Studies on the sintering behaviour of UO2-Gd2O3 fuel pellets

Abstract

The incorporation of gadolinium directly into nuclear power reactor fuel is important from the point of reactivity compensation and adjustment of power distribution enabling thus longer fuel cycles and optimized fuel utilization. The UO2-Gd2O3 poisoned fuel is being proposed to be implanted in Brazil according to the future requirements established for Angra II nuclear power plant. 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 blockages during the sintering process. There is little information in published literature about the possible mechanism for this blockage and this is restricted to the hypothesis based on formation of a low diffusivity Gd rich (U,Gd)O2 phase. The objective of this investigation has been to study the blockage mechanism in this system during the sintering process, contributing thus, to clarify the cause for the blockage. Experimentally it has been shown that the blocking mechanism is based on pore formation because of the Kirkendall effect, instead the formation of low diffusivity phases. The formation of a solid solution during the intermediate stage of sintering leads to the formation of large pores, which are difficult to remove in the final stage of sintering. The phenomenon is better characterized as a concurrence between formation and elimination of pores during sintering than as a sintering blockage.