Determination of Temperature‐Dependent Heat Conductivity and Thermal Diffusivity of Waste Glass Melter Feed

The cold cap is a layer of reacting glass batch floating on the surface of melt in an all‐electric continuous glass melter. The heat needed for the conversion of the melter feed to molten glass must be transferred to and through the cold cap. Since the heat flux into the cold cap influences the rate of melting, the heat conductivity is a key property of the reacting feed. We designed an experimental setup consisting of a large cylindrical crucible with an assembly of thermocouples (TC) that monitors the evolution of the temperature field while the crucible is heated at a constant rate. Then we used two methods to calculate the heat conductivity and thermal diffusivity of the reacting feed: the approximation of the temperature field by polynomial functions and the finite‐volume method (FVM) coupled with least‐squares analysis. Up to 680°C, the heat conductivity of the reacting melter feed was represented by a linear function of temperature.     

Determination of heat conductivity of waste glass feed and its applicability for modeling the batch‐to‐glass conversion

The effective heat conductivity (λ) of reacting melter feed affects the heat transfer and conversion process in the cold cap, a layer of reacting feed floating on molten glass. A heat conductivity meter was used to measure λ of samples of a cold cap retrieved from a laboratory‐scale melter, loose dry powder feed samples, and samples cut from fast‐dried slurry blocks. These blocks were formed to simulate the feed conditions in the cold‐cap by rapidly evaporating water from feed slurry poured onto a 200°C surface. Our study indicates that the effective heat conductivity of the feed in the cold cap is significantly higher than that of loose dry powder feed, which is a result of the feed solidification during the water evaporation from the feed slurry. To assess the heat transfer at higher temperatures when feed turns into foam, we developed a theoretical model that predicts the foam heat conductivity based on morphology data from in‐situ X‐ray computed tomography. The implications for the mathematical modeling of the cold cap are discussed.     

Effects of heating rate,quartz particle size,viscosity, and form of glass additives on high‐level waste melter feed volume expansion

Nuclear waste can be vitrified by mixing it with glass‐forming and ‐modifying additives. The resulting feed is charged into an electric glass melter. To comprehend melting behavior of a high‐alumina melter feed, we monitored the volume expansion of pellets in response to heating at different heating rates. The feeds were prepared with different particle sizes of quartz (the major additive component) and with varied silica‐to‐fluxes ratio to investigate the glass melt viscosity effects. Also, we used additional melter feeds with additives premelted into glass frit. The volume of pellets was nearly constant at temperatures <600°C. After a short period of volume shrinkage at ~600°C‐700°C, foam generation produced massive volume expansion. The low heat conductivity of foam hinders the transfer of heat from molten glass to the reacting feed. The extent of foaming increased with faster heating and higher melt viscosity, and decreased with increasing size of quartz particles and fritting of the additives. Volume expansion data are needed for the mathematical modeling of the cold cap.     

Determination of Heat Conductivity and Thermal Diffusivity of Waste Glass Melter Feed: Extension to High Temperatures

The heat conductivity (λ) and the thermal diffusivity (a) of reacting glass batch, or melter feed, control the heat flux into and within the cold cap, a layer of reacting material floating on the pool of molten glass in an all‐electric continuous waste glass melter. After previously estimating λ of melter feed at temperatures up to 680°C, we focus in this work on the λ(T) function at T > 680°C, at which the feed material becomes foamy. We used a customized experimental setup consisting of a large cylindrical crucible with an assembly of thermocouples, which monitored the evolution of the temperature field while the crucible with feed was heated at a constant rate from room temperature up to 1100°C. Approximating measured temperature profiles by polynomial functions, we used the energy equation to estimate the λ(T) approximation function, which we subsequently optimized using the finite‐volume method combined with least‐squares analysis. The heat conductivity increased as the temperature increased until the feed began to expand into foam, at which point the conductivity dropped. It began to increase again as the foam turned into a bubble‐free glassmelt. We discuss the implications of this behavior for the mathematical modeling of the cold cap.     

