A Better Understanding for Cold-Cap Reactions of Nuclear Waste Feeds through Quantitative Evolved Gas Analysis

In the vitrification of nuclear wastes, waste slurry mixed with glass former chemicals (melter feed) is charged into an electrical glass-melting furnace (a melter) where it becomes a cold-cap, a layer of reacting melter feed floating on a pool of molten glass. Within the cold cap, the feed experiences multiple gas-evolving reactions. Understanding these reactions is critical because the reaction enthalpy and the generation of foam significantly influence the mass and heat transfer within the cold-cap, thus affecting the melting rate. A kinetic model based on thermogravimetric analysis (TGA) provides invaluable insight into the cold-cap reactions, but it does not identify their chemistry. We employed the thermogravimetry-gas chromatography–mass spectrometry (TGA-GC–MS) combination to perform qualitative and quantitative evolved gas analysis (EGA) for the cold-cap reactions. By adding the ‘chemical identity’ to the TGA-based kinetic model, the EGA allows us to recognize possible reactions/reactants over the temperature range of the feed-to-glass conversion. The quantitative analysis was achieved by relating the weighted sum of intensities of individual gases in linear proportion with the differential thermogravimetry. The proportionality coefficients were obtained by three methods based on the stoichiometry, least squares, and calibration. The linearity was shown to be a good first-order approximation, in spite of the complicated overlapping reactions.