Phase studies in the UO2–Gd2O3 system

Abstract

The incorporation of gadolinium directly into nuclear fuel is important regarding reactivity compensation, which enables longer fuel cycles. The incorporation of Gd2O3 powder directly into the UO2 powder by dry mechanical blending is the most attractive process, because of its simplicity. Nevertheless, processing by this method leads to difficulties while obtaining sintered pellets with the minimum required density. This is due to the bad sintering behavior of the UO2–Gd2O3 mixed fuel, which shows a blockage in the sintering process that hinder the densification process. Minimal information exists regarding the possible mechanisms for this blockage and this is restricted to the hypothesis based on the formation of a low diffusivity Gd rich (U,Gd)O2 phase. The objective of this investigation was to study the phase formation in this system, thus contributing to clarifying the causes of the blockage. Experimental evidence indicated the existence of phases in the (U,Gd)O2 system that revealed structures different from the f luorite-type UO2 structure. These phases appear to be isostructural to the phases observed in the rare earth-oxygen system.