Melter feed viscosity during conversion to glass: Comparison between low‐activity waste and high‐level waste feeds

During nuclear waste vitrification, a melter feed (a slurry mixture of a nuclear waste and various glass forming and modifying additives) is charged into the melter where undissolved refractory constituents are suspended together with evolved gas bubbles from complex reactions. Knowledge of flow properties of various reacting melter feeds is necessary to understand their unique feed‐to‐glass conversion processes occurring within a floating layer of melter feed called a cold cap. The viscosity of two low‐activity waste (LAW) melter feeds were studied during heating and correlated with volume fractions of undissolved solid phase and gas phase. In contrast to the high‐level waste (HLW) melter feed, the effects of undissolved solid and gas phases play comparable roles and are required to represent the viscosity of LAW melter feeds. This study can help bring physical insights to feed viscosity of reacting melter feeds with different compositions and foaming behavior in nuclear waste vitrification.     

One‐Dimensional Cold Cap Model for Melters with Bubblers

The rate of glass production during vitrification in an all‐electrical melter greatly impacts the cost and schedule of nuclear waste treatment and immobilization. The feed is charged to the melter on the top of the molten glass, where it forms a layer of reacting and melting material, called the cold cap. During the final stages of the batch‐to‐glass conversion process, gases evolved from reactions produce primary foam, the growth and collapse of which controls the glass production rate. The mathematical model of the cold cap was revised to include functional representation of primary foam behavior and to account for the dry cold cap surface. The melting rate is computed as a response to the dependence of the primary foam collapse temperature on the heating rate and melter operating conditions, including the effect of bubbling on the cold cap bottom and top surface temperatures. The simulation results are in good agreement with experimental data from laboratory‐scale and pilot‐scale melter studies. The cold cap model will become part of the full three‐dimensional mathematical model of the waste glass melter.     

Viscosity of glass-forming melt at the bottom of high-level waste melter-feed cold caps: Effects of temperature and incorporation of solid components

During the final stages of conversion of melter feed (glass batch) to molten glass, the glass-forming melt becomes a continuous liquid phase that encapsulates dissolving solid particles and gas bubbles that produce primary foam at the bottom of the cold cap (the reacting melter feed in an electric glass-melting furnace). The glass-forming melt viscosity plays a dominant role in primary foam formation, stability, and eventual collapse, thus affecting the rate of melting (the glass production rate per cold-cap area). We have traced the glass-forming melt viscosity during the final stages of feed-to-glass conversion as it changes in response to changing temperature and composition (resulting from dissolving solid particles). For this study, we used high-level waste melter feeds—taking advantage of the large amount of data available to us—and a variety of experimental techniques (feed expansion test, evolved gas analysis, thermogravimetric analyzer-differential scanning calorimetry, X-ray diffraction, and viscometer). Starting with a relatively low value at the moment when the melt connects, melt viscosity reached maximum within the primary foam layer and then decreased to its final melter operating temperature value. At the cold-cap bottom—the boundary between the primary foam layer and the thermal boundary layer—where physicochemical reactions of a melter feed influence the driving force of the heat transfer from the melt to the cold cap, the melt viscosity affects the rate of melting predominantly through its effect on the temperature at which primary foam is collapsing.     

X‐ray tomography of feed‐to‐glass transition of simulated borosilicate waste glasses

