Surface Alteration Layers Research Articles

Electron diffraction characterization of nanocrystalline materials using a Rietveld-based approach. Part I. Methodology.

Transmission electron microscopy is a powerful experimental tool, very effective for the complete characterization of nanocrystalline materials by employing a combination of imaging, spectroscopy and diffraction techniques. Electron powder diffraction (EPD) pattern fingerprinting in association with chemical information from spectroscopy can be used to deduce the identity of the crystalline phases. Furthermore, EPD has similar potential to X-ray powder diffraction (XRPD) for extracting additional information regarding material specimens, such as microstructural features and defect structures. The aim of this paper is to extend a full-pattern fitting procedure, broadly used for analysing XRPD patterns, to EPD. The interest of this approach is twofold: in the first place, the relatively short times involved with data acquisition allow one to speed up the characterization procedures. This is a particularly interesting aspect in the case of metastable structures or kinetics studies. Moreover, the reduced sampling volumes involved with electron diffraction analyses can better reveal surface alteration layers in the analysed specimen which might be completely overlooked by conventional bulk techniques. The first step forward to have an effective application of the proposed methodology concerns establishing a reliable calibration protocol to take into correct account the instrumental effects and thus separate them from those determined by the structure, microstructure and texture of the analysed samples. In this paper, the methodology for determining the instrumental broadening of the diffraction lines is demonstrated through a full quantitative analysis based on the Rietveld refinement of the EPD. In this regard, a CeO2 nanopowder reference specimen has been used. The results provide indications also on the specific features that a good calibration standard should have.

Impact of aqueous solution pH on network structure of corrosion‐induced surface layers of boroaluminosilicate glass

AbstractUnderstanding of the long‐term stability of vitrified nuclear waste glass is critical to its safe storage, but knowledge gaps remain regarding the corrosion mechanism, especially with respect to surface alteration layers whose presence changes the observed corrosion rates. Here, we report the aqueous corrosion of a model boroaluminosilicate glass at ground water relevant pHs as investigated by vibrational spectroscopy to understand the network structure of the alteration layer that we hypothesize controls the corrosion rates. Both specular reflection infrared spectroscopy and infrared variable angle spectroscopic ellipsometry were used to elucidate details of the structure of the alteration layers formed at pH 7 and pH 9. After 2312 days, the thickness of the alteration layer is found to be ∼2000 nm for corrosion at pH 7 and ∼300 nm for corrosion at pH 9. The interpretation of the vibrational spectra suggests that alteration layers formed at pH 7 and pH 9 both exhibit a porous network with "silica‐like" structures, but with differences in the bond lengths and bond angles in the glass network. Moreover, the relative abundance of the hydrous species within the porous network appears to be dependent on pH. Additionally, the spectroscopic data suggest that the B and Ca trapped in the alteration layer during the secondary corrosion of the pH 9 pre‐corroded sample in pH 3 solution are hydrated within the nano‐porous layer and not bound to the silicate network. However, the precise chemical form remains to be determined. Based on these findings, we propose that the combination of the network structure of the alteration layer and the relative abundance of hydrous species within the alteration layer act to control the transport of ionic species dissolved from the reaction front to the aqueous solution outside the glass.

Feedbacks and non-linearity of silicate glass alteration in hyperalkaline solution studied by in operando fluid-cell Raman spectroscopy

