PNNL

Abstract

The path for immobilizing low-activity waste (LAW), a mixture of salts, for long-term storage is adding glass-forming and modifying additives and minerals to the solution and vitrifying the resulting slurry in an electric melter. Technetium-99 (99Tc), a long-lived radionuclide present in LAW, is highly volatile at the elevated temperatures in a glass melt. An off-gas system was designed for capturing volatile 99Tc or Re, its nonradioactive surrogate, during continuous charging of a LAW melter feed in a laboratory-scale melter (LSM). This system allowed observation of the cold cap, the layer of reacting feed on top of the molten glass pool, during charging of the melter feed and identifying the quasi-steady state time period at which the cold cap maintained a consistent size and coverage. The Re retention in the glass was determined via the measured amount of volatilized Re in the off-gas. This study was performed in preparation for the LSM system to be applied for melter feeds containing either a LAW simulant spiked with 99Tc or an actual tank waste.