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.

► Batch melting is a significant and yet the least understood step in glass processing.
► Batch melting occurs within the cold cap floating on the surface of molten glass.
► Our study presents a one-dimensional mathematical model of the cold cap.
► The model relates the cold cap thickness to heat fluxes from above and from below.
► Foaming under the cold cap has a decisive influence on the rate of melting.