During the vitrification of radioactive waste in a Joule‐heated melter, aqueous melter feed slurry forms a cold cap, a reacting and melting material, which floats on the surface of the molten glass. The rheological behavior of the feed affects cold cap formation and shape, and is vital for modeling the feed‐to‐melt conversion process. We used slurry feed simulant and fast‐dried slurry solids representing the cold cap to investigate the rheological behavior of the feed as it transforms into glass. Both low‐temperature and high‐temperature rheometry were performed and a new scheme was applied to estimate the feed viscosity. This study shows that the conversion advances in four sequential stages that form distinct regions in the cold cap: (i) a fast‐spreading boiling slurry from which water evaporates, (ii) a porous solid region (viscosity > 108 Pa s) containing reacting solids and molten salts, (iii) a plastic region in which glass‐forming melt connects the refractory solids (~108 to ~106 Pa s), and (iv) a viscous foam layer in which the viscosity drops from ~105 to ~101 Pa s. The implications for the mathematical modeling of the cold cap are discussed.
The effective heat conductivity (k) 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 k 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.
Electrical contact pastes of composition (La 0.90 Sr 0.10 ) 0.98 MnO 3 + δ (LSM-10) formed strong bonds (∼3 MPa) to (Co,Mn) 3 O 4 spinel-coated Crofer 22 APU ferritic steel coupons when exposed to alternating flows of air and nitrogen (10 ppm O 2 ) at 900 • C for 2 h or longer. When held at 900 • C in air only, bond strengths were negligible. Substantial bonds could also be created between LSM-10 contact paste and (La 0.80 Sr 0.20 ) 0.98 MnO 3 + δ (LSM-20) porous cathodes by processing in alternating air and nitrogen, without simultaneous densification of the cathode. Enhanced sintering of LSM-10 is attributed to transients in the defect structure induced by oxygen partial pressure changes.
Exposure of (La0.90Sr0.10)0.98MnO3+δ (LSM‐10) to repeated oxygen partial pressure cycles (air/10 ppm O2) resulted in enhanced densification rates, similar to behavior shown previously due to thermal cycling. Shrinkage rates in the temperature range 700°–1000°C were orders of magnitude higher than Makipirtti–Meng model estimations based on stepwise isothermal dilatometry results at a high temperature. A maximum in enhanced shrinkage due to oxygen partial pressure cycling occurred at 900°C. Shrinkage was the greatest when LSM‐10 bars that were first equilibrated in air were exposed to gas flows of lower oxygen fugacity than in the reverse direction. The former creates transient cation and oxygen vacancies well above the equilibrium concentration, resulting in enhanced mobility. These vacancies annihilate as Schottky equilibria are reestablished, whereas the latter condition does not lead to excess vacancy concentrations.
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.
Intermediate‐stage sintering of lanthanum strontium manganite (LSM, where Sr=0.00, 0.05, 0.10, 0.15, and 0.20) was shown in dilatometry studies to be accelerated when subjected to alternating flows of air and nitrogen. The extent of rate enhancement decreased with increased Sr content, and decreased with increased temperature, which coincides with diminished oxygen nonstoichiometry. Shrinkage rates were further shown to be sensitive to the difference in oxygen content in the alternating gas flows. Baseline air sintering rates were measured using stepwise isothermal dilatometry, from which kinetic parameters were calculated using the Makipirtti–Meng model. Activation energies for sintering in air were determined to be 255 ± 26, 258 ± 28, 308 ± 32, 373 ± 37, and 417 ± 41 kJ/mol for Sr=0.0, 0.05, 0.10, 0.15, and 0.20, respectively. A diffusion‐based model is proposed that is consistent with trends in accelerated shrinkage versus temperature. Transient cation vacancy gradients, which lead to higher cation mobility, were calculated from established oxygen diffusivities and oxygen nonstoichiometry as a function of temperature and time. A potential application of this approach is the processing of LSM‐based cathode‐side contact pastes in solid oxide fuel cells.
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