Mechanisms, feedbacks and resulting non-linearity during silicate glass alteration in a hyperalkaline carbonate solution were studied through hyperspectral Raman imaging of heated fluid-cells. Our experimental setup enabled in operando visualization and rate measurements of glass dissolution and secondary phase precipitation, complemented by spectral characterization of the phases involved and semi-quantitative monitoring of the ionic strength of the solution close to the glass interface. After initial congruent dissolution of the Ba-bearing soda-lime boroaluminosilicate glass, the formation of a crystalline, saponite-based surface alteration layer (SAL), as well as subsequent zeolite precipitation, witherite coating, and carbonate precipitation within pore spaces of the saponite layer were observed. Two in operando experiments were conducted at ∼ 90 °C for 180 and 260 h that otherwise solely differed in the solution volume (SV) while keeping the surface area constant. The high SV experiment exhibited a transient upward excursion of initial dissolution rates, followed by continuously rapid glass dissolution along with slow SAL growth and sustained oscillations in ionic strength. Contrastingly, in the low SV experiment, glass dissolution monotonically decreased after the onset of rapid SAL growth and no sustained oscillations were observed. We find that growth conditions and resulting properties of the SAL exert dominant, non-linear effects on the evolution of glass dissolution rates. In turn, SAL formation depends on nucleation/growth kinetics and the accumulation of glass-derived solutes at the reaction front. Both, dissolution and precipitation, feedback with solution chemistry and transport processes, together controlling the evolution of the corrosion process. Additionally, fracturing, delamination, and the evolution of surface morphology may affect glass dissolution rates and transport pathways. Such interpretations of decelerating reaction rates in response to the growth of a protective layer are consistent in micro-scale experiments and in outcrop- to global-scale observations, as is the accelerating effect of surface area creation by physical disruption and morphology. Thus, these µm-scale mechanistic insights could help elucidating local to global environmental feedbacks (e.g., erosion or weathering patterns) as well as process dynamics in engineered environments (e.g., nuclear waste disposal) and may assist the improvement predictive models.

Dissolution of simplified nuclear waste glass and formation of secondary phases

Abstract. Immobilization of high-level and intermediate-level nuclear wastes by vitrification in borosilicate glass is a well-established process. There is a consensus between the waste management agencies of many countries and many experts that vitrified nuclear waste should be disposed of in a deep geological waste repository and therefore its long-term behavior needs to be taken into account in safety assessments. In contact with water, borosilicate glass is metastable and dissolves. In static dissolution experiments, often a surface alteration layer (SAL) forms on the dissolving glass, and later sometimes secondary phases form. Based on boron or lithium release rates, commonly three stages of glass dissolution are defined as a function of the reaction progress: (I) initial dissolution, described by a congruent glass dissolution at the highest rate, (II) residual dissolution, characterized by a glass dissolution rate several orders of magnitude lower than the initial one, and (III) resumption of glass alteration with initial rates. Microscopically, the formation of a complex SAL has been identified as a prerequisite for the slower dissolution kinetics of stage II. Stage III is typically observed under specific conditions, i.e., high temperature and/or high pH driven by the uptake of Si and Al into secondary phases. Different glass dissolution models explaining the mechanisms of the SAL formation and rate-limiting steps have been proposed and are still under debate. In this article different aspects of glass dissolution from recent studies in the literature and our own work are discussed with a focus on the microscopic aspects of SAL formation, secondary phase formation and the resumption of glass dissolution. Most of the experiments in the literature were performed under near-neutral pH conditions and at 90 ∘C, following standard procedures, to understand the fundamental mechanisms of glass dissolution. The example of interaction of glass and cementitious materials as discussed here is relevant for safety assessments because most international concepts include cement e.g., as lining, for plugs, or as part of the general construction of the repository. The aim of the investigations presented in this paper was to study the combined effect of hyperalkaline conditions and very high surface area/volume ratios (SA/V=264000m-1) on the dissolution of international simplified glass (ISG) and the formation of secondary phases at 70 ∘C in a synthetic young cement water containing Ca (YCWCa). The new results show that the SA/V ratio is a key parameter for the dissolution rate and for the formation of the altered glass surface and secondary phases. A comparison with similar studies in the literature shows that especially on the microscopic and nanoscale, different SA/V ratios lead to different features on the dissolving glass surface, even though the SA-normalized element release rates appear similar. Zeolite and Ca-silicate-hydrate phases (CSH) were identified and play a key role for the evolution of the solution chemistry. A kinetic dissolution model coupled with precipitation of secondary phases can be applied to relate the amount of dissolved glass to the evolution of the solution's pH.

