The problem of high-temperature volatilisation of material from alkali borosilicate glasses, commonly used for the long-term storage of high-level nuclear waste, has been investigated using four mixed-alkali borosilicate glass systems. Each system is based on the mixed alkali borosilicate base-glass (MW) used in the UK and USA, initially being doped with one of three simulated waste oxides in varying quantities; aluminium oxide (0.95 mol%), lanthanum oxide (1.68 mol%) or magnesium oxide (2.55 or 10.2 mol%), before increasing quantities of caesium o~de were added (0-9.66 mol%) [1]. A number of experimental techniques have been employed in this study, including various thermal analysis measurements, nuclear magnetic resonance (MAS NMR) and Raman spectroscopy. From boron-ll and silicon-29 MAS NMR measurements, the fraction of tetrahedral boron (N4) and silicon Q3 and Q4 units (with respective three- and four-bridging oxygen) were resolved and compared against theory [2] and previous experimental measurements [3-6]. Silicon Q3 fractions resolved from NMR measurements [1] were compared against values obtained from quantitative Raman spectroscopy [7]. This approach is shown to be successful in resolving silicon Q3 units and therefore would be particularly useful in studies where the presence of large (> 1 mol%) quantities of paramagnetic species in glasses makes NMR difficult. Further detail resolved in the lIB MAS NMR spectra of these glasses also enabled the fraction of reedmergnerite and danburite medium~~~' order structures to be calculated [8]. The volatilisation of alkali borate and alkali silicate material, identified using EDX measurements, is shown not only to increase as a function of the fraction of Q3 units but is also affected by the presence of these medium-range order structures, where increasing quantities of danburite units leads to a greater degree of volatility [4,8]. Finally, a number of 170-enriched mixed-oxide alkali borosilicate systems have been manufactured, using an 17O-enriched silicon oxide precursor obtained through an established hydrolysis reaction of 20% oxygen-I7 enriched water with silicon tetrachloride [9]. Subsequent thermal analysis and NMR measurements have shown the oxygen-I7 environments of these borosilicate systems to vary significantly based on the presence of caesium, aluminium or lanthanum in quantities realistic to the modified mixed-alkali borosilicate glasses in the remainder of this study.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:487979 |
| Date | January 2007 |
| Creators | Parkinson, Benjamin Graves |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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