PNNL

Abstract

The process of converting high-level waste feed to an immobilized glass form takes place within a cold cap which covers a high-temperature (1150°C) glass melt in a Joule or induction heated melter. Liquid slurry feed is continuously charged through the top of a melter. Within the cold cap, many glass forming reactions occur and the glass-forming melt becomes connected. Gases trapped in the glass-forming melt arrange into bubbles which create a layer of foam below the reacting feed that collapses into cavities. This foam layer thermally insulates the reacting feed limiting heat transfer from the molten glass below, thus affecting the rate of glass formation. Information about the glass formation is desired for incorporation in a mathematical model designed to simulate the melting in a cold cap. To explore this, a set of high-level waste feed simulant samples were heat-treated at 5 K min-1 to temperatures ranging from 400°C to 1200°C for comparison with cold cap sections generated in a laboratory-scale melter. To estimate the temperature distribution in laboratory-produced cold caps, structural (bubble size and shape) and optical (color) properties of heat-treated samples and cold-cap sections were compared along with elemental maps, the shapes and sizes of the silica particles, and the connectivity of the glass matrix. These results will be used to verify the recently developed mathematical model of the cold-cap.