The Chemistry and Physics of Melter Cold Cap

Vitrification converts high-level waste and Hanford low-activity waste to glass. In electric melters, electric power is dissipated within a pool of molten glass, from which the heat is transferred, mostly by natural convection, to the cold cap, a floating layer or pile of unmelted and partially melted feed. The cold cap reduces volatilization, melt-line corrosion, and foaming, but also may result in a slow and unsteady melting as well as sulfate segregation. These problems, which are not necessarily rooted in fundamental material issues, (and thus can be mitigated or avoided), cause decreased melting efficiency and increased operational costs. Slow melting can be caused by bubbles that ascend through the melt and accumulate under the cold cap, creating there a low-density and low-conductivity layer of cold foam that is virtually motionless and effectively hinders heat transfer. The analysis and modeling of cold-cap reactions and the associated reactions in the melt underneath the cold cap, when approached in their full complexity, can be accomplished by expanding the basic field equations to incorporate the cold cap to existing melter models and by applying the recent development experience in numerical methods to solve complex equations. For such an analysis to be successful, cold-cap behavior must be characterized by accurate data. Evolved-gas-analysis data and quantitative x-ray diffraction data of Savannah River Macrobatch 3 with Frit 200 are shown as an example of such a database. In field equations, measured data are represented by rate equations and other response functions.