High‐level waste feed composition affects the overall melting rate by influencing the chemical, thermophysical, and morphological properties of a cold cap layer that floats on the molten glass where most feed‐to‐glass reactions occur. Data from X‐ray computed tomography imaging of melting pellets comprised of a simulated high‐aluminum feed reveal the morphology of bubbles, known as the primary foam, for various feed compositions at temperatures between 600°C and 1040°C. These feeds were formulated to make glasses with viscosities ranging from 0.5 to 9.5 Pa s at 1150°C, which was accomplished by changing the SiO₂/(B₂O₃+Na₂O+Li₂O) ratio in the final glass. Pellet dimensions and profile area, average and maximum bubble areas, bubble diameter, and void fraction were evaluated. The feed viscosity strongly affects the onset of the primary foaming and the foam collapse temperature. Despite the decreasing amount of gas‐evolving components (Li₂CO₃, H₃BO₃, and Na₂CO₃), as the feed viscosity increases, the measured foam expansion rate does not decrease. This suggests that the primary foaming is not only affected by changes in the primary melt viscosity but also by the compositional reaction kinetic effects. The temperature‐dependent foam morphological data will be used to inform cold cap model development for a high‐level radioactive waste glass melter.     

Toward Understanding the Effect of Low‐Activity Waste Glass Composition on Sulfur Solubility

The concentration of sulfur in Hanford low‐activity waste (LAW) glass melter feed will be maintained below the point where the salt accumulates on the melt surface. The allowable concentrations may range from near zero to over 2.05 wt% (of SO₃ on a calcined oxide basis) depending on the composition of the melter feed and processing conditions. If the amount of sulfur exceeds the melt tolerance level, a molten salt will accumulate which may upset melter operations and potentially shorten the useful life of the melter. At the Hanford site, relatively conservative limits have traditionally been placed on sulfur loading in melter feed, which in turn significantly increases the amount of LAW glass that will be produced. Crucible‐scale sulfur solubility data and scaled melter sulfur tolerance data have been collected on simulated Hanford waste glasses over the last 15 years. These data were compiled and analyzed. An empirical model was developed to predict the solubility of SO₃ in glass based on 253 simulated Hanford LAW glass compositions. This model represents the data well, accounting for over 85% of the variation in data, and was well validated. The model was also found to accurately predict the maximum amount of sulfur in melter feed that did not form a salt layer in 13 scaled melter tests of simulated LAW glasses. The model can be used to help estimate glass volumes and make informed decisions on process options (e.g., scale of supplemental LAW treatment facility, and pretreatment facility performance requirements). The model also gives quantitative estimates of component concentration effects on sulfur solubility. The components that increase sulfur solubility most are Li₂O > V₂O₅ > CaO ≈ P₂O₅ > Na₂O ≈ B₂O₃ > K₂O. The components that decrease sulfur solubility most are Cl > Cr₂O₃ > Al₂O₃ > ZrO₂ ≈ SnO₂ > Others (i.e., the sum of minor components) ≈SiO₂. The order of component effects is similar to previous literature data, in most cases.     

Balance of oxygen throughout the conversion of a high-level waste melter feed to glass

Gases evolve from nuclear waste melter feed during conversion to glass in response to heating. This paper is focused on oxygen mass balance based on the stoichiometry of feed melting reactions and evolved-gas analysis data. Whereas O₂-producing and -consuming batch-melting reactions are complete in the reacting and primary-foam layers of the cold cap, O₂ from redox reactions continues to evolve as long as melt temperature increases, and thus generates secondary foam. Also, we discuss the relationship between the oxygen mass balance and the temperature-dependent iron redox ratio and the O₂ partial pressure, as they evolve during the feed-to-glass conversion.     

Conversion of Nuclear Waste to Molten Glass: Cold‐Cap Reactions in Crucible Tests

The feed‐to‐glass conversion, which comprises complex chemical reactions and phase transitions, occurs in the cold cap during nuclear waste vitrification. To investigate the conversion process, we analyzed heat‐treated samples of a simulated high‐level waste feed using X‐ray diffraction, electron probe microanalysis, leaching tests, and residual anion analysis. Feed dehydration, gas evolution, and borate phase formation occurred at temperatures below 700°C before the emerging glass‐forming melt was completely connected. Above 700°C, intermediate aluminosilicate phases and quartz particles gradually dissolved in the continuous borosilicate melt, which expanded with transient foam. Knowledge of the chemistry and physics of feed‐to‐glass conversion will help us control the conversion path by changing the melter feed makeup to maximize the glass production rate.     