Tailoring effect of Y2O3 on water resistance of Na2O–ZnO–Al2O3–B2O3 glasses

To explore the improving effect of Y2O3 on the water resistance of xY2O3-(100–x)(0.05Al2O3-0.15ZnO-0.15Na2O-0.65B2O3) (x = 0, 0.7 mol%, 1.4 mol%, 2.1 mol%, 2.8 mol%) glasses, glass structure and ion migration characteristics were respectively characterized by an infrared spectrometer and an electrochemical workstation. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasm-atomic emission spectrometry (ICP-AES) were used to comprehensively analyze the alteration products. The experimental results show that with the increase of Y2O3 content, the mass loss per unit area of the glass is significantly reduced, and pH in leaching solution has the same change trend with mass loss of the glass, which indicates the improving effect of Y2O3 addition on the studied glass. The content of [BO4] structural unit and dc conductivity of glass increase firstly and then decrease with the increase of Y2O3 content, and reach a maximum at x = 0.7 mol%. Although addition of Y2O3 plays the role in the changes of glass structure and ion migration characteristics, the more important influence of Y2O3 is on the glass surface alteration layer. Y and Zn elements are enriched on the glass surface and finally transformed into an alteration layer, moreover, the adhesion between the alteration layers and the glass matrix is enhanced with the increase of Y2O3 content, which effectively blocks the further interdiffusion between the water molecules and the glass matrix.

Corrosion of ternary borosilicate glass in acidic solution studied in operando by fluid-cell Raman spectroscopy

Fluid-cell Raman spectroscopy is a space and time-resolving application allowing in operando studies of dynamic processes during solution–solid interactions. A currently heavily debated example is the corrosion mechanism of borosilicate glasses, which are the favoured material for the immobilization of high-level nuclear waste. With an upgraded fluid-cell lid design made entirely from the glass sample itself, we present the polymerization of the surface alteration layer over time in an initially acidic environment, including the differentiation between pore and surface-adsorbed water within it. Our results support an interface-coupled dissolution-precipitation model, which opposes traditional ion-exchange models for the corrosion mechanism. A sound description of the corrosion mechanism is essential for reliable numerical models to predict the corrosion rate of nuclear waste glasses during long-term storage in a geological repository.

The hydrothermal alkaline alteration of potassium feldspar: A nanometer-scale investigation of the orthoclase interface

Potassium feldspars (KAlSi3O8) are ubiquitous minerals in the Earth's upper crust. This family of minerals has been the subject of numerous experimental and theoretical investigations concerning their dissolution kinetics and the mechanisms controlling chemical alteration at acid and neutral pH, and at temperatures ranging from ambient to hydrothermal conditions. On the other hand, considerably less research on the dissolution behavior of K-feldspars has been carried out at alkaline conditions, in particular at pH > 9 and elevated temperatures. Filling in this gap in knowledge is the major motivation for this study. More specifically, we wanted to document and understand how the K-feldspar interface structurally and chemically evolves during alteration in order to determine the mechanism of dissolution. In this study we examined interfaces of orthoclase samples that were altered in separate experiments in a Ca(OH)2-H2O solution (pH25°C 12.4) at 190 °C for 24 h. We used a combination of focused ion beam (FIB) milling and advanced analytical transmission electron microscopy (TEM) techniques to investigate the structure and chemistry of the near surface region of post-reaction grains, with particular attention being given to the fluid-solid interface. Even though each grain diminishes in volume due to dissolution, high-resolution TEM imaging indicates that the feldspar structure itself remains completely intact and crystalline, as evidenced by lattice fringes that abruptly terminate at the grain edge. Nanometer-scale chemical composition measurements and mapping by TEM-EDXS (energy dispersive X-ray spectroscopy) and EFTEM (energy filtered TEM) show that the chemistry of the parent feldspar also remains unchanged at the interface. In particular, there is no evidence for the incursion of Ca from the fluid solvent into the structure, either by interdiffusion or by a replacement process. Taken together, the TEM observations point to a sharp chemical reaction front characterized by the congruent (i.e. stoichiometric) release of all elements from the feldspar structure.Nanometer-scale measurements by high resolution analytical TEM also reveal that a surface alteration layer (SAL) of amorphous material forms in situ at the expense of the feldspar structure. The interface demarcates a spatially coincident and nm-sharp chemical and structural discontinuity between the parent feldspar and the amorphous phase. The amorphous SAL has a variable thickness, from under 10 nm up to ~200 nm. This is likely one of the first observed occurrences of a significant surface amorphous layer on feldspar due to alteration in an alkaline solvent. The lack of a gap between the two phases points to an interfacial dissolution-reprecipitation process that continuously operates during hydrothermal alteration, and mostly likely right from the onset of contact with the fluid. After the initial formation of the amorphous layer, a 1–2 μm-thick porous amalgam of secondary crystalline phases comprised of calcite, tobermorite, and hydrogrossular, as well as other minor phases, precipitated over the SAL. These authigenic crystalline minerals formed during the experiment (hydrothermal alteration, followed by fluid loss due to evaporation) by a classical thermodynamically-controlled precipitation process as the reactor bulk fluid became increasingly concentrated.We propose that a coupled interfacial dissolution-reprecipitation (CIDR) mechanism best explains the chemical and structural properties of the interface and the formation of an amorphous surface layer. In fact, many recent studies postulate that a CIDR process controls feldspar dissolution and the formation of SALs at acid and circumneutral pH over a wide range of temperatures. Combining these previous results with our new observations supports the idea that a unique and unifying mechanism likely controls chemical alteration of feldspars in all aqueous fluids.

