Actinide immobilization in crystalline matrix: a study of uranium incorporation in gadolinium zirconate

This paper describes the solid solubility of uranium in gadolinium zirconate (Gd2Zr2O7), a candidate crystalline matrix to immobilize actinide elements present in radioactive wastes. A series of uranium-containing gadolinium zirconate compositions were prepared by the citric acid-ethylene glycol resin precursor method. The final heat treatment was carried out at 1673 K in air. It was found that up to 40 at.% uranium could be incorporated at the Gd position. The pyrochlore ordering of the zirconate phase was lost above approximately 10 at.% uranium, and the structure transformed to fluorite. At the Zr position, the whole of Zr could be replaced by uranium, with its end member of mixed uranium-gadolinium oxide with fluorite structure. The oxidation state of uranium in the solid solutions was investigated by X-ray photoelectron spectroscopy.