Nepheline Crystallization in High-Alumina High-Level Waste Glass
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Nepheline Crystallization in High-Alumina High-Level Waste Glass 1
José Marcial1, John McCloy1, Owen Neill2 School of Mechanical & Materials Engineering & Materials Science and Engineering Program, Washington State University, Pullman, WA 99164-2920, USA 2
Peter Hooper GeoAnalytical Laboratory, School of the Environment Washington State University Pullman, WA 99164-2812, USA
ABSTRACT The understanding of the crystallization of aluminosilicate phases in nuclear waste glasses is a major challenge for nuclear waste vitrification. Robust studies on the compositional dependence of nepheline formation have focused on large compositional spaces with hundreds of glass compositions. However, there are clear benefits to obtaining complete descriptions of the conditions under which crystallization occurs for specific glasses, adding to the understanding of nucleation and growth kinetics and interfacial conditions. The focus of this work was the investigation of the microstructure and composition of one simulant high-level nuclear waste glass crystallized under isothermal and continuous cooling schedules. It was observed that conditions of low undercooling, nepheline was the most abundant aluminosilicate phase. Further undercooling led to the formation of additional phases such as calcium phosphate. Nepheline composition was independent of thermal history. INTRODUCTION In typical nuclear waste vitrification, multiphase solid/liquid waste feed is mixed with glass-forming additives (e.g., SiO2 and H3BO3) and converted to glass for storage in a permanent geological repository. After feed-to-glass conversion, molten glass is poured into stainless steel canisters and air quenched [1]. The cooling profile of the canister allows for rapid cooling of the outer melt but results in slow cooling of melt in the canister center [1]. Often this behavior is simulated with benchtop-scale testing by heat treating glasses following a canister-centerline cooling (CCC) schedule. As dictated by Johnson-Mehl-Avrami-Kolmogarov kinetics, crystallization of aluminosilicate phases will proceed given sufficient time and temperature for lattice arrangement of [SiO4]4- and [AlO4]5- tetrahedra [1]. The aluminosilicate phase of interest is nepheline, NaAlSiO4, which has been previously shown to be deleterious to chemical durability due to the extraction of alumina and silica from the glass-forming matrix, leaving a residual glass of less-durable components [2-5]. Nepheline can accept up to 0.25 atomic% K as Na0.75K0.25AlSiO4. The long-term corrosion resistance is significant because vitrified waste must tolerate subterranean storage conditions for ≥10 6 years with limited radioisotope transport. To understand the crystallization behavior of glass, isothermal heat treatments (IHT) were performed at 1050-750°C for 3-30 hours. Compositional analysis of crystallized glasses was performed through wavelength-dispersive spectroscopy (WDS). Phase identification was performed through X-ray diffraction (XRD). Scanning electron microscopy energy-dispersive spectrosc
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