Study on Variation in Resistance with a Change in Sodium Concentration (Na2O) During the Vitrification of Radioactive Liquid Waste (RLW) in Joule Heated Ceramic Melter

The Radioactive Liquid Waste (RLW) generated during the reprocessing of spent nuclear fuel from commercial nuclear power reactors is conditioned by immobilizing it in an inert glass matrix by a process called “Vitrification.” Borosilicate-based glass is the matrix of choice due to its long-term chemical durability and stability under a high radiation field. Vitrification is carried out in glass melter. In India, the vitrification of RLW is being carried out in a Waste Immobilization Plant (WIP) using Joule Heated Ceramic Melter (JHCM). RLW being conditioned in WIP contains, in addition to long and short-lived fission products, inactive process chemicals like nitric acid, sodium nitrate, cladding dissolved material like zirconium, corrosion products, and high concentrations of sodium. Sodium borosilicate glass in the form of glass beads with the composition of SiO2—48%, B2O3—26.5%, Na2O—11.5%, TiO2—9.5%, and Fe2O3—4.5% is used as host glass matrix. The alkali modifier Na2O behaves like bond breakers in the silica network, decreasing the viscosity and pouring temperature. JHCM incorporates a process of heating the glass by conducting electric current through molten glass. Glass starts conducting electrically at elevated temperatures of above 650 °C. As the resistance of molten glass is used for Joule heating, its specific conductivity plays a vital role in determining the power fed to the melter. In this study, variation in resistance of molten glass pool is determined for sodium concentration in the melter. Na2O content from the waste oxide brought down the resistance in the melter, which resulted in the fall of pouring temperature of the base glass frit. Conditioning monitoring of melter operation is monitored based on Na2O and resistance across the electrode. Based on this experience, real-time monitoring of melter transients was ascertained, and correlations were developed for variation of sodium concentration on electrical conductivity and viscosity.