Vitrification of nuclear-contaminated HEPA filter media: A study on the viscosity-component correlation and the volatilization of simulated radionuclides

High-efficiency particulate air (HEPA) filters are commonly used in nuclear facilities for ventilation systems, which are mainly made of glass fiber. At present, the nuclear-contaminated filter media is compressed to reduce the volume and then immobilized in cement before disposal. Vitrification is a widely accepted technology in the world to immobilize nuclear wastes due to the relative simplicity of process and the excellent chemical durability of waste glass. However, the glass fiber of the filter media becomes highly viscous at the melting temperature used for nuclear waste vitrification as it contains a high concentration of SiO2 (> 50 wt%). This makes the treatment of filter media practically impossible using the existing vitrification process. Thus, in this work, emphasis has been given to the formulation of a filter media that can adapt to the existing melting process. Extreme vertices design was employed to formulate three-component (heat-treated filter mediaCaONa2O) waste glasses, and the Scheffé model was used to figure out the viscosity-component correlation of waste glasses. Moreover, the effect of each component was quantitatively determined at the temperature at which viscosity was 10 Pa·s. The volatilization experiment revealed that at 1200 °C, Co was nonvolatile, and Cs was much easier to volatilize than Sr. Furthermore, the losses of Sr and Cs during glass melting were estimated from their rates of diffusion and chemical reaction. This work would provide the fundamental basis to formulate the filter media waste glass and control the loss of radionuclides for the existing vitrification process.