Electron Microscopy Study of Reaction Sintered HLW Glass

Abstract

ABSTRACTReaction sintering under pressure can be used to vitrify radioactive waste. We used defense waste simulate with high concentrations of refractory oxide ZrO2 (∼36 wt%). Silicate glasses with 30 to 50 wt% of waste loading were prepared by hot isostatic pressing (800 °C, 28 MPa). Amorphous silica (50 to 70 wt%) was added to calcined waste simulants. The reaction sintered final products were characterized by scanning and analytical electron microscopy. Our results showed that a continuous glass phase formed. Waste components such as Na2O, CaO, MnO2, Fe2O3, and Al2O3 dissolved in the glass phase. ZrO2 dissolved as well but reached its solubility limit. Nanometer size crystals of baddeleyite (ZrO2) suggest that their presence is a result of super-saturation of the glass phase rather than incomplete dissolution. The sodium concentration increases in the early stages of reaction sintering and decreases towards the end and so does solubility of ZrO2 and other oxides. The microstructure of reaction sintered glasses is determined by a) the concentrations of refractory oxides, b) the concentration of reaction sintering agents, c) the particle size of glass formers, and d) sintering conditions, e.g., temperature and time. Evidence is provided by respective analytical data and micrographs.