipitation rate constant. From the simulations we conclude, firstly, that the rate of glass dissolution is dependent on the kinetics of formation of the zeolitic phase. Therefore, the kinetics of secondary phase formation is an important parameter that should be taken into account in future glass dissolution modeling efforts. Secondly, the results indicate that, in the absence of a gel layer, the glass dissolution rate controls the rate of analcime precipitation in the long term. The meaning of these results pertinent to long-term glass durability is discussed.

Compositional Effects on the Long-Term Durability of Nuclear Waste Glasses: A Statistical Approach

AbstractAs a result of the development of a reference glass formulation for the immobilization of West-Valley nuclear waste, a large number of glass compositions have been tested under a variety of leaching conditions for extended durations. In this work, data from the standard PCT leaching procedure (deionized water, 90°C, S/V = 2000 m-1) are evaluated for a subset of 98 of these glasses, for which the alteration times extend up to fifteen years. Analysis of the leachate data over this extended period has led to a classification of the glass compositions according to their PCT behavior, as characterized by the boron release in solution, the decrease in the rate of alteration, or the occurrence of a resumption of alteration. Three types of behavior are associated with a normalized mass loss of boron after 100 days of around 1, 2, or 5 g m-2 respectively, and a ratio of the rate of alteration at 7 days to the rate at 1000 days of about 30. A fourth type is characterized by a much larger decrease between the rates at 7 and 1000 days with a ratio exceeding 120. Four further categories are associated with late-stage resumption of glass alteration. A statistical analysis of the data has been used to model the leachate data as a function of glass composition. The effects of seven major elements (Si, B, Al, Li, Na, K, Fe) on the long-term glass durability have been studied. As a result of this analysis, the different types of leaching behaviors can be predicted with a good reliability from the composition of the pristine glass.

Raman spectra, structural units and durability of nuclear waste glasses with variations in composition and crystallization: implications for intermediate order in the

The Raman spectra of nuclear waste glasses are composed of large variations in half-width and intensity for the commonly observed bridging (Q 0 ) and non-bridging (Q 1 to Q 4 ) bands in silicate structures. With increase in waste concentration in a boroaluminosilicate melt, the bands of quenched glasses are distinctly localized with half-width and intensity indicative of increase in atomic order. Since the nuclear waste glasses contain disparate components, and since the bands depart from the typical random network, a systematic study for the origin of these bands as a function of composition and crystallization was undertaken. From a comparative study of Raman spectra of boroaluminosilicate glasses containing Na 2 O-ZrO 2 , Na 2 O-MgO, MgO-Na 2 O-ZrO 2 , Na 2 O-CaO-ZrO 2 , Na 2 O-CaO, and Na 2 O-MgO-CaF 2 component sets and orthosilicate crystals of zircon and forsterite, intermediate order is inferred. An edge-sharing polyhedral structural unit is proposed to account for narrow bandwidth and high intensity for Q 2 antisymmetric modes, and decreased leaching of sodium with ZrO 2 concentration in glass. The intense Q 4 band in nuclear waste glass is similar to the intertetrahedral antisymmetric modes in forsterite. The Raman spectra of zircon contains intratetrahedral quartz-like peaks and intertetrahedral non-bridging silicate peaks. The quartz-like peaks nearly vanish in the background of forsterite spectrum. This difference between the Raman spectra of the two orthosilicate crystals presumably results from their biaxial and uniaxial effects on polarizability ellipsoids. The results also reveal formation of 604, 956 and 961 cm -1 defect bands with composition and crystallization.

Raman spectra, structural units and durability of nuclear waste glasses with variations in composition and crystallization: Implications for intermediate order in the glass network

Abstract The Raman spectra of nuclear waste glasses are composed of large variations in half-width and intensity for the commonly observed bridging (Q0) and non-bridging (Q1 to Q4) bands in silicate structures. With increase in waste concentration in a boroaluminosilicate melt, the bands of quenched glasses are distinctly localized with half-width and intensity indicative of increase in atomic order. Since the nuclear waste glasses contain disparate components, and since the bands depart from the typical random network, a systematic study for the origin of these bands as a function of composition and crystallization was undertaken. From a comparative study of Raman spectra of boroaluminosilicate glasses containing Na2O-ZrO2, Na2O-MgO, MgO-Na2O-ZrO2, Na2O-CaO-ZrO2, Na2O-CaO, and Na2O-MgO-CaF2 component sets and orthosilicate crystals of zircon and forsterite, intermediate order is inferred. An edge-sharing polyhedral structural unit is proposed to account for narrow bandwidth and high intensity for Q2 antisymmetric modes, and decreased leaching of sodium with ZrO2 concentration in glass. The intense Q4 band in nuclear waste glass is similar to the intertetrahedral antisymmetric modes in forsterite. The Raman spectra of zircon contains intratetrahedral quartz-like peaks and intertetrahedral non-bridging silicate peaks. The quartz-like peaks nearly vanish in the background of forsterite spectrum. This difference between the Raman spectra of the two orthosilicate crystals presumably results from their biaxial and uniaxial effects on polarizability ellipsoids. The results also reveal formation of 604, 956 and 961 cm−1 defect bands with composition and crystallization.

