Nuclear waste vitrification efficiency: Cold cap reactions

a r t i c l e i n f o 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. The cold cap, or batch blanket, is the layer of glass batch floating on molten glass in an electrical glass-melting furnace (a melter). In melters producing commercial glasses, the batch is typically spread in a layer of uniform thickness on the whole top surface area of the melt. In melters for nuclear waste glass, the melter feed, typically a slurry containing 40–60% water, is charged through one or more nozzles. The cold cap covers 90–95% of the melt surface. Mathematical models of melters have been well developed [1–5] in all aspects except for the batch melting, which has rarely been addressed in other than a simplified manner [6–12]. This work presents an initial step toward the modeling of a cold cap in a melter for high-level-waste glass while taking advantage of the availability of data for the key properties and the reaction kinetics for a high-alumina melter feed [13–16] considered for the Waste Treatment and Immobilization Plant, currently under construction at the Hanford Site in Washington State, USA. The 1D model is based on the ideas by Hrma [9] and Schill [10,11]. Though the 1D modeling greatly simplifies mathematical treatment , …