Retention analysis from vitrified low-activity waste and simulants in a laboratory-scale melter

The Waste Treatment and Immobilization Plant (WTP) will process and stabilize nuclear waste stored in tanks on the Hanford Site. At the WTP, the tank waste will be combined with glass-forming chemicals to make a melter feed slurry that can be vitrified in joule-heated melters. Technetium-99 (⁹⁹Tc), a long-lived radionuclide present in tank waste, is semi-volatile from a glass melt at elevated temperatures. A small laboratory-scale melter system has been designed by PNNL to operate under radioactive conditions, giving it the ability to vitrify actual tank waste and gain information about melter feed processability and the partitioning of components of interest. This study describes two runs performed in a duplicate system with non-radioactive simulants of Hanford tanks 241-AP-107 and 241-AN-105 to gain insight into the relationship between the retentions of ⁹⁹Tc and its non-radioactive surrogate Re while also investigating the effects of increasing the reducing agent such as sucrose during vitrification on processing and the retention of semi-volatile components in the glass product. The results from these runs align with the general trends of greater retention in the glass of Re compared to ⁹⁹Tc and the improved retention in the glass of Re with increased reducing agent.     

High Chrome Refractory Characterization: Part II. Accumulation of Spinel Corrosion Deposits in Radioactive Waste Glass Melters

High Cr₂O₃ containing Monofrax^™ K-3 is a robust refractory that is used in radioactive waste glass melters worldwide. Monofrax^™ K-3 contains highly reduced phases. Conversely, many of the radioactive feeds being processed are highly oxidizing. The K-3 refractory corrosion rates in oxidizing (high nitrate) feeds were ~1.8–2.8 times higher than the rates determined using reducing feeds. The corrosion product formed is a mixture of spinel and glass (slag) that can accumulate on the melter floor. A methodology to calculate the depth of slag deposits from refractory corrosion is presented and verified with slag measurements from the Defense Waste Processing Facility (DWPF) melter after it had processed oxidized feeds for 1.75 years. The calculations show that had the facility continued to process oxidized feeds the melter lifetime (based on when the deposits could have reached and blocked the pour spout riser) would have been ~4.5 years. The DWPF changed to a reducing flow sheet after ~3 years of operation. The lifetimes of Melter #1 and Melter #2, assuming a failure due to pour spout blockage, are calculated as 7.7–12 years based on corrosion rates measured with reducing feeds. Lifetimes of 9 and >11 years have actually been achieved.     

High Chrome Refractory Characterization: Part I. Impact of Melt Reduction/Oxidation on the Corrosion Mechanism

High Cr₂O₃ containing Monofrax^™ K-3 is a robust refractory that is used in the fiberglass industry and used in radioactive waste glass melters worldwide. Monofrax^™ K-3 is tolerant of transition metal oxides but contains highly reduced solid solutions of spinels, that is, (Mg,Fe²⁺)(Al,Cr)₂O₃. Conversely, many of the waste feeds being processed are highly oxidizing. The K-3 refractory corrosion was tested in sealed crucibles starting with slurried melter feed instead of prereacted glass called for by ASTM C621. Testing the refractory coupon during the feed-to-glass conversion exposes the refractory to the oxidizing and reducing species being released during vitrification, for example, NO₃⁻, NO₂⁻, CO₂, CO, O₂. Corrosion rates measured in highly oxidizing (high nitrate) feeds 3 3 Oxidized feeds give glass Fe⁺²/∑Fe of ~0.
were ~1.8–2.8 times higher than those determined using prereacted glass or reduced feeds. 4 4 Reduced feeds give glass Fe⁺²/∑Fe of > 0.09.
Confirmatory corrosion rates were measured on Monofrax^™ K-3 coupons immersed in oxidizing feed in a 1/100th-scale HLW pilot-scale melter. Corrosion is heterogeneous or incongruent as Ni and Fe in the waste glass exchange with Mg and Al in the refractory. An insoluble NiFe₂O₄ spinel corrosion product is formed that can build up a protective layer along the refractory walls or spall and settle to the melter floor depending on melt pool convection/agitation.     

