Remediation on off-gas system deposits in a radioactive waste glass melter

Since the early 1980's, research glass melters have been used at the Savannah River Laboratory (SRL) to develop the reference vitrification process for immobilization of high level radioactive waste. One of the operating concerns for these melters has been the pluggage of the off-gas system with solid deposits. Samples of these deposits were analyzed to be mixture of alkali-rich chlorides, sulfates, borates, and fluorides with entrained Fe2O3 spinel, and frit particles. The spatial distribution of these deposits throughout the off-gas system indicates that they form by vapor-phase transport and subsequently condensation. Condensation of the alkali-rich phases cements entrained particulates causing the off-gas line to plug. It is concluded that off-gas system pluggage can be effectively controlled by maintaining the off-gas velocity above 16 m/s, while maintaining the off-gas temperature as high as practical below the glass softening point. This paper summarizes the results of chemical and physical analyses of off-gas deposit samples from various melters at SRL. Recent design changes made to the Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS) to alleviate the pluggage problem are also discussed