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There is wide industrial interest in developing robust models of long‐term (>100 years) glass durability. Archeological glass analogs, glasses of similar composition, and alteration conditions to those being tested for durability can be used to evaluate and inform such models. Two such analog glasses from a 1500‐year‐old vitrified hillfort near Uppsala, Sweden have previously been identified as potential analogs for low concentration Fe‐bearing aluminosilicate nuclear waste glasses. However, open questions remain regarding the melting environment from which these historic glasses were formed and the effect of these conditions on their chemical durability. A key factor to answering the previous melting and durability questions is the redox state of Fe in the starting and final materials. Past work has shown that the melting conditions of a glass‐forming melt may influence the redox ratio value (Fe+3/∑Fe), a measure of a glass's redox state, and both melting conditions and the redox ratio may influence the glass alteration behavior. Synthetic analogs of the hillfort glasses have been produced using either fully oxidized or reduced Fe precursors to address this question. In this study, the melting behavior, glass transition temperature, oxidation state, network structure, and chemical durability of these synthesized glass analogs is presented. Resulting data suggests that the degree of network connectivity as impacted by the oxidation state of iron impacted the behavior of the glass‐forming melt but in this case does not affect the chemical durability of the final glass. Glasses with a lower degree of melt connectivity were found to have a lower viscosity, resulting in a lower glass transition temperature and softening temperature, as well as in a lower temperature of foam onset and temperature of foam maximum. This lower degree of network connectivity most likely played a more significant role in accelerating the conversion of batch chemicals into glass than the presence of water vapor in the furnace's atmosphere. Future work will focus on using the results from this work with outcomes from other aspects of this project to evaluate long‐term glass alteration models. [ABSTRACT FROM AUTHOR]

Understanding the impact of individual compositional components on the devitrification of complex multicomponent glasses, for example, 10–50+ oxides, typically requires numerous studies to examine each component's impact. Here we apply exploratory data analysis (EDA) to a heterogeneous data set of silicate glasses to determine the cations' individual and interacting effects on the crystallization of nepheline (nominally NaAlSiO4). Our data consisted of 795 simulated high‐level nuclear waste glasses composed of, on average, 50 oxide components. We determine the interactions in the heterogeneous data that cause deviations from the behavior found in simplified composition studies. Using both univariate and bivariate EDA techniques, we demonstrate the importance of including calculated structural glass parameters on nepheline's devitrification, including field strength, cation‐to‐anion radius ratio, and single‐bond strength. We also show that studies with simplified glass compositions may fall short in generating knowledge directly transferrable to complex glass compositions. The method used in this study has the potential to inform experimental design for simplified compositions (∼6+ oxides) that can generate knowledge directly transferrable to complex, multivariable compositions. The observations reported here have broad implications for any study attempting to map the physical properties of a complex glass containing numerous cations. [ABSTRACT FROM AUTHOR]

This study shows the effects of nuclear waste glass production on Monofrax K‐3 refractory corrosion. A continuously fed research‐scale melter containing an Fe‐ and Ni‐rich simulated nuclear waste feed with borosilicate glass‐forming chemicals was cyclically melted at 1150°C and idled at 1050°C for a total of 11 weeks. Chemical maps using scanning electron microscopy show the interactions between the quenched melt and the refractory. Nanoscale X‐ray‐computed tomography was used for a three‐dimensional visualization of certain parts of the interface. Unreacted K‐3 consists of primarily corundum (Al,Cr)2O3 and spinel (Fe2+,Mg)(Al,Cr)2O4 interlocking crystalline phases. Some of the Cr from the refractory interacts with the Ni and Fe from the melt to form a reaction layer comprising (Ni,Fe2+)(Cr,Fe3+)2O4 spinel crystals. Simultaneously, melt components (Na,Si) infiltrate into the refractory. This interaction proceeds at the expense of the integrity of the refractory structure. Intact refractory grains (e.g., (Al,Cr)2O3) as well as the reaction layer itself can lose mechanical integrity and spall off into the melt, especially near the top of the melter. As the reaction layer can be a protective boundary for the refractory against further melt infiltration, a reduction in the reaction layer thickness allows an increase in refractory corrosion. [ABSTRACT FROM AUTHOR]