Corrosion interactions between stainless steel and lead vanado-iodoapatite nuclear waste form part II

This paper studied the release of iodine from lead vanado-iodoapatite (I-APT, Pb9.85(VO4)6I1.7), a potential nuclear waste form for the radioactive waste element of I-129, which can be enhanced when crevice corrosion of stainless steel (SS) occurring nearby. Reference corrosion studies of I-APT were performed in different bulk solutions including DI water, 0.6 M and 6 M NaCl, and 0.1 M HNO3 without metal crevice corrosion interactions. The localized enrichment of Cl−, one of the major consequences of SS crevice corrosion, was found to be the decisive factor that led to the enhanced release of iodine. A surface alteration layer consisting of a mixture of nanocrystalline I-APT and Cl-rich apatite (Cl-APT) formed on I-APT surface. Meanwhile, large Cl-APT crystals formed at the crevice mouth on the I-APT surface. This study reveals a new near-field corrosion mechanism for ceramic waste forms when they are exposed to aggressive local corrosive conditions created by the electrochemical reactions of nearby metals. The insight gained in this study could be beneficial for a more accurate prediction of waste form degradation.

Real-time in situ observations of reaction and transport phenomena during silicate glass corrosion by fluid-cell Raman spectroscopy.

Borosilicate glass is an important material used in various industries due to its chemical durability, such as for the immobilization of high-level nuclear waste. However, it is susceptible to aqueous corrosion, recognizable by the formation of surface alteration layers (SALs). Here, we report in situ fluid-cell Raman spectroscopic experiments providing real-time insights into reaction and transport processes during the aqueous corrosion of a borosilicate glass. The formation of a several-micrometre-thick water-rich zone between the SAL and the glass, interpreted as an interface solution, is detected, as well as pH gradients at the glass surface and within the SAL. By replacing the solution with a deuterated solution, it is observed that water transport through the SAL is not rate-limiting. The data support an interface-coupled dissolution-reprecipitation process for SAL formation. Fluid-cell Raman spectroscopic experiments open up new avenues for studying solid-water reactions, with the ability to in situ trace specific sub-processes in real time by using stable isotopes.

