Vitrification of lead–bismuth alloy nuclear waste into a glass waste form

Abstract

Lead–bismuth eutectic (LBE), a promising coolant in advanced nuclear systems, can be activated by neutrons during nuclear reactor operations. The decommissioning of nuclear facilities would generate lead–bismuth (Pb–Bi) alloy-contaminated nuclear waste. The current metallic nuclear waste treatment approach involves remelting followed by cementitious solidification. This increases the waste volume and the risk of radionuclide migration in groundwater. Therefore, this study developed a method for vitrification of Pb–Bi alloy waste. Different amounts of SiO₂ were added at 750–1100°C in the air to turn the simulated LBE waste into glass waste form. The values of the normalized elemental leaching rates (Pb, Bi, Si, Te, and Ni) determined using the 28-day static leaching test were less than .2 g m⁻² d⁻¹ and varied with SiO₂ addition. Furthermore, a three-stage evolution of the glass structure with SiO₂ addition was proposed according to the structural analysis performed using Raman and X-ray photoelectron spectroscopies. The evolution stages were as follows: (i) the stage of heavy metal transition from covalent to ionic heavy metals (7.5 wt% < SiO₂ < 15 wt%), (ii) the stage of increase in bridging oxygen (15 wt% < SiO₂ < 20 wt%), and (iii) the stage of domination of the Si–O network (20 wt% < SiO₂ < 25 wt%). The evolution of the glass structure resulted in varying glass chemical durability. Finally, the glass-forming region of (20–48)PbO–(35–70)Bi₂O₃–(7.5–25)SiO₂ (wt%) and the temperature needed to melt those glasses were determined through the melting test, where radionuclides and toxic heavy metals showed undetectable volatilization during vitrification. Hence, turning LBE waste into glass waste form will be a potential approach for treating Pb–Bi alloy nuclear waste.