The Structural Characterization of a Series of Uranium-containing Gadolinium Zirconates
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The Structural Characterization of a Series of Uranium-containing Gadolinium Zirconates Daniel J. Gregg, Yingjie Zhang, Zhaoming Zhang, Inna Karatchevtseva, Mark G. Blackford, Gerry Triani, Gregory R. Lumpkin and Eric R. Vance. Institute of Materials Engineering, ANSTO, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ABSTRACT A series of uranium-containing gadolinium zirconate samples have been fabricated at 1723 K in air. X-ray diffraction and Raman spectroscopy have confirmed pyrochlore or defect fluorite structures, while diffuse reflectance, X-ray absorption near edge structure and X-ray photoelectron spectroscopies indicate a predominantly U6+ oxidation state, even when Ca2+ was added to charge balance for U4+. The results demonstrate the potential of gadolinium zirconates as host materials for actinides. INTRODUCTION Recently, there has been renewed interest in the potential use of ceramics for the conditioning and transmutation of long-lived waste isotopes generated from nuclear power and weapons programs. In particular, pyrochlores and related fluorite phases are likely to incorporate plutonium and minor actinides within durable ceramic waste forms for long-term geologic disposal or as inert matrices for actinide transmutation [1-4]. It has been claimed [5] that pyrochlore-structured zirconates have increased resistance to radiation damage arising from the decay of incorporated actinides when compared with titanate pyrochlores. For example, gadolinium zirconate (Gd2Zr2O7) has been found to have remarkable resistance to amorphization under ion beam irradiation as well as high chemical durability [6]. The pyrochlore crystal structure has the general formula A2B2O7 where A and B are cations, e.g., La3+, Zr4+. Pyrochlore is a superstructure (2 × 2 × 2) of the fluorite structure (AO2), with the A and B cations ordered on two distinct cation sites, and one-eighth of the oxygen anions absent to maintain charge balance [6,7]. It has been shown that significant amounts of actinide elements can be incorporated within a Gd2Zr2O7 matrix, on both the A- and B-cation positions [7]. For example, the synthesis of Pu- and U-doped zirconate pyrochlores has recently been reported [8,9]. Further, U4+, U5+ and U6+ have previously been observed in air sintered pyrochlore and fluorite samples [8,10]. As Pu and U are likely to be immobilized together to reduce proliferation concerns, it is necessary to confirm the speciation and valence state of uranium in such matrices. In this work we concentrate on the incorporation of U in pyrochlorestructured Gd2Zr2O7, with different compositions e.g., Gd1.6U0.2Ca0.2Zr2O7 (targeting U4+), Gd1.4U0.2Ca0.4Zr2O7 (targeting U5+), and Gd1.2U0.2Ca0.6Zr2O7 (targeting U6+). The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy and the uranium oxidation states investigated by diffuse reflectance (DR), X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopies (XPS).
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EXPERIMENT The samples (~20 g on an
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