Natural alteration of 6Li alumino-silicate glass

Understanding the chemical durability of neutron shielding materials is necessary when assessing their long-term service potential. In this study, the chemical durability of a 6Li enriched neutron shielding glass that has been exposed to natural, near-operational conditions is assessed by Prompt Gamma Activation Analysis (PGAA) and Neutron Depth Profiling (NDP). These non-destructive, nuclear analysis techniques are sensitive to 6Li, and PGAA is uniquely able to detect H in low quantities in solids. It was determined that the enriched alumino-silicate glass can alter within 2 months of exposure to the natural environment. This exposure resulted in an average surface alteration layer thickness of ≈22 μm. The alteration layer contained ≈47% less 6Li than the bulk glass. Alternatively, a 3 years exposed sample of the glass had a surface alteration depth of ≈30 μm and 6Li depletion levels in the alteration layer were between 47% and 75% less 6Li than the bulk glass. When the alteration layer on the 3 years sample was removed, the H content of the glass's surface was nearly eliminated. This sample also showed variable Li concentrations throughout the alteration volume, which contrasts with near static Li concentration in the alteration volume of the 2 months sample. From these findings it was determined that the depletion in Li at the surface of the glass will not affect the glass's neutron shielding properties, but it may change the mechanical stability of the glass's surface and, due to increased H content in the alteration layer, make it an inappropriate material for the lining of certain neutron analysis instruments.

Towards a unifying mechanistic model for silicate glass corrosion

Borosilicate glasses are currently used for the immobilization of highly radioactive waste and are materials of choice for many biomedical and research industries. They are metastable materials that corrode in aqueous solutions, reflected by the formation of silica-rich surface alteration layers (SAL). Until now, there is no consensus in the scientific community about the reaction and transport mechanism(s) and the rate-limiting steps involved in the formation of SALs. Here we report the results of multi-isotope tracer (2H,18O,10B, 30Si, 44Ca) corrosion experiments that were performed with precorroded and pristine glass monoliths prepared from the six-component international simple glass and a quaternary aluminum borosilicate glass. Results of transmission electron microscopy and nanoscale analyses by secondary ion mass spectrometry reveal a nanometer-sharp interface between the SAL and the glass, where decoupling of isotope tracer occurs, while proton diffusion and ion exchange can be observed within the glass. We propose a unifying mechanistic model that accounts for all critical observations so far made on naturally and experimentally corroded glasses. It is based on an interface-coupled glass dissolution-silica precipitation reaction as the main SAL forming process. However, a diffusion-controlled ion exchange front may evolve in the glass ahead of the dissolution front if SAL formation at the reaction interface significantly slows down due to transport limitations.

Chemical durability of vitrified wasteforms: effects of pH and solution composition

AbstractVitrification is used for the immobilization and conditioning of high-level waste (HLW) arising from the reprocessing of spent nuclear fuel in the UK. Vitrification is also under consideration for the immobilization of certain intermediate-level wastes (ILW), where there may be advantages of volume reduction and removal of uncertainties in long-term waste behaviour, compared to encapsulation in a cement grout. This paper gives an overview of recent work into the chemical durability of UK vitrified wasteforms to inform the technical specification for the disposal facilities for these waste products and the treatment of their long-term behaviour in post-closure performance assessment. This has included: (1) measurements of the initial glass dissolution rates of a simulated HLW Magnox waste glass in a range of groundwater types representative of potential UK host geologies and in simulated high pH near-field porewaters relevant to potential disposal concepts, using Product Consistency Test type-B (PCT-B) at 40°C; and (2) durability testing of three simulant ILW glasses in a saturated calcium hydroxide buffered solution to simulate conditions in cement-based disposal vaults, using PCT-B tests at 50°C.The experimentally defined initial rate of HLW Magnox waste glass dissolution in a range of simulated groundwater compositions appears to be similar regardless of the ionic strength and major element composition of the solution. The release of caesium from HLW Magnox waste glass appears to be sensitive to solution composition. Caesium is selectively retained in the glass compared to other soluble components in the two low ionic strength solutions, but is released at similar rates to other soluble components in the three groundwaters and Ca(OH)2 solution. Whether this change in caesium retention is an ionic strength effect or is related to changes in the nature of the surface alteration layer formed on the glass, has yet to be established. For HLW Magnox waste glass, dissolution is accelerated at high pH in NaOH solution, however, the presence of calcium acts to mitigate the effects of high pH, at least initially. In Ca(OH)2 solution, calcium is found to react with all the glasses studied leading to the formation of calcium-containing alteration products. The initial dissolution behaviour in Ca(OH)2 solution varies with glass composition and in particular appears to be sensitive to the boron content.