A Method for Determining Bulk Density,Material Density,and Porosity of Melter Feed During Nuclear Waste Vitrification

Glassmelting efficiency largely depends on heat transfer to reacting glass batch (melter feed), which in turn is influenced by the bulk density (ρ_b) and porosity (?) of the reacting feed as functions of temperature (T). Neither ρ_b(T) nor ?(T) functions are readily accessible from direct measurements. For the determination of ρ_b, we monitored the profile area of heated feed pellets and calculated the pellet volume using numerical integration. For the determination of ?, we measured the material density of feeds quenched at various stages of conversion via pycnometry and then computed the feed density at heat‐treatment temperature using thermal expansion values of basic feed constituents.     

Polymer-metal slurry reactive melt infiltration: A flexible and controllable ceramic modification strategy for irregular C/C components

Herein, we investigate the applicability of the polycarbosilane (PCS)–metal slurry reactive melt infiltration (RMI) process to various metals. The slurry exhibiting the best ceramized ability was used to examine the relationship between the ceramic thickness and reactive time, ceramic thickness and reactive temperature, and infiltration depth and slurry-coating thickness. The results show that the thickness of the ceramic layer increases with reactive time and temperature and the infiltration depth increases with the coating thickness. PCS–Si₉₀Zr₁₀ slurry RMI was selected to modify cylindrical nozzle C/C preforms, and dense C/C–SiC–ZrC composites with a density of ~2.05 g cm⁻³ were obtained. Owing to the good control of the PCS–Si₉₀Zr₁₀ slurry RMI on the interface, matrix, and carbon fiber of the as-received cylindrical composites, the bending strength of the C/C–SiC–ZrC composites was as high as 306.4 MPa, which is considerably higher than that of a C/C preforms (70.4 MPa). Considering the ablation resistance, the mass and linear ablation rates of the C/C–SiC–ZrC composite (~0.29 mg s⁻¹ and ~2.48 × 10⁻³ mm s⁻¹, respectively) were similar to those of the composites prepared using traditional RMI (~0.23 mg s⁻¹ and ~2.29 × 10⁻³ mm s⁻¹). The proposed polymer–metal RMI is more suitable for the modification of C/C preforms with thin-wall structures owing to its advantages including precise control of infiltration dose and flexible operation of slurry coating. Furthermore, it is suitable for the local modification of C/C components.     

Investigation on properties of Al2O3–SiO2 complex shaped refractory fabricated by layered extrusion forming

As one of the 3D printing methods, layered extrusion forming (LEF) has distinct advantages to form complex configuration ceramics directly. The feasibility of using LEF to make refractory products with complex shapes was explored by this work, using water-based Al₂O₃–SiO₂ ceramic slurry and specially equipped device. By measuring rheological parameters, the effects of binder addition, dispersant addition and volume proportion of the solid portion composed of α-Al₂O₃ ultrafine powder (92 wt%) and silica fume (8 wt%) on rheological behavior of the slurry were investigated. The green body specimens prepared by the LEF were fired at 1400°C–1600 °C for 3h. The influence of firing temperature on phase composition, microstructure, sintering degree and comprehensive properties of the specimens was investigated. At 2.5 wt% addition of aluminum dihydrogen phosphate as binder, 0.2 wt% addition of sodium hexametaphosphate as dispersant and with solid portion between 56 vol% and 58 vol%, required pseudoplastic behavior of the slurry can be achieved, suitable for the LEF. With the increase of heating temperature, mullitization by the reaction between the α-Al₂O₃ ultrafine powder and silica fume becomes stronger and sintering gets enhanced, leading to improved comprehensive properties of the specimens. Fired at 1600 °C, properties in terms of bulk density 3.03g/cm³, cold compressive strength 190.5 MPa and refractoriness under load 1598 °C are achieved. Crucible slag test shows a good resistance to the glass melt corrosion. Good feasibility of fabricating some complex shaped refractory products by LEF as a novel forming approach has been confirmed by the present work.     

