Nuclear Waste Glass Research Articles

The Effect of Heavy Ion Irradiation on the Forward Dissolution Rate of Borosilicate Glasses Studied in Situ and Real Time by Fluid-Cell Raman Spectroscopy.

Borosilicate glasses are the favored material for immobilization of high-level nuclear waste (HLW) from the reprocessing of spent fuel used in nuclear power plants. To assess the long-term stability of nuclear waste glasses, it is crucial to understand how self-irradiation affects the structural state of the glass and influences its dissolution behavior. In this study, we focus on the effect of heavy ion irradiation on the forward dissolution rate of a non-radioactive ternary borosilicate glass. To create extended radiation defects, the glass was subjected to heavy ion irradiation using 197Au ions that penetrated ~50 µm deep into the glass. The structural damage was characterized by Raman spectroscopy, revealing a significant depolymerization of the silicate and borate network in the irradiated glass and a reduction of the average boron coordination number. Real time, in situ fluid-cell Raman spectroscopic corrosion experiments were performed with the irradiated glass in a silica-undersaturated, 0.5 M NaHCO3 solution at temperatures between 80 and 85 °C (initial pH = 7.1). The time- and space-resolved in situ Raman data revealed a 3.7 ± 0.5 times increased forward dissolution rate for the irradiated glass compared to the non-irradiated glass, demonstrating a significant impact of irradiation-induced structural damage on the dissolution kinetics.

Preparation and Properties of Lead-Iron-Phosphate Nuclear Waste Glasses

Lead-iron phosphate (LIP) glasses are a promising new waste form for safe immobilization of both high level defence and high level commercial radioactive waste for long term disposal. LIP glasses have several advantages such as lower aqueous corrosion rate, lower processing temperature, etc. The durability test for LIP glasses showed that the alkaline earth oxides BaO, SrO and CaO decrease the durability while PbO, Fe2O3 and CeO2 improves the corrosion resistance. For some LIP glass samples containing uranium the leach rates as calculated from BET surface areas were in the range of 8.2×10–4 to 4.3×10–3 g.m–2.h–1 at 90°C which is 1000 times lower than that of the borosilicate glasses. Thermal study shows that the glass transition temperature was in the range of 360°-400°C for the LIP glasses. IR studies show absorptions at 520, 1025, 1700, 2225 and 3450 cm–1. The observed IR and Raman scattering spectra are representative of a mixture of chain terminating Q1 species and chain forming Q2 species. From the radiotracer leaching studies the role of Ce4+ as good binder-modifier is established. The values of isomer shift in the Mössbauer spectra increases with increasing Fe2O3 content.

Investigation of the role of Mg and Ca in the structure and durability of aluminoborosilicate glass

The structure and dissolution behaviour of Na2O·CaO·(15–x)Al2O3·xB2O3·SiO2 and Na2O·MgO·(15–x)Al2O3·xB2O3·SiO2 glasses, relevant to compositions of UK nuclear waste glass, have been investigated using nuclear magnetic resonance (NMR) spectroscopy and static dissolution experiments using the PCT protocol. Structural data from 11B, 27Al and 29Si NMR analyses show that increasing the [B2O3]/([Al2O3] + [B2O3]) ratio of the alkali-alkaline-earth aluminoborosilicate glasses led to an overall decrease in the proportion of non-silicate tetrahedral species (IVAl + IVB) and a decrease in SiOX bonds (XB, Al). The Mg-containing glasses exhibited lower IVB fractions than their Ca-containing counterparts, which is thought to be due to the presence of IVMg tetrahedra in the network. The measured corrosion rates were similar for both Ca and Mg-containing glasses although unexpectedly some Ca-containing glasses exhibited higher corrosion losses than the Mg-containing ones for time periods up to 112 d. However, there was evidence of a greater tendency to rate resumption in the Mg containing than the Ca containing ones. Alteration products were found to contain Ca, Si and Al with the Ca containing glasses and Ca, Mg, Si and Al with the Mg containing glasses; Na was not detected in the alteration products although its presence cannot be ruled out based on the data obtained.

