Pyrochlores (<i>A=2<sub>B</sub>2<sub>O</sub>7<sub>, </sub>Fd3m</i>) have been proposed as candidate phases for the immobilisation of actinides from high level nuclear waste. They may form part of a multi-barrier approach to final waste disposition in an underground repository. Titanate and zirconate pyrochlores are particularly interesting; titanates show high chemical durability, whilst some zirconate pyrochlores display very high resistance to structural damage by radiation. To fully evaluate the likely performance of a pyrochlore-based waste-form, a comprehensive understanding of its structure and behaviour must be developed. Related phases, which might exsolve, must also be characterised, to test whether their presence will have a deleterious effect. Various zirconate and titanate oxides including (La<sub>1-x</sub>Nd<sub>x</sub>)<sub>2</sub>Zr<sub>2</sub>O<sub>7, </sub>Nd<sub>2</sub>(Zr<sub>1-x</sub>Ti<sub>x</sub>)<sub>2</sub>O<sub>7</sub>, La<sub>2</sub>(Zr<sub>1-x</sub>Ti<sub>x</sub>)<sub>2</sub>O<sub>7</sub> and (Y<sub>1-x</sub>La<sub>x</sub>)<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> were synthesized and characterised by X-ray and neutron powder diffraction, electron microscopy and electron probe micro-analysis. Rare earth elements acted as non-radioactive analogues for actinide behaviour. The extent of solid solubility and factors governing exsolution were explored. The conductivity of Nd<sub>2</sub>(Zr<sub>1-x</sub>Ti<sub>x</sub>)<sub>2</sub>O<sub>7</sub> ceramics was measured using impedance spectroscopy. Pyrochlores find application in a wide variety of technologies, including those which exploit the electrical properties of the pyrochlore structure. The effect of Ti substitution in Nd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> was studied and compared to the conductivity of monoclinic Nd<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub>. The correlation between ionic mobility and critical radiation dose is discussed. Leaching from (Gd<sub>1.4</sub>Ce<sub>0.2</sub>La<sub>0.1</sub>Sm<sub>0.1</sub>Eu<sub>0.1</sub>)(Zr<sub>0.9</sub>Ti<sub>0.9</sub>Sc<sub>0.1</sub>In<sub>0.1</sub>)O<sub>7</sub> was investigated. This composition facilitated comparison of numerous elemental dissolution rates from a single sample. Experimental conditions mimicked those likely in a repository at 1000m depth. Alteration of the solid was characterised by light and electron microscopy. The dissolved concentrations of leached elements were determined by inductively coupled plasma-mass spectrometry. Various synthetic routes were considered, to maximise the sample density.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:603819 |
| Date | January 2007 |
| Creators | Harvey, E. J. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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