Nuclear Waste Glass Durability: I, Predicting Environmental Response from Thermodynamic (Pourbaix) Diagrams

This paper reports that Pourbaix diagrams are shown to conceptually explain dissolution of glasses in terms of their thermodynamic stability and surface layer formation. To predict the long-term effects of ground-water contact on the durability of nuclear waste glass in a geologic repository, the nature of the glass dissolution process has been extensively examined. Hydration thermodynamics has been used to quantify the role of glass composition and the effect of solution pH. The glass compositions examined vary widely in composition and include natural, lunar, medieval, and nuclear waste glasses, as well as some glass-ceramics. The known effects of solution pH and oxidation potential (Eh) on glass dissolution are empirically described by thermodynamically calculated Pourbaix diagrams. Statistical analysis of over 300 glass durability tests demonstrates that the Pourbaix diagram can be quantified because of the colinearity of the hydration free energy with solution pH defined by the Nernst equation and the colinearity of the solution pH with the concentration of dissolved silicon and boron in the solution defined by the pH dependence of the ion activities. Construction of Pourbaix diagrams to describe glass dissolution serves to unify and categorize various existing experimental glass dissolution data.

Observation by Sem/edxs of Leached Layers on SRL-165 Glass from 5-Year MIIT Test

ABSTRACTLong-term tests on the durability of nuclear waste glasses, both under laboratory conditions and in actual burial environments, have shown that after periods of several months or years the rates of glass corrosion can show unexpected increases. In burial tests in brine environments the growth of the thickness of the surface layer during years 3 to 5 as revealed by SIMS and SEM measurements was found to be faster than expected on the basis of findings during the first 2 years. In this paper we report on our SEM-EDXS observations of a 5 year burial sample of SRL-165 glass from the MIIT test, and compare our findings to those of other studies, especially SIMS measurements. Our average thickness measurement for the leached layer at the glass-metal interface is 3.70 μm, but wide variation is observed, as well as evidence of partial loss of the leached layer in some areas. A thicker, better retained layer is observed on the glass-brine interface near the core heater averaging 8 to 12 μm in thickness with occasional local attack producing leached layers of greater depth, in one case nearly 40 μm. These findings show that it is important to extend durability tests to long periods in order to establish a reliable basis for predictive modeling of long-term glass durability.

The effect of adsorbed lead on the chemical durability of nuclear waste glasses

The chemical durability of two types of nuclear waste glasses was measured by the MCC-1 method, with and without the presence of lead in the leachant. This presence has a significant effect on the chemical durability of the sample with the initially bad durability (T-60) and a minor effect on the other glass (C-411), initially more durable. Surface analysis using X-ray Photoelectron Spectroscopy (XPS) was performed on both samples. The O 1s spectra yield a correlation between the number of non-bridging oxygens (NBOs) on the surface and the chemical durability, the latter decreasing with increasing the numbers of NBOs. These NBOs are created in the rate limiting step of the silica-water reaction by breaking the structural Si-O bonds. A lead ion can react with an NBO and inhibit the reaction that destroys the structural component in the glass. This effect is dominant in the less durable T-60 but is only minor for C-411, where most of the surface is composed of Al 2O 3 which is non soluble, giving the glass its initial good durability.

Effects of Surface Area-to-Solution Volume Ratio on Chemical Durability of Nuclear Waste Glasses

ABSTRACTWe report data on the relationship between nuclear waste glass durability, as measured by leachate concentrations and leach rates, and the ratio of glass surface area to solution volume, S/V. The study includes West Valley non-radioactive production glasses (SF6 and SF10), West Valley glasses containing U and Th designed at CUA (WVCM47, WVCM50, and WVCM59), Savannah River SRLTDS-131 glass, and Hanford waste glass HW-39 (for which existing literature data is used). While some of these glasses show departures of leachate concentrations from simple (S/V)t scaling others conform well. The departures are, in all cases, most evident at high values of S/V. It is therefore important, therefore to understand how glass composition determines which corrosion mechanism is dominant in order to assess the region of validity of extrapolations based on (S/V)t scaling. While leach rates show a general tendency to decrease with S/V and time, exceptions are evident for the less durable glasses which show minima and maxima in the S/V-dependence at fixed time.

