Chen, Tzu-Yu (2012)
Ph.D. thesis, University of Birmingham.
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The main aim of this project was to develop a durable ceramic wasteform by HIPing Cs-exchanged crystalline silicotitanate (CST) used for nuclear waste clean-up. The sodium form (Na-CST) and niobium substituted sodium form (Na-Nb/CST) CST were hydrothermally synthesised and characterised. The synthesised CSTs and a commercial CST containing material, IONSIV®, were subjected to ion exchange studies and then the crystal phases present after HIPing were investigated. Cs-IONSIV® was thermally decomposed and converted to two major Cs-containing phases, Cs2TiNb6O18 and Cs2ZrSi6O15, and a series of other phases. Additionally the effect of metal addition on phase formation under HIP conditions was explored. The microstructure and phase assemblage of HIPed Cs-IONSIV® samples as a function of Cs content were examined using XRD, XRF, SEM and TEM/EDX.
To understand the Cs bonding environment in these Cs-containing phases, structural characterisation was undertaken using Rietveld analysis of synchrotron X-ray powder diffraction data and neutron diffraction data. The potential of these phases for hosting Cs+ and its decay product Ba2+ was also studied.
This thesis is also concerned with determining the aqueous durability of these HIPed samples by carrying out MCC-1 and PCT-B leach tests. These show very low Cs leach rates and the promise of safe long-term immobilisation of Cs from CSTs as well as suggesting these phases are more leach resistant than hollandite - the material targeted for Cs sequestration in Synroc.
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