Spinel crystallization is known to be detrimental to the operation of Joule heated ceramic melters during the vitrification of iron‐rich high‐level nuclear wastes (HLW) into borosilicate glasses. The literature on this subject focuses on tackling the problem by developing empirical constraints to design compositions, which limit the fraction of spinels formed in the melter or by developing empirical models to predict the settling behavior of spinels in the melter as a function of the glass composition. While these empirical models can predict the behavior of most of the compositions, they are not failsafe as there are always some compositions, whose behavior is beyond the predictive ability of these models. This can lead to undesirable situations during the vitrification of the nuclear waste, and therefore an in‐depth investigation of the chemo‐structural descriptors controlling the crystallization behavior in these glasses is warranted. Accordingly, the present study aims to understand the impact of non‐framework cation mixing (i.e., Li+/Na+ and Ca2+/Na+) on the structure (through Raman spectroscopy and Mössbauer spectroscopy) and crystallization behavior (through XRD, SEM‐EDS, and vibrating sample magnetometry) of iron‐rich model HLW glasses in the system: (mol.%) x MyO–(25−x) Na2O–9.12 B2O3–6.4 Al2O3–51.25 SiO2–7.22 Fe2O3–0.38 MnO–0.08 Cr2O3–0.55NiO (MyO = Li2O or CaO). [ABSTRACT FROM AUTHOR]

Because microbes cannot be eliminated from radioactive waste disposal facilities, the consequences of bio-colonization must be understood. At a pre-Viking era vitrified hillfort, Broborg, Sweden, anthropogenic glass has been subjected to bio-colonization for over 1,500 years. Broborg is used as a habitat analogue for disposed radioactive waste glass to inform how microbial processes might influence long-term glass durability. Electron microscopy and DNA sequencing of surficial material from the Broborg vitrified wall, adjacent soil, and general topsoil show that the ancient glass supports a niche microbial community of bacteria, fungi, and protists potentially involved in glass alteration. Communities associated with the vitrified wall are distinct and less diverse than soil communities. The vitrified niche of the wall and adjacent soil are dominated by lichens, lichen-associated microbes, and other epilithic, endolithic, and epigeic organisms. These organisms exhibit potential bio-corrosive properties, including silicate dissolution, extraction of essential elements, and secretion of geochemically reactive organic acids, that could be detrimental to glass durability. However, long-term biofilms can also possess a homeostatic function that could limit glass alteration. This study documents potential impacts that microbial colonization and niche partitioning can have on glass alteration, and subsequent release of radionuclides from a disposal facility for vitrified radioactive waste. [ABSTRACT FROM AUTHOR]

High‐level waste (HLW) glasses with high alumina content are prone to nepheline crystallization during the slow canister cooling that is experienced during large‐scale production. Because of its detrimental effects on glass durability, nepheline (NaAlSiO4) precipitation must be avoided; however, developing robust, predictive models for nepheline crystallization behavior in compositionally complex HLW glasses is difficult. Using overly conservative constraints to predict nepheline formation can limit the waste loading to lower than the achievable capacity. In this study, a robust data‐driven model using five compositional features has been developed to predict nepheline formation. A new descriptor is introduced called the "difference based on correlation", which has higher accuracy compared to previous descriptors and also has more balanced false positive and false negative rates. The analysis of the model and the data show an overlap, instead of a distinct compositional boundary, between glasses that form and do not form nepheline. As a result, the model's predictive accuracy is not the same throughout the feature space and instead is dependent on the location of the glass composition in the dimensionally reduced feature space. [ABSTRACT FROM AUTHOR]

A 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. [ABSTRACT FROM AUTHOR]

Abstract: Models for long‐term glass alteration are required to satisfy performance predictions of vitrified nuclear waste in various disposal scenarios. Durability parameters are usually extracted from short‐term laboratory tests, and sometimes checked with long‐term natural experiments on glasses, termed analogues. In this paper, a unique potential ancient glass analogue from Sweden is discussed. The hillfort glass found at Broborg represents a unique case study as a vitrified waste glass analogue to compare to Low Activity Waste glass to be emplaced in near surface conditions at Hanford (USA). Glasses at Broborg have similar and dissimilar compositions to LAW glasses, allowing the testing of long‐term alteration of different glass chemistries. In addition, the environmental history of the site is reasonably well documented. Initial investigations on previously collected samples established methodologies for handling and characterizing these artifacts by laboratory methods while preserving their alteration layers and cultural context. Evidence of possible biologically influenced glass alteration, and differential alteration in the 2 types of glass found at the Broborg site is presented. [ABSTRACT FROM AUTHOR]

