Melter Operation Research Articles

In-line detection of salt formation during vitrification using millimeter wave radiometry and interferometry

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 (Na2SO4), 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.

Conversion degree and heat transfer in the cold cap and their effect on glass production rate in an electric melter

AbstractA predictive model of melt rate in waste glass vitrification operations is needed to inform melter operations during normal and off‐normal operations. This paper describes the development of a model of the cold cap (the reacting melter feed floating on molten glass in a glass melter) that couples heat transfer with the feed‐to‐glass conversion kinetics. The model was applied to four melter feeds designed for high‐level and low‐activity nuclear waste feeds using the material properties, either measured or estimated, to obtain temperature and conversion distribution within the cold cap. The cold cap model, when coupled with a computational fluid dynamics model of a Joule‐heated glass melter, allows the prediction of the glass production rate and power consumption. The results show reasonable agreement with the melting rates measured during pilot‐scale melter tests.

Impacts of temperature on sulfur solubility in low‐activity waste glasses

AbstractSolubility of sulfur in glass melts has been studied to estimate the maximum sulfur tolerance in nuclear waste glass melter operations. Sulfur solubility data at various temperatures are needed to investigate the possible sulfate segregation in different stages of the melting process. In this work, a crucible salt saturation method was applied to measure the sulfur solubility in three low‐activity glasses at different temperatures ranging from 950°C to 1200°C. The results show that temperature has little impact on sulfur solubility, with one exception that the sulfur solubility increases slightly from 950°C to 1100°C. The temperature effect of solubility data using SO2/O2 gas and sulfate salt as the sulfur source are compared. The decreased melter tolerance with decreasing temperature is likely caused by multiple factors including sulfur solubility.

Effect of cold cap coverage and emissivity on the plenum temperature in a pilot‐scale waste vitrification melter

AbstractIntegrated computational fluid dynamics (CFD) models are being developed to model the complex physics occurring within the high‐level waste melter for vitrification of legacy tank waste at the Hanford site. This study presents a validation of the integrated CFD model by using data from two experimental runs in a pilot‐scale melter. While the model uses several simplifying assumptions (such as constant heat sinks from a cooling jacket and inleakage of ambient air, steady state feed‐to‐batch conversion heat, and a cold cap model with a simplified shape), it closely predicts the molten glass (1150°C and 1175°C) and plenum temperatures (550°C) obtained from thermocouples during two pilot‐scale tests, with an average cold cap coverage of 80%. Additional simulations were performed to explore the sensitivity of the predicted plenum temperatures to variations in cold cap coverage (fraction of melt surface covered by the glass batch) and batch emissivity. The plenum temperature was found to be in the range of 606°C when cold cap coverage decreased from 95% to 70%. Cold cap emissivity had a smaller effect, increasing the plenum temperature by as much as 179°C when cold cap emissivity increased from 0.2 to 0.8. Maintaining a high cold cap coverage without overfeeding is important for a sustained melter operation with high glass throughput. This work provides a tool for achieving that goal in terms of correlating the plenum temperature with the cold cap coverage.

Sulfur solubility in low activity waste glass and its correlation to melter tolerance

AbstractHanford low‐activity waste (LAW) glasses with high sulfur concentrations are subject to salt segregation in the melter, which hinders melter operation by corroding components and shortening the melter life. To better predict the point at which salt accumulates on the melt surface, the development of sulfate solubility models is needed. Using a sulfur saturation method, crucible scale melts for 13 LAW glasses with varying sulfur solubilities were conducted. The resulting salt and glass compositions were reported and the change in component partitioning following the saturation process was examined to better understand potential changes in overall glass composition. It was shown that both Cr2O3 and Cl experience significant losses, with ~28% of Cr2O3 partitioning into the salt phase and Cl primarily volatilizing out of the melt (~23% partitioned to salt and ~40% lost as off gas). These patterns can be accounted for during model development. Measured sulfur solubilities were also compared to previously reported data. It was found that crucible sulfur solubility ranged from 0.95 to 2.14 wt% SO3 with a high correlation () between crucible solubility and melter tolerance. These results suggest that crucible scale sulfate solubility data can be used to predict SO3 tolerance in the melter feed.

