Photon Factory Activity Report 2019 #37 (2020)

BL-27B/2019G586

Characterisation of zirconolites prepared under different processing atmospheres
Shi-Kuan Sun*, Lewis R. Blackburn, Martin C. Stennett, and Neil C. Hyatt
Immobilisation Science Laboratory, Department of Materials and Science Engineering, The
University of Sheffield, Sheffield.
1 Introduction
The inventory of UK-owned plutonium (Pu) isforecast to peak at 140 teHM. A fraction of the Pu
inventory has been declared unsuitable for MOX
fabrication, due to chlorine contamination arising fromlong term storage in polyvinyl chloride (PVC) packagingcontainment [1]. An alternative approach is theconditioning of the Pu stockpile and waste material via
immobilisation within a ceramic matrix.
Zirconolite(prototypicallyCaZrTi2O7)
isbeing considered as a potential ceramic wasteform for theimmobilisation of plutonium due to its aqueous durability
and chemical flexibility [2]. The understanding of theoxidation state of Pu within zirconolite is essential topredict the long-term stability and feasibility ofzirconolite. In this work, two atmospheres were employed
to prepare zirconolite wasteforms with composition
Ca0.8Ce0.2ZrTi1.6Cr0.4O7, using Ce as the surrogate of Pu
and Cr for charge compensation. The oxidation state ofCe in the samples was examined and compared.
2 Experiment
Reagent grade oxides (CaTiO3, ZrO2, Cr2O3, TiO2 andCeO2) were batched and mixed, according to the target
composition (Ca0.8Ce0.2ZrTi1.6Cr0.4O7). The oxide
precursors were reaction sintered under two atmospheres:(1) at 1350oC for 20 h in air; and (2) at 1320oC for 30 minin a reducing environment using spark plasma sintering(SPS) [3]. Specimens were prepared for XANES
measurement by homogenising and suspending inpolyethylene glycol to produce samples with a thicknessof ca. one absorption length. Ce LIII edge XANES spectra
were acquired in transmission mode on BL-27 at thePhoton Factory (Tsukuba, Japan). Spectra were alsoacquired on CeO2 and CePO4 standards. Subsequent data
reduction and analysis, was performed using the Athenasoftware package [4].
3 Results and Discussion
A zirconolite-2M phase (space group C2/c) wasdetected as the major product after sintering
Ca0.8Ce0.2ZrTi1.6Cr0.4O7 in air. The presence of CaTiO3(1.2 ± 0.2 wt%) and Cr2O3 (0.6 ± 0.2 wt%) were
determined by Rietveld refinement. In the SPSed sample,
19.3 ± 0.4 wt% CaTiO3 was detected alongside the major
zirconolite-2M phase.
Fig. 1 shows the Ce LIII edge XANES data for thezirconolite samples overlaid with the standards. It wasapparent from visual inspection that Ce was present
mainly as Ce4+ in the air-sintered sample, which is a goodanalogue for Pu4+. Closer inspection of the data revealed asmall shift in the edge position toward lower energy (withrespect to CeO2) and a small change in the relative
intensity of the asymmetric doublet feature on the crest of
the absorption edge, consistent with the presence of aminor fraction of Ce3+. Linear combination fitting using
the CeO2 and CePO4 standards, indicated the presence of
16.4 ± 1.1% Ce3+ and 83.6 ± 1.5 % Ce4+. The spectrum for
the SPS sample consisted of a single intense feature
consistent with the Ce3+ standard. Linear combination
analysis determined that a complete reduction to the Ce3+
species had been achieved. These results indicates that theselection of the processing atmosphere has a significant
impact on phase assemblage and Ce oxidation state.
Further investigation on the minimisation of thesecondary phase, which is undesirable, and the aqueous
durability of zirconolite is underway.

Fig. 1: Ce LIII edge XANES data for Ca0.8Ce0.2ZrTi1.6Cr0.4O7
samples processing under different atmosphere. Ce standards ofknown oxidation state are also shown for comparison.

Acknowledgement
We acknowledge Dr. Y. Okamoto and Dr. M. Hondafrom JAEA for the assistance during beamtime. S.K Sun
and M. C. Stennett are grateful for financial support fromthe Daiwa Foundation. We are grateful for financial
support from the Nuclear Decommissioning Authority
and EPSRC under grants EP/M026566/1, EP/S01019X/1,
EP/N017870/1, EP/R511754/1 and EP/T011424/1
References[1] NDA plutonium topic strategy: Credible options
technical analysis, SAF/081208/006.2 (2009).
[2] R. Ewing, Prog. Nucl. Energy, 49, 635 (2007).
[3] L. Blackburn et al., J. Eur. Ceram. Soc., accepted,(2020) DOI: 10.1016/j.jeurceramsoc.2020.05.066.
[4] B. Ravel et al., J. Synchrotron Rad., 12, 537 (2005).
* shikuan.sun@sheffield.ac.uk