Sims Depth Profiling Studies of Sphene-Based Ceramics and Glass Ceramics Leached in Synthetic Groundwater
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SCIENTIFIC BASIS FOR RADIOACTIVE WASSE MLAM4MW -V Werner.TLnize, editor
SIMS DEPTH PROFILING STUDIES OF SPHENE-BASED CERAMICS AND GLASS CERAMICS LEACHED IN SYNTHETIC GROUNDWATER P.J. HAYWARD, W.H. HOCKING,
F.E. DOERN and E.V.
CECCHETTO
Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba, Canada, ROE iLO ABSTRACT Glass ceramics and ceramics based on the mineral sphene (CaTiSiO5 ) are being developed to host the wastes arising from possible future CANDU* fuel reprocessing. Results from leaching tests in deionized water and in synthetic groundwater indicate that these materials are highly durable. Secondary Ion Mass Spectrometry (SIMS) depth profiling of leached specimens suggests that leaching in the glass ceramics is predominantly confined to the glass phase. The high ionic strength and composition of the groundwater have a significant passivating effect on leaching and surface alteration phenomena, and encourage the precipitation of new phases on the ceramic surface. Leaching results, scanning electron microscope (SEM) observations and SIMS depth profile measurements are compared and discussed.
INTRODUCTION Atomic Energy of Canada Limited is developing the technology to immobilize and isolate the high-level wastes that would arise from possible future reprocessing of irradiated uranium and thorium fuels from CANDU* nuclear reactors. The waste immobilization program includes the development of durable matrices to host these wastes, including boroaluminosilicate glasses, sphene-based glass ceramics and ceramics. All are being examined for thermal, mechanical, geochemical and radiation stability in the postulated hydrothermal environment of a flooded underground vault in the Canadian Shield [ii. Recent analyses [2] of deep groundwater samples from representative+ sins within the Shield ind~ate that saline groundwaters with high Na Ca and Cl and low K , Mg and HCO 3 concentrations are to be expected at depths greater than P 500 m. Below ^ 1000 m, the total dissolved solids in these waters may exceed 200 g.L . It has been suggested [3] that sphene, CaTiSiO0, should be a thermodynamically stable phase under anticipated vault Konditions. Thermodynamic calculations and leaching experiments using synthetic groundwaters and both natural and synthesized CaTiSiO5 [4] appear to support this prediction. The purpose of the present work is to examine the nature of * CANada Deuterium Uranium
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the surface interactions that occur when sphene-based materials, containing simulated fission products and actinides, are leached in a synthetic saline groundwater of high ionic strength. Under these circumstances, it becomes impractical to rely on solution analyses alone for an understanding of the leaching processes. Analytical techniques such as Electron Spectroscopy for Chemical Analysis (ESCA) and Auger Electron Spectroscopy (AES), in combination with ion sputtering, or Secondary Ion Mass Spectrometry (SIMS) are, therefore, used to profile the outermost layers of the waste form chemicall
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