Synthesis and Characterization of Brannerite Compositions for MOX Residue Disposal
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Synthesis and Characterization of Brannerite Compositions for MOX Residue Disposal D.J. Bailey1*, M.C. Stennett1 and N.C. Hyatt1 1 Immobilisation Science Laboratory, Department of Materials Science and Engineering, Univeristy of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom
ABSTRACT Due to their high actinide content MOX residues require immobilization within a robust host matrix. Although it is possible to immobilize actinides in vitreous wasteforms; ceramic phases, such as brannerite (UTi2O6), are attractive due to their high waste loading capacity and relative insolubility. Brannerites Gd0.1U0.9Ti2O6, Ce0.1U0.9Ti2O6 and Gd0.1U0.81Ce0.09Ti2O6 were prepared using an oxide route. Charge compensation of trivalent cations was expected to occur via the oxidation of U (IV) to higher valence states (U (V) or U (VI)). Gd was added to act as a neutron absorber in the final Pu bearing wasteform and Ce was used as a structural surrogate for Pu. Xray absorption spectroscopy showed that Ce (IV) was reduced to Ce (III) in all cases. X-ray powder diffraction of synthesized specimens found that the final phase assemblage was strongly affected by processing atmosphere (air or argon). Prototypical brannerite was formed in all compositions, secondary phases observed were found to vary according to processing atmosphere and stoichiometry. Microstructural analysis (SEM) of the sintered samples confirmed the results of the X-ray powder diffraction. INTRODUCTION The accumulation of plutonium stockpiles as a result of civil reprocessing of spent nuclear fuel (SNF) presents a significant proliferation risk [1]. As a result, the reduction of Pu stockpiles is an issue of great importance. A possible method for the reduction of Pu stockpiles is re-use of Pu in mixed oxide fuel (MOX) to generate electricity. MOX fuels are composed of a mixture of uranium and plutonium oxides, typically 90-95 % U and 5 - 10% Pu [2]. Residues arising from the production of MOX fuels will require disposal and, consequently, require the design of a robust host matrix. Ceramic phases are particularly attractive for actinide-bearing wastes due to their ability to incorporate high actinide waste loadings. Brannerite, UTi2O6, is particularly attractive due to its high actinide content (~ 55 wt% U). A monoclinic phase with space group C2/m, brannerite is often an accessory mineral in uranium deposits and multiphase ceramic wasteforms for disposal of actinide-rich wastes [3], [4]. The brannerite structure consists of layers of TiO6 octahedra with larger cations, also in octahedral co-ordination, located between the layers. Naturally occurring brannerites have been shown to be chemically flexible with elements such as Ca, Y, Pb, Ce and Th incorporated on the A site (U) and Fe, Si and Al substituting on the B site (Ti)[5], [6]. Although natural samples are often found to be completely metamict, the presence of brannerite in alluvial sediments after the weathering of host rocks indicates that brannerites possess sufficient aqueous durability for considerati
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