Influence of ZrO₂ on Structure and Properties of Simulated High Level Liquid Waste Glasses

Vitrification is worldwide recognized as the most reliable technology for high level liquid waste(HLLW) immobilization. At present, the mainstream technologies for vitrification of high level liquid waste(HLLW) worldwide include joule-heated ceramic melter vitrification and two-step cold crucible induction melter(CCIM) vitrification. In the two-step cold crucible vitrification process, HLLW undergoes denitration and calcination sequentially in a rotary calciner to form calcined waste. The calcine is then fed into a cold crucible induction melter, where it is melted together with a glass-forming batch. Upon cooling, a homogeneous waste form is produced, achieving safe immobilization of radioactive waste. This study is based on the two-step cold crucible induction melter vitrification technology. Previous studies regarding glass formulation and process optimization have mainly focused on the direct treatment of high-level radioactive liquid waste. However, to meet practical engineering operation requirements, slag flushing water co-generated during reprocessing must also be co-treated. According to source-term characterization, the main component of slag flushing water is metallic zirconium scraps(accounting for more than 90%(mass fraction, the same below)), accompanied by minor insoluble radionuclides including U, Pu, Ru, and Tc. Therefore, during the co-treatment of HLLW and slag flushing water, the effects of metallic zirconium scraps on the vitrification process and glass formulation must be carefully evaluated. In the two-step cold crucible vitrification process, metallic zirconium scraps are converted into zirconia(ZrO₂) during the first-stage rotary calcination. From the perspective of glass formulation design, the influence of slag flushing water is thus dominated by the effects of ZrO₂ on the structure and performance of the final glass waste form. In this study, glasses containing simulated HLLW and different amounts of ZrO₂ were prepared. The results show that the glass wasteform structure is able to accommodate up to 6% ZrO₂ content. With the increase of ZrO₂ doping content in the borosilicate glass matrix, the glass density rises obviously while the molar volume decreases; meanwhile, both the chemical durability and high-temperature viscosity of the glass are enhanced. However, when 2% RuO₂ coexists in glass, the glass density and molar volume slightly change with ZrO₂ content while the glass viscosity largely increases. Moreover, the nuclear magnetic resonance(NMR) ²⁹Si and ¹¹B results suggest that ZrO₂ will depolymerize the SiO₄ connections in network backbone, whereas increase the ratio of BO₄/BO₃, thereby increasing the stability of glass network.