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Title: Actinide immobilisation in zirconate and titanate ceramics
Author: Harvey, E. J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2007
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Pyrochlores (A=2B2O7, Fd3m) 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 (La1-xNdx)2Zr2O7, Nd2(Zr1-xTix)2O7, La2(Zr1-xTix)2O7 and (Y1-xLax)2Ti2O7 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 Nd2(Zr1-xTix)2O7 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 Nd2Zr2O7 was studied and compared to the conductivity of monoclinic Nd2Ti2O7. The correlation between ionic mobility and critical radiation dose is discussed. Leaching from (Gd1.4Ce0.2La0.1Sm0.1Eu0.1)(Zr0.9Ti0.9Sc0.1In0.1)O7 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available