Computational Studies of the Substitution of Early Actinides and Cerium into Zirconolite

Abstract

The work described in this thesis utilises density functional theory (DFT), in conjunction with the periodic electrostatic embedded cluster method (PEECM), to analyse changes to a series of embedded clusters of the ceramic zirconolite (CaZrTi2O7) following substitution of early actinides (Th-Am) and Ce onto Zr- and Ca-sites. The large amounts of plutonium currently stockpiled by the United Kingdom requires immobilisation and geological disposal at the end of the nuclear fuel cycle, potentially following reuse as a mixed oxide fuel. Zirconolite is a candidate waste form able to immobilise and sequester early actinides from the biosphere. Developing more understanding of chemical behaviour of the zirconolite lattice following substitution can provide insight into specific electronic behaviour in these systems. Further discussion of the use of cerium as a non-radioactive surrogate for Pu, focussing on redox behaviour, provides greater context for experimental work. The first three chapters of this thesis will discuss the experimental and theoretical research which has been performed on zirconolite, its intended use-case and characteristics before describing theoretical and computational methods used. Chapter 4 firstly describes the unsubstituted zirconolite cluster used throughout the rest of the results. Following sections detail the geometric, electronic and energetic changes following isovalent and altervalent substitutions of early actinides (An = Th-Am) and Ce onto Zr- and Ca-sites with Fe, Al, Ga, V, Cr, Nb, Cu, Mo and Co being used as charge balancers when necessary. Mean displacements of a central region of ions indicated redox behaviour which was confirmed through analysis of spin density data. Strong correlations were found between the substitution energies of these ions and the ratio of the ionic radii of substituent An and substituted ions in most cases. Spin density data of molecular M(OH)n (M = Ce, Th - Am, n = 3 - 5) showed a preference for M(III) for later actinides - as seen in the zirconolite clusters. Calculated helium migration through pristine and substituted zirconolite clusters indicate migration pathways and defect sites in agreement with previous work. Results further infer zirconolite's high stability and suitability as a candidate high level waste form at low doping levels.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Francis Livens (Supervisor) & Nikolas Kaltsoyannis (Supervisor)