The sintering blocking mechanism on the UO2-GD2O3 system. Part 1: the hypothesis of diffusion barrier

The direct incorporation of gadolinium into nuclear power reactor fuel is important to the reactivity compensation and adjustment of power distribution thus enabling longer fuel cycles and optimized fuel utilization. Dry mechanical blending of Gd2O3 and UO2 powders is commercially the most attractive process route due to its simplicity. Nevertheless, processing by this route leads to difficulties in getting sintered pellets with the minimum required density due to a sintering blocking mechanism. Regarding this, there s little published information and the explanations are focused on the formation of a low diffusivity Gd-rich (U, Gd)O2 phase during sintering process which decreases pellets density. An attempt to understand the mechanism for this effect was done in this work. Experimental evidences indicated the existence of phases in the (U, Gd)O2 system with structure different from the fluorite-type UO2 structure. These new phases were found for Gd molar fractions higher than 0,5, which coincide with the lowering of both the sintered density and the interdiffusion coefficient. However, it has been also shown that these new phases cannot be itself the cause for the density decrease observed. (author)