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

Abstract

This paper describes the solid solubility of uranium in gadolinium zirconate (Gd 2Zr 2O 7), 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.