Micron-sized spinel crystals in high level waste glass compositions: Determination of crystal size and crystal fraction

The compositions utilized for immobilization of high-level nuclear wastes (HLW) are controlled using glass property models to avoid the deleterious effects of crystallization in the high-level waste (HLW) vitrification melters. The type and size of the crystals that precipitate during melter operations (typically at 1150 °C) and idling (∼1000 °C) are significantly impacted by glass composition and thermal history. This study was conducted to measure the impact of melt composition and heat treatment temperature on crystal size and fraction. A matrix of 31 multi-component glasses canvasing the expected Hanford HLW compositional space was developed and the glasses fabricated and heat treated at 850, 900, and 950 °C. The crystal amounts, as determined by X-ray diffraction, varied from 0.2 to 41.0 wt%. Spinel concentrations ranged from 0.0 to 13.8 wt%. One glass of the matrix did not precipitate spinel and contained 0.2 wt% RuO2, which was assumed to be undissolved from the melting process. All compositions contained crystals in the as-quenched glass. All of the spinel based crystals present in the glasses were less than 10 μm in diameter, as determined by scanning electron microscopy with image analysis. Composition and temperature dependent models were generated using the resulting data and the best model fit was obtained by only considering spinel concentrations (R2 = 0.87). Two glasses were unable to be characterized because of an inability to process the glass under the conditions of this study. Those glasses were utilized to give insight into a potential multi-component constraint to aid in future statistical composition designs.

Effect of Na2O on aqueous dissolution of nuclear waste glasses

Sodium oxide is present in the majority of commercial and waste glasses as a viscosity-reducing component. In some nuclear waste glasses, its source is the waste itself. As such, it can limit the waste loading because of its deleterious effect on the resistance of the glass to attack by aqueous media. The maximum tolerable content of Na2O in glass depends on the presence and concentration of components that interact with it. To assess the acceptability limits of Na2O in the composition region of nuclear waste glasses, we formulated 11 baseline compositions by varying the content of oxides of Si, B, Al, Ca, Zr, and Li. In each of these compositions, we varied the Na2O fraction from 8–16 mass% to 23–30 mass%. To each of 146 glasses thus formulated, we applied the seven-day Product Consistency Test (PCT) to determine normalized B and Na releases (ri, where i ≡ B or Na). Fitting approximation functions ln(ri/gm−2) = Σbijgj to ri data (gj is the j-th component mass fraction and bij the corresponding component coefficient), we showed that the rB (and, consequently, the initial glass alteration rate) was proportional to the glass component mass fractions in the order Al2O3<CaO<SiO2<ZrO2<B2O3<Na2O<Li2O. No threshold was detected at which glass structure would fall apart or beyond which a continuous nondurable phase would be separated. Specific examples are given to demonstrate restrictions imposed on the boundary of the composition region of acceptable glasses by the maximum allowable rB and by the melt viscosity required for glass melter operation. Finally, the role that PCT data may play in understanding the evolution of the glass alteration process is discussed.

ICONE23-1419 THE ACHIEVEMENT OF THE STABLE K-MELTER OPERATIONS IN ACTIVE TESTS AT ROKKASHO REPROCESSING PLANT

The KA facility in Rokkasho Reprocessing Plant started the active tests to solidify HAW into the glass in 2007 which was the examination of the final stage before the operation, but the active test had to be discontinued due to the trouble of glass melter operation with down of pouring by deposit of noble metals on the melter bottom. After the K-melter equipment and operating conditions were improved in response to the result of the mock-up tests, a series of active tests were restarted including pre-confirmation tests in May, 2012 by operating each one of two K-melters for the purpose of confirming the stable condition before having the pre-use inspection. One year later, these tests were finished with enough confirmation of stability in the state such as glass temperature and controlling the noble metals, and also confirmation of the processing capacity. This paper gives the successful situation in latest active tests of the KA facility by processing the actual waste including the implementation contents in the full scale mock-up tests using KMOC-melter that examined to identify the cause and measure for the troubles in K-melter operation.

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 &gt; V2O5 &gt; CaO ≈ P2O5 &gt; Na2O ≈ B2O3 &gt; K2O. The components that decrease sulfur solubility most are Cl &gt; Cr2O3 &gt; Al2O3 &gt; ZrO2 ≈ SnO2 &gt; Others (i.e., the sum of minor components) ≈SiO2. The order of component effects is similar to previous literature data, in most cases.