Crystallization study of rare earth and molybdenum containing nuclear waste glass ceramics

AbstractA glass‐ceramic waste form is being developed for immobilization of waste streams of alkali (A), alkaline‐earth (AE), rare earth (RE), and transition metals generated by transuranic extraction for reprocessing of used nuclear fuel. Benefits over an alkali borosilicate waste form are realized by the partitioning of the fission product fraction insoluble in glass into a suite of chemically durable crystalline phases through controlled cooling, including (AE,A,RE)MoO4 (powellite) and (RE,A,AE)10Si6O26 (oxyapatite). In this study, a simplified 8‐oxide system (SiO2‐Nd2O3‐CaO‐Na2O‐B2O3‐Al2O3‐MoO3‐ZrO2) was melted, then soaked at various temperatures from 1450 to 1150°C, and subsequently quenched, in order to obtain snapshots into the phase distribution at these temperatures. For these samples, small angle X‐ray and neutron scattering, quantitative X‐ray diffraction, electron microscopy, 23Na nuclear magnetic resonance, Nd3+ visible absorption, and temperature‐dependent viscosity were characterized. In this composition, soak temperatures of 1250°C were necessary to nucleate calcium molybdate (~10‐20 nm in diameter). Further cooling produced oxyapatite and total crystallization increased with lower soak temperatures. Both Na and Nd entered the crystalline phases with lower‐temperature soak conditions. Slow cooling or long isothermal treatments at ~975°C produced significantly higher crystal fractions.

Interfacial structures of spinel crystals with borosilicate nuclear waste glasses from molecular dynamics simulations

AbstractSpinel crystal formation presents a critical issue and glass formulation in nuclear waste glass processing. In this paper, the interfacial structures of the model borosilicate nuclear waste glasses, the international simple glass (ISG), with two types of spinel crystals, namely the MgAl2O4 and NiFe2O4, were studied using classical molecular dynamics simulations with effective partial charge potentials and recently developed composition‐dependent boron‐related parameters. The simulation results revealed the structural features of the borosilicate nuclear waste glasses and their interfaces with the two types of spinel crystals. It was found that there exist notable structural changes of glasses close to the interfacial region, affected by the adjacent crystal structures, terms of preferential segregation and ordering of cations, as well as ctaion coordination numbers. Specifically, the fraction of fourfold coordinated boron (B3) in glass near the interface decreases as compared to the bulk glass. In addition, the amount of fourfold coordinated Al decreases while fivefold Al increases in the glass region close to the glass‐crystal interface, which suggests indication of initial stage of crystal growth as Al adopts higher (sixfold) coordination like in the crystal as compared to majority of fourfold coordination in the glass. These interfacial structure changes obtained from MD simulations provide evidence of the influence of the precipitated crystals on the surrounding melt and glass and the initial stage of crystal growth.

Heat transfer from glass melt to cold cap: Effect of heating rate

AbstractConversion of melter feed to glass occurs in the cold cap that floats on the melt pool in a nuclear waste glass melter. The conversion rate (the melting rate or the glass production rate) is controlled by the heat flux delivered to the cold cap from the molten glass. In an attempt to analyze the intricate relationship between the rate of heating, the feed foaming response, and the rate of melting, we measured the change in feed volume at different heating rates by using several melter feeds known to exhibit a wide range of melting rates under identical melter operating conditions. As expected, the maximum foam porosity increased as the heating rate increased. However, contrary to expectation, the temperature at which the foam reached maximum volume either decreased or increased with the heating rate, depending on the feed composition. A change in maximum foam temperature from the feed volume expansion test indicates a similar change of the cold‐cap bottom temperature, which influences the heat flow to the cold cap, and thus the rate of melting.

Study the effect of gamma radiation on the some properties of glass and glass-ceramic immobilize nuclear waste

Vitrifications process one of the important methods to immobilize nuclear waste. In this research nuclear waste (Strontium Oxides) with molecular weight (5%) was immobilized by vitrification methods in two types of borosilicate glass (c-type) which are glass and glass-ceramics. To investigate the physical, chemical and mechanical properties of glass and glass-ceramic after immobilize nuclear waste these samples irradiated by gamma ray radiation. Co-60 was used as gamma a irradiation with dose rate 0.38 kGy/hr for different period of time. It’s found that gamma radiation affected the glass and glass-ceramic properties. From phase analysis by the x-ray diffraction for glass-ceramic samples proved that at doses 343kGy change the crystalline glass to amorphous glass. A conclusion from these ionization studies is that the limited magnitude of Strontium ion leaching associated with ionization damage does not appear to pose any direct problems for the safe storage of nuclear waste glass.

