Temperature Distribution within a Cold Cap during Nuclear Waste Vitrification

Abstract

The kinetics of the feed-to-glass conversion affects the waste vitrification rate in an electric glass melter. The primary area of interest in this conversion process is the cold cap, a layer of reacting feed on top of the molten glass. The work presented here provides an experimental determination of the temperature distribution within the cold cap. Because direct measurement of the temperature field within the cold cap is impracticable, an indirect method was developed in which the textural features in a laboratory-made cold cap with a simulated high-level waste feed were mapped as a function of position using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The temperature distribution within the cold cap was established by correlating microstructures of cold-cap regions with heat-treated feed samples of nearly identical structures at known temperatures. This temperature profile was compared with a mathematically simulated profile generated by a cold-cap model that has been developed to assess the rate of glass production in a melter.

Modeling batch melting: Roles of heat transfer and reaction kinetics

“…In addition, Guillen et al reported that the thickness of these cavities sliding along the base of the batch blanket is approximately 6 mm. This well agrees with the earlier theoretical value of 6.6 mm reported by Pokorny and Hrma, and with the ~ 0.5−1.0 cm sized bubbles escaping at the sides of a cold cap in a laboratory‐scale melter vessel and visible at the cold cap‐melt boundary when quenching the laboratory‐scale melter cold cap . Guillen et al also concluded that the moving cavities enhance the heat transfer by stirring the melt in the thermal boundary layer, which is, as will be shown below, of a similar thickness.…”

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

“…We used a melter feed for a simulated high‐alumina HLW glass (Table ) that has been characterized in previous studies . The feed was prepared as slurry, as described in a previous paper .…”

“…The feed‐to‐glass conversion occurs in the cold cap—a layer of reacting feed floating on the pool of molten glass . As the feed moves down through the cold cap from the top, where the temperature can be as low as 100°C, to the bottom, where the temperature typically reaches to 1100°C, it is converted to molten glass through complex chemical reactions and phase transitions . Feed‐to‐glass conversion usually consists of five stages: (i) evaporation and release of water, (ii) melting and decomposition of oxy‐ionic salts, including mainly carbonates and nitrates/nitrites, (iii) formation of an early glass‐forming (borate) melt and intermediate crystalline phases, (iv) dissolution of refractory particles, such as quartz and zirconia, and (v) expansion and collapse of foam…”

“…The reaction kinetics of feed‐to‐glass conversion was investigated using thermal analysis techniques including differential scanning calorimetry‐thermogravimetric analysis (DSC‐TGA) and gas chromatography‐mass spectrometry (GC‐MS) . Further information regarding feed reactions and phase transitions that cannot be identified with those techniques were investigated using laboratory‐scale melter tests and crucible tests . Detailed understanding of all stages of feed‐to‐glass conversion as they occur over the temperature range in the cold cap is needed for advanced melter feed formulation to enhance the glass production rate …”

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

“…Glass‐forming melt is a product of reactions of feed components, such as amorphous hydroxides and oxyhydrates, first with borates, and then with quartz and silicates. Intermediate crystalline phases, such as hematite, various spinels, nepheline, or crystals with sodalite structure, are common …”