Investigating the Effect of Fe₂O₃ on the Zirconolite Phase Assemblage

The UK has one of the world’s largest inventory of civil separated plutonium (PuO₂). There are two potential Pu management strategies being investigated in the UK. One option is re-use within mixed oxide fuel (which is the current UK policy) or alternatively, immobilisation in a ceramic or glass host matrix. The latter could be an option for contaminated PuO₂ stocks (e.g. chlorine contamination) that could be unsuitable for MOX fabrication or a larger portion of the inventory if deemed necessary [1].<br/><br/>In this study, the efficacy of Fe₂O₃ as a cation charge compensator within zirconolite was investigated, with particular focus on the phase assemblage and microstructure (without the presence of Pu or a surrogate in order to isolate the effect of Fe³⁺ on the zirconolite structure). The Fe-doped zirconolite ceramic structures were fabricated via cold press and sintering method. The conceptual ceramic wasteforms were analyzed <i>via</i> X-ray diffraction (XRD) and Scanning Electron Microscopy/ Energy Dispersive X-ray Spectroscopy (SEM /EDX) methods. Rietveld refinement was performed to determine the effect of Fe³⁺ incorporation on the unit cell parameters of the zirconolite matrix, whilst Mössbauer spectroscopy and X-ray absorption near edge spectroscopy (XANES) data revealed the coordination environment and oxidation state of Fe substitution in which Ti site into the structure. As a result, XRD revealed the phase transformation from zirconolite-2M to -3T, with the increase of Fe concentration and there was an increase in the occurrence of secondary phases, mainly perovskite. The successful incorporation of the charge compensator cation into the zirconolite structure was proved with the change of unit cell parameters, associating with the reduction in volume, and SEM/ EDX micrograph. XANES analysis showed that Fe was uniformly present in the Fe³⁺ oxidation state in octahedral coordination.<br/><br/>[1] N.C. Hyatt, Safe management of the UK separated plutonium inventory: a challenge of materials degradation, npj Materials Degradation 4(28) (2020).