Prediction of Glass Durability as a Function of Environmental Conditions

ABSTRACTA thermodynamic model of glass durability has been applied to natural, ancient, and nuclear waste glasses. The durabilities of over 150 different natural and man-made glasses, including actual ancient Roman and Islamic glasses (Jalame ca. 350 A. D., Nishapur 10-11th century A. D., and Gorgon 9-11th century A.D.), have been compared. Glass durability has been shown to be a function of the thermodynamic hydration free energy, δGhvd, which can be calculated from glass composition and solution pH. Using this approach the durability of the most durable nuclear waste glasses examined was ˜106 years by comparison with the durability of the natural basalts of ˜106 years. The least durable waste glass formulations were comparable in durability to the most durable simulated medieval window glasses of ˜103 years. In this manner, the durability of nuclear waste glasses has been interpolated to be ˜106 years and no less than 103 years.Hydration thermodynamics have been shown to be applicable to the dissolution of glass in various natural environments. Groundwater-glass interactions relative to geologic disposal of nuclear waste, hydration rind dating of obsidians, and/or other archeological studies can be modeled, e.g. the relative durabilities of six simulated medieval window glasses have been correctly predicted for both laboratory (one month) and burial (5 year) experiments.The effects of solution pH on glass dissolution has been determined experimentally for the 150 different glasses and can be predicted theoretically by hydration thermodynamics. The effects of solution redox (oxidation potential expressed as Eh) on dissolution of glass matrix elements such as Si and B have been shown to be minimal. The combined effects of solution pH and Eh have been described and unified by construction of thermodynamically calculated Pourbaix (pH-Eh) diagrams for glass dissolution. The Pourbaix diagrams have been quantified to describe glass dissolution as a function of environmental conditions by use of the data derived from hydration thermodynamics.

Pourbaix Diagram for The Prediction of Waste Glass Durability in Geologic Environments

AbstractDissolution of nuclear waste glass occurs by corrosion mechanisms similar to those of metallurgical and mineralogic systems albeit on different time scales. The effects of imposed pH and oxidation potential (Eh) conditions existing in natural environments on metals and minerals have been quantatively and phenomenologically described in compendiums of Pourbaix (pH-potential) diagrams. Construction of Pourbaix diagrams to quantify the response of nuclear waste glasses to repository specific pH and Eh conditions is demonstrated. The expected long-term effects of groundwater contact on the durability of nuclear waste glasses can then be unified.

Influence of simulated waste oxides on the durability of a borosilicate glass

Although the numerous papers on the chemical durability of nuclear waste glasses, more work is needed to understand the effects of waste oxides on the glass durability. This is due to the great variety of possible compositions and to the wide leaching conditions [1]. We have prepared the N. 189 glass (England), B, to which we have added 10% and 20% of SrO, Cs 2O, U 3O 8. Furthermore we studied the effects on B of an addition of 4% ZnO [2]. All the glasses were grounded and passed through 40 and 60 mesh sieves to obtain approximately the same surface for the same weight. Powders were leached by water at 70 °C for times varying from 1 day to 24 days and the solutions were analyzed by means of conventional methods. SiO 2 B 2O 3 Li 2O Na 2O SrO Cn 2O U 3O 4 ZnO Loss% B 55.5 29.2 5.0 10.3 - - - - 72.9 SR1 50.4 26.6 4.5 8.4 8.1 - - - 83.1 SR2 46.2 24.3 4.2 8.6 16.7 - - - 87.7 CS1 50.4 26.5 4.6 9.4 - 9.1 - - 91.7 CS2 46.2 24.3 4.2 8.6 - 16.7 - - 97.1 U1 50.4 26.6 4.5 9.4 - - 9.1 - 73.5 U2 46.2 24.3 4.2 8.6 - - 16.7 - 75.9 ZN4 53.3 28.0 4.8 9.8 - - - 4.0 15.3 In the Table I are reported the sample compositions and the percentage of mass leached out after 24 days. In any case the waste ions make poorer the base glass with a maximum for the cesium containing glass. On the contrary the zinc ions, even if in a very low amount, allow very increased resistance to the corrosion [3]. As example in Figs. 1 and 2 are reported the quantitites of SiO 2 and Na species leached out in the course of the experiment. It must be pointed out that the zinc containing glass continues to dissolve in an appreciable way. This is due to the fact that after 24 days the glass mass is ▪ ▪ not negligible. On the contrary, the other glasses after 24 days are dissolved up to 97%. The different behaviour of the cesium and zinc containing glasses is evidenced also by Figs. 3, and 4, which show the SEM photographs of leached glasses. IR technique can show the effect of leaching on the glass structure. In Fig. 5 we can see that water transforms the silicate amorphous structure of the glass B in a structure very similar to the vitreous silica [4].

Effects of Radiation-Induced Stress on the Chemical Durability of Nuclear Waste Glasses

AbstractThe growth of stresses in irradiated glasses is modeled, and the resulting effects on chemical durability are evaluated. It is shown that in actinide-doped glasses, maximum stresses are small when compared to those that act on a plane parallel to the ion tracks of an implanted glass, at equivalent fluences. The theory also offers an explanation for the apparent change in the dissolution enhancement factor with fluence in ion-implantation experiments.

Static and dynamic tests for the chemical durability of nuclear waste glass

Different types of durability tests are analyzed in terms of their relevance to flow conditions and their ability to predict the long-term corrosion rates under the most adverse flow and dilution conditions. Results obtained by applying various types of tests to glasses developed for use in nuclear waste fixation show that carefully controlled flow tests are most useful in modelling the entire dependence of corrosion on flow rates and surface-to-volume ratios. Constant-medium tests carried out in buffered media and with limited silica accumulation (up to 10 mg/ L) are easier to perform and are useful both in predicting worst-risk corrosion rates and in supplying data for modelling repository situations. Results of independent constant-medium tests, stagnant tests, and flow tests on simulated nuclear waste glasses show excellent agreement in the determination of the maximum corrosion rates.