The distribution and physical form of technetium in a Hanford low-activity waste (LAW) glass was examined with scanning electron microscopy ( SEM) and X-ray diffraction ( XRD). A simulated Hanford LAW glass was spiked with varying amounts of potassium pertechnetate and melted at 1000°C. The glass was melted in a sealed quartz ampoule with the air pumped out, so that volatile material could leave the glass but would not be lost from the system. Previous studies have shown that technetium remains in the glass up to about 2000 ppm, but rises to the top of the melt as a separate salt phase above this concentration. Examination by SEM shows that crystals of technetium compounds appear to grow out of the hot glass, which implies that the hot glass was supersaturated in technetium salts. Some of the technetium compound crystals had apparently melted, but other crystals had obviously not melted and must have formed after the glass had partially cooled. The technetium compounds in the salt layer are KTcO4 and NaTcO4, according to SEM and XRD. No TcO2 was found in the salt phase, even though Tc( IV) has been previously reported in the glass. [ABSTRACT FROM AUTHOR]

Historical data have been critically compiled and analyzed for investigating the quantity of nepheline ( NaAlSiO4) precipitated as a function of composition in simulated nuclear waste glasses. To understand compositional effects two primary methods were used: (1) investigating the Al2O3- SiO2- Na2O ternary while filtering for different B2O3 levels and (2) creating a quadrant system consisting of compositions reduced to two representations: (i) the nepheline discriminator ( ND) which depends only on the SiO2 content by weight normalized to the total weight of the Al2O3- SiO2- Na2O submixture and (ii) the optical basicity ( OB) which contains contributions from all constituents in the glass. Nepheline precipitation is expected to be suppressed at high SiO2 levels ( ND > 0.62) or at low basicities ( OB < 0.55-0.57). Changes in sodium aluminosilicate glass OB values due to additions of CaO and B2O3 correlate with the observed effects on nepheline formation. It is proposed that additional composition space is available for formulating high waste loading, high- Al2O3 nuclear waste glasses when consideration is given to location on the Al2O3- SiO2- Na2O submixture as well as OB. [ABSTRACT FROM AUTHOR]

Crystallization of rhenium salts in a simulated low-activity waste borosilicate glass: Erratum Corresponding author: Brian Riley; email id: brian.riley@pnnl.gov There is a typographical error in the introduction of this paper. Crystallization of rhenium salts in a simulated low-activity waste borosilicate glass. [Extracted from the article]

[Display omitted] • Millimeter wave technology demonstrated for high temperature in-line monitoring. • MMW/THz radiometry used to identify phase change and optical properties of melts. • Na 2 SO 4 salt formation unexpectedly identified in minuscule quantities. • MMW/THz interferometry observed volatilization and foaming. • Broad potential manufacturing applications discussed. Vitrification is an internationally significant industrial process used for the treatment and immobilization of hazardous and radioactive waste. For successful silicate melt vitrification, molten salt formation during melter operation must be avoided. As such, proper process controls and continuous monitoring are critical to minimizing melting problems. Several in-situ process technologies for glass melters are well-developed; however, the lack of in-situ surface salt formation detection methods presents a risk to vitrification at the Waste Treatment & Immobilization Plant (WTP) on the US Hanford Site. While proposed previously, millimeter wave (MMW) radiometry and interferometry are demonstrated for the first time for in-line detection of salt formation in simulated nuclear waste glass melts. The experimental radiometer and interferometer setup uses the optical properties of the melt and a dual receiver operating at ∼ 137 GHz to elucidate melting behavior. A series of previously characterized glasses supersaturated with sulfate (Na 2 SO 4), chloride (NaCl), or fluoride (NaF) salts are analyzed using the MMW system. This provides insight into volatile losses, fining, salt formation, salt identity, crystallization, and optical properties of a heterogeneous melt. Relevant terahertz (MMW/THz) optical properties are also compiled. Millimeter wave measurements are evaluated here for the ability to detect phase changes in salt-forming glass melt compositions without opaque body radiometry assumptions. This contribution demonstrates MMW radiometry with interferometry as a useful method for in-situ salt detection, enabling risk reduction in nuclear waste vitrification melters. [ABSTRACT FROM AUTHOR]

