Electron Microscopy Study of Reaction Sintered HLW Glass
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ELECTRON MICROSCOPY STUDY OF REACTION SINTERED HLW GLASS Weiliang Gong and Werner Lutze Vitreous State Laboratory, The Catholic University of America Washington, D.C. 20064, USA ABSTRACT Reaction 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.
INTRODUCTION Currently, vitrification by melting is the only technology used to convert high-level radioactive waste into a disposable solid. In this process, the waste is mixed with glass-forming additives such as SiO2 and B2O3 or a glass frit, melted and poured into steel containers to produce a borosilicate glass. Vitrification requires that a homogeneous, single-phase glass melt forms. Waste loading for the Joule heated ceramic melter is limited by the solubility of certain, sometimes minor waste constituents or their reaction products. At Hanford, for example, waste streams contain various critical constituents, depending on the tank farm (concentrations in wt%): ZrO2 (≤35), Na2O (≤60), Cr2O3 (≤4), Fe2O3 (≤60), U3O8 (≤24), Al2O3 (≤23), Ce2O3 (≤5), and NiO (≤12). Most of these constituents are refractory oxides with high liquidus temperatures imposing limitations on waste loading in the glass through crystallization of phases such as spinel, baddeleyite, Na-Zr-silicate, eskolaite, and possibly nepheline in the melter [1, 2]. Given the large amounts of waste to be melted, sedimentation of solids on the melter’s bottom may shorten the lifetime of state-of-art melters. One way to address the problems associated with vitrification is to change the process by which the glass is made. Instead of melting, glasses can be made by sintering under pressure. Vitrification of High-level Liquid Waste (HLLW) by conventional sintering is not new [3-5]. Fabrication of sintered glass using hot uniaxial pressing and simulated waste has been demonstrated in a pilot p
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