Processing of alumina‐coated glass‐bonded silicon carbide membranes for oily wastewater treatment

Crack‐free γ‐Al₂O₃‐coated glass‐bonded SiC membranes were successfully prepared using a simple heat‐treatment and dip‐coating process at a temperature as low as 850°C in air. The changes in the porosity, flexural strength, flux, and oil rejection rate of the membranes were investigated while changing the initial SiC particle size. Larger SiC particles led to bigger pores, resulting in higher flux in the oily water and a lower oil rejection rate. The SiC membranes with a support prepared from 10 μm SiC powder showed an exceptionally high oil rejection rate (99.9%) with a feed oil concentration of 600 mg/L at an applied pressure of 101 kPa. The typical porosity, flexural strength, steady state flux, and oil rejection rate of the alumina‐coated SiC membrane were ~45%, ~81 MPa, 1.78×10^?5 m³ m^?2s^?1, and 99.9%, respectively.     

Observation of Particle Motion in High‐Concentration Ceramic Slurries Under Low Shear Rate

The dispersed state of particles in high‐concentration slurries has a significant effect on the development of the particle packing structure of the powder compact. We observed individual particle motion directly in highly concentrated ceramic slurries under slow flow through confocal laser scanning fluorescent microscopy using transparent slurries. In particular, we focused on particle motion soon after the application of a constant low shear rate. Measurements of the shear stresses indicated that the stress of a 30 vol% slurry gradually increased from 0.018 Pa over time, whereas the stress of a 40 vol% slurry soon reached 0.03 Pa and remained constant. For direct observation, shear stress was applied to the slurry, which was set between two glass plates, by moving one unilateral glass plate at 0.8 μm/s, to create a shear rate of 0.01 s^?1. The motion of the particles could be observed continuously. In the early stage, particles in the 30 vol% slurry near the moving plate were united as one body and did not alternate positions. Then, a speed incline developed gradually over time. For the 40 vol% case, the particles moved at the same time and together. Increases in the shear stress were caused by increased interaction when the particles were close together.     

Instant precipitation of KMgF3:Ni2+ nanocrystals with broad emission (1.3‐2.2 μm) for potential combustion gas sensors

Optical gas sensors present fundamental and industrial importance considering their broad applications. Challenges remain to obtain new photonic materials with broadband emission covering the absorption spectrum of typical combustion gases. Here, broadband near‐infrared (NIR) photoluminescence (PL) spanning the wide absorption spectrum of typical combustion products is realized through instant precipitation of stable cubic perovskite KMgF₃:Ni²⁺ nanocrystals inside an aluminosilicate glass matrix after melt‐quenching. Excited by an 808 nm laser diode, NIR luminescence with a peak centered at ~1624 nm and a bandwidth (FWHM) greater than 315 nm is observed, originating from ³T_2g(³F) → ³A_2g(³F) electronic transition of octahedral coordinated Ni²⁺ in KMgF₃ GC. Controlled precipitation of these perovskite crystals from a supercooled aluminosilicate melt enables immediate encapsulation and, hence, stabilization in an inorganic glass phase. While the precipitation temperature has only a small effect on crystallite size, it controls the redox state of the melt and the degree of dopant incorporation into the crystalline phase so that PL performance can be optimized. Spontaneous crystallization of perovskite nanocrystals inside glass may offer a new way to stabilize these novel nanocrystals. Moreover, spontaneous crystallization can be attractive in the control of activator partitioning and in the fabrication of composite fiber devices with high transparency and emission gain. In the present case, this offers a potential platform for broadly tunable gain media, for example, for combustion gas sensing.