PNNL

Abstract

During the vitrification of radioactive waste in a Joule-heated melter, aqueous melter feed slurry forms a cold cap, a reacting and melting material, which floats on the surface of the molten glass. The rheological behavior of the feed affects cold cap formation and shape, and is vital for modeling the feed-to-melt conversion process. We used slurry feed simulant and fast-dried slurry blocks representing the cold cap to investigate the rheological behavior of the feed as it transforms into glass. Both low-temperature and high-temperature rheometry were performed and applied to a new scheme for obtaining rheological responses for the feed with ongoing reactions leading to the estimation of feed viscosity. This study shows that the conversion advances in four sequential stages that form distinct regions in the cold cap: (i) a fast-spreading boiling slurry from which water evaporates, (ii) a porous solid region (viscosity > 108 Pa s) containing reacting solids and molten salts, (iii) a plastic region in which glass-forming melt connects the refractory solids (~108 to ~106 Pa s), and (iv) a viscous foam layer in which the viscosity drops from ~105 to ~101 Pa s. The implications for the mathematical modeling of the cold cap are discussed.