Towards Increased Waste Loading in High Level Waste Glasses: Developing a Better Understanding of Crystallization Behavior

A number of waste components in US defense high level radioactive wastes (HLW) have proven challenging for current Joule heated ceramic melter (JCHM) operations and have limited the ability to increase waste loadings beyond already realized levels. Many of these “troublesome” waste species cause crystallization in the glass melt that can negatively impact product quality or have a deleterious effect on melter processing. Recent efforts at US Department of Energy laboratories have focused on understanding crystallization behavior within HLW glass melts and investigating approaches to mitigate the impacts of crystallization so that increases in waste loading can be realized. Advanced glass formulations have been developed to highlight the unique benefits of next-generation melter technologies such as the Cold Crucible Induction Melter (CCIM). Crystal-tolerant HLW glasses have been investigated to allow sparingly soluble components such as chromium to crystallize in the melter but pass out of the melter before accumulating. The Hanford site AZ-101 tank waste composition represents a waste group that is waste loading limited primarily due to high concentrations of Fe2O3 (with higher Al2O3). Systematic glass formulation development utilizing slightly higher process temperatures and higher tolerance to spinel crystals demonstrated that an increase in waste loading of more than 20% could be achieved for this waste composition, and by extension higher loadings for wastes in the same group.

Cold crucible induction melter studies for making glass ceramic waste forms: A feasibility assessment

Glass ceramics are being developed to immobilize fission products, separated from used nuclear fuel by aqueous reprocessing, into a stable waste form suitable for disposal in a geological repository. This work documents the glass ceramic formulation at bench scale and for a scaled melter test performed in a pilot-scale (∼1/4 scale) cold crucible induction melter (CCIM). Melt viscosity, electrical conductivity, and crystallization behavior upon cooling were measured on a small set of compositions to select a formulation for melter testing. Property measurements also identified a temperature range for melter operation and cooling profiles necessary to crystallize the targeted phases in the waste form. Bench scale and melter run results successfully demonstrate the processability of the glass ceramic using the CCIM melter technology.

Nuclear waste vitrification efficiency: Cold cap reactions

Batch melting takes place within the cold cap, i.e., a batch layer floating on the surface of molten glass in a glass-melting furnace. The conversion of batch to glass consists of various chemical reactions, phase transitions, and diffusion-controlled processes. This study introduces a one-dimensional (1D) mathematical model of the cold cap that describes the batch-to-glass conversion within the cold cap as it progresses in a vertical direction. With constitutive equations and key parameters based on measured data, and simplified boundary conditions on the cold-cap interfaces with the glass melt and the plenum space of the melter, the model provides sensitivity analysis of the response of the cold cap to the batch makeup and melter conditions. The model demonstrates that batch foaming has a decisive influence on the rate of melting. Understanding the dynamics of the foam layer at the bottom of the cold cap and the heat transfer through it appears crucial for a reliable prediction of the rate of melting as a function of the melter-feed makeup and melter operation parameters. Although the study is focused on a batch for waste vitrification, the authors expect that the outcome will also be relevant for commercial glass melting.

Numerical Analysis of Platinum Group Particle Behavior in Vitrification Melter

In the vitrification of high-level radioactive liquid wastes, platinum group particles originally included in the liquid wastes are mixed into molted glass and convected with the glass flow in vitrification melters. Though behaviors of the platinum group particles in the melters should be investigated to establish efficient melter operation methods, difficulties in observations or measurements of the particles in the actual melters due to high radioactivity have prevented enough investigation. In this paper, we present a numerical analysis method to evaluate the behaviors of the particles in the melters. Since the thermal convection field of the molten glass, electrical potential and particle distribution are formed by the interaction of the particles in the melters, we coupled these physical behaviors in the numerical method to evaluate the melter status with high accuracy. A numerical simulation was conducted and transient behaviors of the particles in the melters were determined. The results of simulation for ten batches of operation (each batch consists of melting, bottom heating and discharging operations) showed that an effective particle discharge can be achieved, as observed in mock-up experiments. These results imply that our numerical method can evaluate transient behaviors of the platinum group particles in vitrification melters.

Electromagnetic and thermal-flow modeling of a cold-wall crucible induction melter

An approach for modeling cold-wall crucible induction melters is described. Materials in the melt and melter are nonferromagnetic. In contrast to other modeling studies reported in the literature, the numerical models use commercial codes. The ANSYS finite-element code[1] is employed for electromagnetic field simulations and the STAR-CD finite-volume code[2] for thermal-flow calculations. Results from the electromagnetic calculations in the form of local Joule heat and Lorentz force distributions are included as source terms in the thermal-flow analysis. This loosely coupled approach is made possible by the small variation in temperature and, consequently, small variation in electrical properties across the melt as well as the quasi-steady-state nature of the thermal-flow calculations. A three-dimensional finite-element grid for electromagnetic calculations is adapted to a similar axisymmetric finite-volume grid for data transfer to the thermal-flow model. Results from the electromagnetic model compare well with operational data from a 175-mm-diameter melter. REsults from the thermal-flow simulation provide insight about molten metal circulation patterns, temperature variations, and velocity magnitudes. Initial results are included for a model that simulates the formation of a solid (skull) layer on the crucible base and wall. Overall, the modeling approach is shown to produce useful results that relate operational parameters to the physics of steady-state melter operation.