Iron-hydroxide, iron-sulfate and hydrous-silica coatings in acid-mine tailings facilities: A comparative study of their trace-element composition

Abstract Surface alteration-layers often coat minerals in acid-mine drainage systems and the characterization of their chemical composition is required to understand the uptake or release of potentially toxic elements. Samples with micrometer-thick rock coatings were collected from bedrock in contact with three acidic tailings ponds and a small lake, all located within the Copper Cliff mine tailings disposal area in Sudbury, Ontario, Canada. Distribution and concentration of trace-metals in the rock coatings were characterized with Laser-Ablation Inductively-Coupled Plasma Mass Spectroscopy and Micro X-ray Fluorescence Spectroscopy. The rock coatings are composed of goethite, ferrihydrite, schwertmannite, jarosite and amorphous silica. The latter phase is a product of the non-stoichiometric weathering of the underlying siliceous rock. Layers within the coatings are distinguished on the basis of their atomic Fe:Si ratios: FeO x coatings have Fe:Si > 4:1, Si–FeO x coatings have Fe:Si = 4:1 to 1:1 and SiO x coatings have Si > Fe. Iron-rich coatings (FeO x ) in contact with acidic tailings ponds (pH x in contact with lake water at near neutral pH have similar trace-metal concentrations than Si–FeO x and SiO x , most likely the result of higher adsorption rates of metals at near neutral pH conditions. High trace-metal concentrations in Si–FeO x and SiO x are explained by the presence of jarosite-group minerals, which formed within Si-rich alteration layers through mixing of leached alkaline cations and trace elements from the underlying rock and Fe 3+ -sulfate solutions from the pond. Calculated enrichment factors for trace metals and metalloids in the coatings (relative to the pond) indicate that the mobility for Pb, As, Cr and Cu in the upper part of tailings ponds is commonly lower than the mobility for Zn, Mn, Co and Ni. The environmental significance of these findings is discussed in terms of the attenuation of trace metals in the coatings and the widespread occurrences of silica gels and jarosite-group minerals.

Texture analysis of bronze age axes by neutron diffraction

Neutron diffraction has been utilised as a non-invasive diagnostic tool to gain insight into the ancient metallurgic and manufacturing techniques employed for the production of three bronze age axes from archaic to late bronze age (20th to 13th Century BC). The analysed bronze artefacts are from the “Terramare” and other bronze age settlements near Modena, Italy. Neutron diffraction provides the alloy and phase compositions in a totally non destructive approach, without interference from surface alteration and corrosion layers. Furthermore, neutron based texture analysis, with the advantage of large grain statistics deep into the bulk, provides details of the production techniques that can complement traditional metallographic examinations, and may provide unique information for samples that cannot suffer invasive treatments.

Surface Layer Effects on Waste Glass Corrosion

ABSTRACTWater contact subjects waste glass to chemical attack that results in the formation of surface alteration layers. Two principal hypotheses have been advanced concerning the effect of surface alteration layers on continued glass corrosion: (1) they act as a mass transport barrier and (2) they influence the chemical affinity of the glass reaction. In general, transport barrier effects have been found to be less important than affinity effects in the corrosion of most high-level nuclear waste glasses. However, they can be important under some circumstances, for example, in a very alkaline solution, in leachants containing Mg ions, or under conditions where the matrix dissolution rate is very low. The latter suggests that physical barrier effect may affect the long-term glass dissolution rate. Surface layers influence glass reaction affinity through the effects of the altered glass and secondary phases on the solution chemistry. The reaction affinity may be controlled by various precipitates and crystalline phases, amorphous silica phases, gel layer, or all the components of the glass. The surface alteration layers influence radionuclide release mainly through colloid formation, crystalline phase incorporation, and gel layer retention. This paper reviews current understanding and uncertainties.