Effect of sulfate sequestration by Ba[sbnd]Sn composite material on Re retention in low-activity waste glass

Incorporating radioactive Tc-99 into glass during vitrification of low-activity waste (LAW) at the U.S. Department of Energy's Hanford Site is a scientific challenge. Previous studies have suggested that the presence of sulfate salts in the melt hinder the incorporation of Re, a non-radioactive surrogate for Tc-99, into nuclear waste glass. In this study, we assessed the capability of a BaSn composite material to sequester sulfate during vitrification of AN-102 LAW feed and the resulting effects on Re retention in the glass. We also investigated a feed in which the initial soluble sulfate source in the simulated LAW (Na2SO4) was replaced with insoluble BaSO4. The BaSn composite-treated feed and BaSO4-substitute feed were compared to a control, AN-102 baseline. The feeds were prepared and then heated to temperatures ranging from 400 to 1000 °C at a 5 °C/min rate and then air-quenched to room temperature. The heated samples and dried feeds were leached in deionized water. The solutions from the leach test were analyzed by inductively coupled plasma optical emission spectroscopy to determine the amount of soluble sulfate as a function of temperature, and the resulting solids were analyzed for Re retention. Although the BaSn composite material successfully sequestered sulfate in the as-prepared LAW feed, heating to 400 °C resulted in most of the sulfate partitioning to a soluble salt phase. By 700 °C, sulfate partitioning in the composite-treated feed was identical to that of the baseline feed. Throughout the entire temperature range, the BaSO4-substitute feed behaved similarly to the composite-treated feed. The results of the study show that sequestering sulfate in a BaSO4 phase is not effective for Re retention as the sulfate salt behavior was nearly identical in the three feeds at ≥700 °C. The fraction of Re retained in the glass at 1000 °C varied from 39.4 to 46.6% for the three feeds, which is shown to be a statistically insignificant difference.

Glass structure and crystallization in boro-alumino-silicate glasses containing rare earth and transition metal cations: a US-UK collaborative program

ABSTRACTNuclear wastes generated from reprocessing of used nuclear fuel tend to contain a large fraction of rare earth (RE, e.g., Nd3+), transition (TM, e.g., Mo6+, Zr4+), alkali (A, e.g., Cs+), and alkaline earth cations (AE, e.g., Ba2+, Sr2+). Various strategies have been considered for immobilizing such waste streams, varying from nominally crystal-free glass to glass-ceramic to multi-phase ceramic waste forms. For glass and glass-ceramic waste forms, the added glass-forming system is generally alkali-alkaline earth-aluminoborosilicate (i.e., Na-Ca-Al-B-Si oxide). In a US-UK collaborative project, summarized here, we investigated the glass structure and crystallization dependence on compositional changes in simulated nuclear waste glasses and glass-ceramics. Compositions ranged in complexity from five – to – eight oxides. Specifically, the roles of Mo and rare earths are investigated, since a proposed glass-ceramic waste form contains crystalline phases such as powellite [(AE,A,RE)MoO4] and oxyapatite [(RE,AE,A)10Si6O26], and the precipitation of molybdenum phases is known to be affected by the rare earth concentration in the glass. Additionally, the effects of other chemical additions have been systematically investigated, including Zr, Ru, P, and Ti. A series of studies were also undertaken to ascertain the effect of the RE size on glass structure and on partitioning to crystal phases, investigating similarities and differences in glasses containing single RE oxides of Sc, Y, La, Ce, Nd, Sm, Er, Yb, or Lu. Finally, the effect of charge compensation was investigated by considering not only the commonly assessed peralkaline glass but also metaluminous and peraluminous compositions. Glass structure and crystallization studies were conducted by spectroscopic methods (i.e., Raman, X-ray absorption, nuclear magnetic resonance (NMR), optical absorption, photoluminescence, photoluminescence excitation, X-ray photoelectron spectroscopy), microscopy (i.e., scanning electron microscopy, transmission electron microscopy, electron probe microanalysis), scattering (i.e., X-ray and neutron diffraction, small angle measurements), and physical characterization (i.e., differential thermal analysis, liquidus, viscosity, density). This paper will give an overview of the research program and some example unpublished results on glass-ceramic crystallization kinetics, microstructure, and Raman spectra, as well as some examples of the effects of rare earths on the absorption, luminescence, and NMR spectra of starting glasses. The formal collaboration described here has resulted in the generation of a large number of results, some of which are still in the process of being published as separate studies.