• Structures of fluorine containing aluminosilicate glasses were modeled using MD simulations. • CaO to Na 2 O substitution effect on structural changes and fluorine distribution studied. • The melt structure during glass formation were monitored as a function of temperature. • Ca was found to bond strongly to fluorine than Na, both in the glass and in the melt. • Results provide insight on enhancing fluorine retention in nuclear waste glasses. Fluorine and other halides commonly exist in nuclear waste forms, and due to their volatile nature, halide retention poses an issue affecting waste loading during vitrification. The compositional effect on fluorine incorporation in aluminosilicate glasses is investigated through molecular dynamics simulations. Oxygen and fluorine coordination numbers around glass former and modifier cations, bond angle distributions, and medium range structure features such as Q n distributions, ring size distributions and neutron diffraction structure factors were calculated. It was found that fluorine has higher preference to bond to Ca2+ than to Na+, both in the melt and the glass, and there is no Si-F bond formation in the glass but they do exist in the melt. Consequently, CaO for Na 2 O substitution can be an effective way to help fluorine retention without significantly changing the glass chemistry. These results thus provide insights on fluorine incorporation in the aluminosilicate nuclear waste glasses and the strategy on how to improve fluorine retention both in the glass and the melt. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Molybdenum oxides are an integral component of the high-level waste streams being generated from the nuclear reactors in several countries. Although borosilicate glass has been chosen as the baseline waste form by most of the countries to immobilize these waste streams, molybdate oxyanions (MoO42-) exhibit very low solubility (ͬ41 mol %) in these glass matrices. In the past three to four decades, several studies describing the compositional and structural dependence of molybdate anions in borosilicate and aluminoborosilicate glasses have been reported in the literature, providing a basis for our understanding of fundamental science that governs the solubility and retention of these species in the nuclear waste glasses. However, there are still several open questions that need to be answered to gain an in-depth understanding of the mechanisms that control the solubility and retention of these oxyanions in glassy waste forms. This article is focused on finding answers to two such questions: (1) What are the solubility and retention limits of MoO3 in aluminoborosilicate glasses as a function of chemical composition? (2) Why is there a considerable increase in the solubility of MoO3 with incorporation of rare-earth oxides (for example, Nd2O3) in aluminoborosilicate glasses? Accordingly, three different series of aluminoborosilicate glasses (compositional complexity being added in a tiered approach) with varying MoO3 concentrations have been synthesized and characterized for their ability to accommodate molybdate ions in their structure (solubility) and as a glass-ceramic (retention). The contradictory viewpoints (between different research groups) pertaining to the impact of rare-earth cations on the structure of aluminoborosilicate glasses are discussed and their implications on the solubility of MoO3 in these glasses are evaluated. A novel hypothesis explaining the mechanism governing the solubility of MoO3 in rare-earth containing aluminoborosilicate glasses has been proposed. [ABSTRACT FROM AUTHOR]

• X-ray nano-computed tomography (nano-CT) is powerful technique to explore glasses. • Complex features were imaged in nuclear waste, natural, and chalcogenide glasses. • Nano-CT enabled 3D study of crystal, phase separation, pore, & fracture morphology. • Features from 100 to 20 µm were visualized with 3D videos and 2D images. • 3D measurements included dimensions, orientation, shape, and crystal number density. X-ray nano-computed tomography (nano-CT) is a powerful technique to characterize and visualize 3 dimensional (3D) phenomena in complex glasses at the nanoscale. This technique can offer a unique opportunity to explore the intricate morphology of multicomponent glass and glass-ceramic samples, due to its low X-ray energy and high spatial resolution (down to 50 nm). In the current demonstration paper, nano-CT provided insight into crystallization and phase separation, as well as a variety of other phenomena, including corrosion, fracture, and porosity. At scales ranging from 100 to 20 μm, the microstructures of phase separation in borosilicate, complex silicate, and chalcogenide glass compositions were examined. In addition, crystallites present in simulant nuclear waste glasses, volcanic glasses, and other related samples were explored. Nano-CT analysis can add to the understanding of (1) the formation processes, (2) distributions, interactions, and compositions of phases, and (3) the mechanisms, structures, and growth of crystallization and phase separation. Nano-CT offers wide potential in the field of glass science, especially when (1) common characterization methods are insufficient, (2) simple sample preparation is required, (3) 3D rendering is needed, or (4) compelling images of small, complex features are desired. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the solubility of iodine in a low-activity waste borosilicate glass when heated inside an evacuated and sealed fused quartz ampoule. The iodine was added to glass frit as KI in quantities of 99.4-24,005 ppm iodine (by mass). Each mixture was added to an ampoule, heated at 1000°C for 2h, and then air quenched. In samples with ≥11,999 ppm iodine, low viscosity salt phases were observed on the surface of the melts that solidified into a white coating upon cooling. These salts were identified by X-ray diffraction as mixtures of KI, NaI, and Na2SO4. Iodine concentrations in glass specimens were analyzed with inductively-coupled plasma mass spectrometry, and the overall iodine solubility was determined to be 10,000 ppm. Several crystalline inclusions of iodine sodalite, Na8(AlSiO4)6I2, were observed in the 24,005 ppm specimen. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hanford low activity waste glass spiked with KTcO4 500–6000ppm Tc by mass. [•] Tc solubility varied with glass redox, 2000– 2800ppm, higher when slightly reduced. [•] Tc mostly present as Tc(IV) with some Tc(VII) and rarely TcO2 inclusions. [•] Small amounts of WC from ball mills can be strongly reducing for Fe and Tc. [ABSTRACT FROM AUTHOR]