Foaming in simulated radioactive waste.

Radioactive waste treatment process usually involves concentration of radionuclides before waste can be immobilized by storing it in stable solid form. Foaming is observed at various stages of waste processing like SRAT (sludge receipt and adjustment tank) and melter operations. This kind of foaming greatly limits the process efficiency. The foam encountered can be characterized as a three-phase foam that incorporates finely divided solids (colloidal particles). The solid particles stabilize foaminess in two ways: by adsorption of biphilic particles at the surfaces of foam lamella and by layering of particles trapped inside the foam lamella. During bubble generation and rise, solid particles organize themselves into a layered structure due to confinement inside the foam lamella, and this structure provides a barrier against the coalescence of the bubbles, thereby causing foaming. Our novel capillary force balance apparatus was used to examine the particle-particle interactions, which affect particle layer formation in the foam lamella. Moreover, foaminess shows a maximum with increasing solid particle concentration. To explain the maximum in foaminess, a study was carried out on the simulated sludge, a non-radioactive simulant of the radioactive waste sludge at SRS, to identify the parameters that affect the foaming in a system characterized by the absence of surface-active agents. This three-phase foam does not show any foam stability unlike surfactant-stabilized foam. The parameters investigated were solid particle concentration, heating flux, and electrolyte concentration. The maximum in foaminess was found to be a net result of two countereffects that arise due to particle-particle interactions: structural stabilization and depletion destabilization. It was found that higher electrolyte concentration causes a reduction in foaminess and leads to a smaller bubble size. Higher heating fluxes lead to greater foaminess due to an increased rate of foam lamella generation in the sludge system.

Development of glasses for the vitrification of high level liquid waste (HLLW) in a joule heated ceramic melter

A vitrification process was developed at Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgungstechnik (INE), for solidifying in borosilicate glasses High Level Waste (HLW) solutions from the nuclear fuel cycle. To optimise melter operation the glass melt should have a flat viscosity curve and a relatively high specific electrical resistance of ≤6.5 ω·cm at 1150°C. Further requirements are: no liquid-liquid immiscibility and no crystallization of the glass, waste loading ≥15 wt% and, in view of repository storage of the HLW glass, a chemical durability comparable to that of other HLW glasses. The main emphasis of experimental work was put on finding out how the viscosity, the slope of the viscosity curve, the specific electric resistance and the chemical durability depend on the chemical composition of the glasses. Especially, the effect of the mixed alkalis Li and Na on the glass properties was studied. It was found that by increasing from 0 to 1 the Li 2O/(Na 2O + Li 2O) molar ratio of the glass FRIT WAW, the viscosity of the melt decreases roughly linearly and the slope of the viscosity curve decreases as well. The specific electric resistance passes through a maximum and the Soxhlet leach rate through a minimum at an alkali ratio of about 0.5. As a final result, a range of optimum glass compositions was determined which meet the required properties.

The Platinum Metals in Hllw — Glass Products

ABSTRACTThe behaviour of the platinum metals in nuclear waste glass produced in a lab scale liquid fed ceramic melter was investigated. The platinum metals form two kinds of metallic conductive particles in the glass melt: RuO2 (preferentially needle shaped) and a Pd—Rh—Te alloy (tear—shaped). These phases decrease the electrical resistivity of the glass melt. Resistivities were measured at 950º and 1150 ºC. Powdered glass was used as starting material. For the same temperatures resistivities were calculated applying an equation derived for an embedment structure of highly conductive particles in a matrix of low conductivity. To match calculated and measuredresistivities one must assume needle—shaped particles. This gives evidence that mainly RuO2 contributes to the observed decrease in resistivity. If the measurements are started with glass blocks instead of glass powder, remarkably lower resistivities are measured in the melt. A comparison of both results indicates a partial destruction of particles or particle contacts by grinding. Thus, resistivities measured with ground samples are not representative of those effective in a melter.The investigation of solid glass samples yielded the following results: Within a few hours, Pd—Rh—Te and RuO2 particles form fine sediments in the melter. The development of stable coarse sediments needs some 1000 h of melter operation. The Pd—Rh—Te alloy particles reach a mean diameter of 5 μm in the fine sediment and of 20 μm in the coarse one. With time, the alloy dissolves more Rhand Te. Finally, the alloy separates into two phases. RuO2 dissolves Cr and Rh.