Boron-speciation and aluminosilicate crystallization in alkali boroaluminosilicate glasses along the NaAl1-xBxSiO4 and LiAl1-xBxSiO4 joins

The influences of boron on silicate glass dissolution behavior and crystallization have long been important in the nuclear waste glass and technical glass communities. Boron is a challenging component to model due to this ability to assume three- and four-fold coordinations with oxygen, as well as its ability to form complex structural units with silicate tetrahedra. Furthermore, the low atomic mass limits the available characterization techniques. In the present work, recent literature findings on the compositional influence on boron speciation are implemented to interpret new data on alkali boroaluminosilicate glasses studied with 11B nuclear magnetic resonance (NMR), synchrotron X-ray scattering (s-XRS), and chemical durability measurements. The purpose of this study is to understand the influence of boron on the crystallization behavior of sodium and lithium aluminosilicate systems, NaAl1-xBxSiO4 and LiAl1-xBxSiO4, as applied to problems in nuclear waste vitrification. Comparisons are drawn between the alkali coordination number and crystallization behavior as a function of boron addition, as obtained through modeling of synchrotron X-ray total scattering using Empirical Potential Structural Refinement.

Influence of young cement water on the corrosion of the International Simple Glass

Understanding the corrosion of nuclear waste glass is critical to predicting its safe disposal within a geological facility. The corrosion mechanisms and kinetics of the International Simple Glass, a simplified version of high-level nuclear waste glass, was shown to be significantly influenced by a high pH cement solution representative of disposal conditions. We provide the first microscopic characterisation of the porous, Zr-rich aluminoalkali-silica gel corrosion layer that was observed. Ca, Na and K from the cement solution were incorporated into the corrosion layer to charge compensate Si, Al and Zr species; the incorporation of Al was postulated to result in precipitation of an aluminosilicate-rich gel with large voids, facilitating rapid transport of species through the gel layer and significantly enhancing the corrosion rate. Precipitation of Al-containing zeolite and phyllosilicate phases was also observed, indicating that cementitious solutions may promote the detrimental ‘rate resumption’ stage of glass corrosion.

Structural features of ISG borosilicate nuclear waste glasses revealed from high-energy X-ray diffraction and molecular dynamics simulations

Vitrification is a widely accepted method to immobilize nuclear waste. Detailed structural information is critical to understand the physical and chemical behaviors, including the long-term chemical durability of these glasses that are to be stored in geological storage sites. High-energy X-ray diffraction studies are used to obtain accurate structural information of the International Simple Glass (ISG), a six-component borosilicate model nuclear waste glass with a composition (mol%) of 60.2SiO2-16.0B2O3-12.6Na2O-3.8Al2O3-5.7CaO-1.7ZrO2. Classical molecular dynamics (MD) simulations with recently developed effective potentials are utilized to generate structural models of the ISG glasses to provide interpretation of the high-energy X-ray structural data and additional short and medium range structural details. The atomic structure model generated from MD simulations shows an excellent agreement with high-energy X-ray diffraction, measured for the first time for ISG, with an Rx value of 5.5%. Systematic investigations of the medium-range structural features of ISG are performed, built upon recent study of the short-range structure. In particular, the glass former cation oxygen polyhedral analysis show that [SiO4]-[SiO4] and [BO4]-[SiO4] connections are the most abundant while [ZrO6] octohedra having higher probability to connect to another [SiO4], [BO4] and [BO3]. Moreover, we have studied the ISG surface structure using MD simulations. Noticeable differences in terms of both chemical compositions and structural features between the bulk and surface are observed. There exist increased small-sized rings, under coordinated Si species, high 3-coordinated boron content (86.5%) on the surface. Very importantly, sodium ions are found to be enriched on the glass surface, consistent with recent experimental results. Both bulk and surface structural information are discussed in terms of the dissolution and corrosion mechanisms of ISG and related nuclear waste glasses.

Molecular dynamics simulation of ballistic effects in simplified nuclear waste glasses

Ballistic effects in simple sodium borosilicate (Na2OB2O3=1) and sodium alumino-borosilicate glasses (Na2O−Al2O3B2O3=1) were investigated using molecular dynamics simulations. Specifically, the glasses were irradiated with heavy projectiles that caused atomic displacements by elastic collisions (displacement cascades) and progressively damaged the bulk glass. The accumulated pressure and internal energy inside the glass were found to saturate with deposited energy. Furthermore, structural analysis of the irradiated glasses revealed several important ballistic effects including a decrease in glass density, depolymerization of the borosilicate network, and increase in chemical mixing, short range and intermediate disorder. The magnitude of radiation damage was found to depend on the glass composition and, in general, alumino-borosilicate glasses were found to be slightly less damaged, after irradiation, as compared to borosilicate glasses.

