Simulated high-level waste (HLW) feed, prepared with co-precipitated Fe, Ni, Cr, and Mn hydroxides to simulate the formation of spinel, was heated up at temperature-increase rates believed to be close to those the feed experiences in the HLW glass melter. Decomposition, melting, and dissolution of the feed components (such as nitrates and silica) and the formation of intermediate crystalline phases (spinel, sodalite (Na_8(AlSiO_4)_6(NO_2)_2), and Zr-containing minerals) were observed and characterized using evolved gas analysis, volume-expansion measurement, optical microscope, scanning electron microscope, thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction. Nitrates and quartz, the major HLW feed components, were converted to a glass-forming melt by 880 deg C. The conversion heat for this reaction was about 280 J/g-feed. A chromium-free spinel formed in the nitrate melt starting from 520 deg C and eventually dissolved in the borosilicate melt by 1050 deg C. Sodalite, a transient product of corundum dissolution, appeared above 600 deg C and eventually dissolved in glass. Large sparse particles (50 #mu#m) of zirconium silicate, a product of a reaction between zirconium nitrate and silica, disproportionated to sodium zirconium silicate and zirconium-oxide before dissolving. Volume expansion (primary foam generation) started at 560 deg C, peaked at 865 deg C (with a volume 10 times larger than that of glass), and collapsed by 910 deg C.
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