Reaction sintered glass: a durable matrix for spinel-forming nuclear waste compositions

Glass formation by reaction sintering under isostatic pressure is an innovative process to vitrify refractory-rich high-level radioactive waste. We used a typical defense waste composition, containing spinel-forming components such as similar to 4 wt% of Cr2O3, similar to 23 wt% Al2O3, similar to 13 wt% Fe2O3, and similar to 9 wt% UO2, with CeO2 simulating UO2. Reaction sintered silicate glasses with waste loading up to 45 wt% were prepared within three hours, by hot pressing at 800 degrees C. The glass former was amorphous silica. Simulated waste was added as calcined oxides. The reaction sintered glass samples were characterized using scanning and analytical electron microscopy. The results show that extensive reaction sintering took place and a continuous glass phase formed. Waste components such as Na2O, CaO, MnO2, and Fe2O3, dissolved completely in the continuous glass phase. Cr2O3, Al2O3, and CeO2 were only partially dissolved due to incomplete dissolution (Al2O3) or super-saturation and reprecipitation (Cr2O3 and CeO2). The precipitation mechanism is related to a time dependent alkali content in the developing glass phase. Short-term corrosion tests in water showed that the glasses are chemically more durable than melted nuclear waste glasses. Based on hydration energies calculations, the long-term chemical durability of our reaction sintered glasses is expected to be comparable to that of rhyolitic and tektite glasses. (C) 2000 Elsevier Science B.V. All rights reserved.