Adaptation of the GRAAL model of Glass Reactivity to accommodate non-linear diffusivity

The empirical GRAAL (Glass Reactivity with Allowance for the Alteration Layer) model, developed to capture the essential corrosion behavior of nuclear waste glasses, was modified in a simple manner by raising the unitless diffusion term by a power factor p. With values of p > 1.0, the diffusivity is a non-linear, power function of the altered layer thickness. The modified model, GRAALP, was used to fit the long-term alteration data for a series of sodium borosilicate glasses reported by Gin et al. J. Non-Cryst. Solids, 358 (2012) 2559 where glass alteration is dominated by growth of the hydrolysis layer. The optimal value for the parameter p was found to vary significantly and could be correlated with glass composition. A value of p = 3.0 was found to be most suitable for the glasses containing both Ca and Al, while p = 2.6 was more appropriate if Ce was present in the glass. For glasses without Ca, the optimal fit to the data followed the original GRAAL model with p = 1.0. In the absence of Al, glasses containing Ca and/or Zr were not as easily characterized. These findings point to a relationship between the residual alteration rate and glass composition. Elements present in the contacting solution may also play a similar role but for this data set only pure water was used. A power law dependence of diffusivity on local water content is proposed which can partially account for the formation of an altered layer with a relatively sharp interface with the pristine glass. The empirical models do not account for the removal of oxide species due to hydrolysis and the associated influx of water and this, in combination with a diffusivity dependent on local water content, may be required to understand the results of depth profiling studies.

Non-linear effects of alumina concentration on Product Consistency Test response of waste glasses

The effect of alumina mole fraction on the 7-day Product Consistency Test (PCT) responses, ln[NLα], of simulated nuclear waste glasses was examined. It was found that the effect is highly non-linear. At low mole fractions of Al2O3 (xAl2O3 ≤ 0.035) the effect of Al2O3 additions is highly negative (dlnNLαdxAl2O3 = −72). At intermediate concentrations (0.035 <xAl2O3 ≤ 0.19) the effect of Al2O3 additions is moderately negative (dlnNLαdxAl2O3 = −11). At high concentrations (0.19 <xAl2O3) the effect of Al2O3 additions is highly positive (dlnNLαdxAl2O3 = +21). This variable impact of Al2O3 on ln[NLα] is speculated to be caused by the rates at which the glass corrosion process changes through various reaction regimes in static conditions at 90 °C. A model for prediction of ln[NLα] as a function of glass composition is presented. This model represents the data from 2669 glass compositions spanning a broad nuclear waste glass composition region and it is useful up to much higher Al2O3 concentrations than previous models (from ∼10 to 25 mol% Al2O3). The model was validated using data subset validation methods and shown to predict validation data in the same composition region with roughly equal certainty as the model fit data. The potential causes for non-linear effects of Al2O3 on ln[NLα] were discussed in context of the progression of glass corrosion rates. Time resolved static corrosion data is needed to better understand these non-linear effects.

Wet chemical processing for nuclear waste glass to retrieve radionuclides.

Here, we show unexpected and significant elution behavior of various elements from simulated nuclear waste glass (NWG) in ∼10° mol dm-3 acidic solutions below 100 °C, where a borosilicate-based glass matrix has been believed to be chemically durable. Most elements like glass main components (Li, B, Na, Ca, Al, and Zn, but except for Si) and simulated radionuclides (Rb, Cs, Sr, Ba, Se, Te, Mn, Pd, Mo, rare earths, Cr, Fe, and Ni) were remarkably eluted from the simulated NWG in ∼10° M HNO3 aq with Cl- at 90 °C. Especially, the elution of Pd is governed by its coordination chemistry including a redox reaction, because Pd(0) present in the simulated NWG has to be oxidized to Pd2+ which forms [PdCl4]2- for its dissolution. While Zr in simulated NWG is sparingly eluted even in this treatment, its elution actually proceeds in 1-3 M H2SO4 aq at 90 °C thanks to strong coordination of Zr(IV) with SO42-. Through design and optimization of the leaching conditions, a protocol of the wet chemical process to retrieve the radionuclides from simulated NWG has been proposed and demonstrated.