The development of surface alteration layers on SRL-165 nuclear waste glasses

Abstract A series of experiments were conducted to investigate the rate of formation of reaction layers on samples of simulated SRL-165 nuclear waste glass with as-cast surfaces. Reaction times from 1 to 91 days at 90°C were used with deionized water or sodium silicate solution as the leachant. Results are presented for the leaching solutions at the end of the reaction periods as well as for the chemical composition of the reaction layer developed on the glass. Layers developed in silicate solution were thin, simple in structure, and very reproducible from place to place on the same sample and between samples reacted for the same time period. In contrast, layers developed in deionized water were complicated in structure and showed extreme variability even within a single sample. Mass-balance calculations show that dissolution of the glass continues even though the alteration layer adheres firmly to the glass.

Does Fully Radioactive Glass Behave Differently than Simulated Waste Glass?

ABSTRACTStatic tests at SA/V (ratio of surface area of glass to solution volume) 20,000 m−1 on SRL 200 glass compositions show that, at long test periods, the simulated nuclear waste glass (nonradioactive) leaches faster than the corresponding radioactive glass by a factor of about 40, although comparative tests, done through 560 days, at lower SA/V, 2000 m−1, indicate little difference in the leach behavior of the two types of glasses. The similarity in leach behavior between radioactive and simulated glasses at SAN of 2000 m−1 or lower is also observed for SRL 165/42 and 131/11 compositions. The accelerated glass reaction with the simulated glass 200S is associated with the formation of crystalline phases such as clinoptilolite (or potassium feldspar), and a pH excursion. The radiation field generated by the fully radioactive glass reduces the solution pH. This lower pH, in turn, may retard the onset of increased reaction rate. The radiation field generated by the radioactive glasses does not directly affect the stability of the glass surface alteration layer under those conditions where the radioactive and simulated glasses react at the same rate. These results suggest that the fully radioactive nuclear waste glass 200R may maintain a much lower leach rate than the simulated 200S, if the lower pH in the 200R leachate can be sustained. Meaningful comparison tests between radioactive and simulated nuclear waste glasses should include long-term and high SA/V tests.

A Kinetic Model for Borosilicate Glass Dissolution Based on the Dissolution Affinity of a Surface Alteration Layer

ABSTRACTA kinetic model for the dissolution of borosilicate glass, incorporated into the EQ3/6 geochemical modeling code, is used to predict the dissolution rate of a nuclear waste glass. In the model, the glass dissolution rate is controlled by the rate of dissolution of an alkalidepleted amorphous surface (gel) layer. Assuming that the gel layer dissolution affinity controls glass dissolution rates is similar to the silica saturation concept of Grambow [1] except that our model predicts that all components concentrated in the surface layer, not just silica, affect glass dissolution rates. The good agreement between predicted and observed elemental dissolution rates suggests that the dissolution rate of the gel layer limits the overall rate of glass dissolution. The model predicts that the long-term rate of glass dissolution will depend mainly on ion concentrations in solution, and therefore on the secondary phases which precipitate and control ion concentrations.

The Formation of Surface Layers and Reaction Products in the Leaching of Defense Borosilicate Nuclear Waste Glass

The development of surface alteration layers and solid products in the reaction of borosilicate nuclear waste glass with aqueous solutions has been studied as a function of time, surface morphology, water flow rates, and solution composition. A physical description of the growth of the surface layers has been developed, and several solid reaction products have been identified. The results of the study support a saturation based description of long-term radionuclide release from glass waste forms and demonstrate the complex dependence of short-term leaching experiments upon both surface and solution variables.

Waste Glass Leaching: Chemistry and Kinetics

AbstractNuclear waste glass leaching has been studied extensively during the past ten years. Although much has been learned concerning the kinetics and mechanisms of glass dissolution, it does not appear that accurate predictions can yet be made concerning the release kinetics for specific elements from a given glass as a function of environmental conditions. In order to reliably predict elemental release rates, one needs to know: 1) how a given element is incorporated into the glass structure, 2) how specific sites in the glass react with water, 3) how the composition and reactivity of the leachate influence glass reactivity, 4) how the structure and reactivity of the glass changes in surface alteration layers, and 5) how glass dissolution modifies the chemistry of the leachate. At our current level of understanding, we are only able to make qualitative predictions concerning each of the above factors which allow us to make “order of magnitude” or “upper limit” predictions for radionuclide release rates.