Effects of surface initial condition on aqueous corrosion of glass—A study with a model nuclear waste glass

AbstractBeing a nonequilibrium material, the structure of glass varies with the sample history. Thus, the initial surface condition of a glass can vary with the preparation condition and have a large impact on its reactivity. This paper shows that the aqueous corrosion behavior of international simple glass (ISG) varies depending on the initial surface state. The ISG glass samples were prepared as‐polished‐only and polished‐then‐annealed and they were immersed in aqueous solution saturated with soluble SiO2 at 30°C (modeling a mild condition) and at 90°C (modeling a severe condition). Molecular dynamics simulations were performed to obtain coordination numbers of network formers of ISG to assist oxygen speciation calculations. The surface structures of as‐prepared and corroded ISG samples were analyzed using various imaging and spectroscopic techniques. Among these analyses, only the oxygen speciation with x‐ray photoelectron spectroscopy showed discernable differences between two uncorroded surfaces with different preparation histories; all other methods could not differentiate the surface preparation history before aqueous corrosion. Such minor difference in chemical structures was found to have a profound impact on corrosion behaviors in the mild condition. In the harsh condition, the surface history dependence was not as drastic as the corrosion in the mild condition. The analysis results of the corroded surfaces suggested that the thickness and structure of the alteration layer formed on ISG in aqueous corrosion can vary with the initial surface state.

Discovery of a maximum damage structure for Xe-irradiated borosilicate glass ceramics containing powellite

In order to increase the waste loading efficiency in nuclear waste glasses, alternate glass ceramic (GC) materials are sought that trap problematic molybdenum in a water-durable CaMoO4 phase within a borosilicate glass matrix. In order to test the radiation resistance of these candidate wasteforms, accelerated external radiation can be employed to replicate long-term damage. In this study, several glasses and GCs were synthesized with up to 10 mol% MoO3 and subjected to 92 MeV Xe ions with fluences ranging between 5 × 1012 to 1.8 × 1014 ions/cm2. The main mechanisms of modification following irradiation involve: (i) thermal and defect-assisted diffusion, (ii) relaxation from the ion's added energy, (iii) localized damage recovery from overlapping ion tracks, and (iv) the accumulation of point defects or the formation of voids that created significant strain and led to longer-range modifications. Most significantly, a saturation in alteration could be detected for fluences greater than 4 × 1013 ions/cm2, which represents an average structure that is representative of the maximum damage state from these competing mechanisms. The results from this study can therefore be used for long-term structural projections in the development of more complex GCs for nuclear waste applications.

Impacts of glass composition, pH, and temperature on glass forward dissolution rate

Nuclear waste glasses dissolve at the forward dissolution rate (rf) 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 rf, the impacts of glass composition have remained uncertain. In this work, rf data from 19 nuclear waste glasses were used with the aim of identifying the effect of glass composition on rf. The rf values were modeled as: rf = k010–ηpHexp(−Ea/RT), with k0, η, Ea, 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[k0], η, and Ea) and glass composition for the glasses considered in this study, an outcome that was attributed to the strong positive correlation between the log[k0] and Ea parameters. Therefore, a model was fitted directly to the combined rf from all 19 glasses. This approach showed that 90% of the variation in rf 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 rf 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 rf are predicted with no discernable composition effects below the threshold.

Crystallization behavior of iron‐ and boron‐containing nepheline (Na2O·Al2O3·2SiO2) based model high‐level nuclear waste glasses

AbstractThis study focuses on understanding the relationship between iron redox, composition, and heat‐treatment atmosphere in nepheline‐based model high‐level nuclear waste glasses. Glasses in the Na2O–Al2O3–B2O3–Fe2O3–SiO2 system with varying Al2O3/Fe2O3 and Na2O/Fe2O3 ratios have been synthesized by melt‐quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N2), and reducing (96%N2–4%H2). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass‐ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass‐ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass‐ceramics produced as a result of heat treatment in air and N2 atmospheres developed a golden/brown colored iron‐rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron‐rich layer was observed in glass‐ceramics with varying Al2O3/Fe2O3 ratio, it was absent from glass‐ceramics with varying Na2O/Fe2O3 ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron‐rich layer on the surface of glass‐ceramics on the chemical durability of high‐level nuclear waste glasses have been discussed.