The role of pore formation on sintering UO₂-Gd₂O₃ fuel

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 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. Experimentally it has been shown that the blocking mechanism is based on pore formation because of the Kirkendall effect. The formation of a solid solution during the intermediate stage of sintering leads to formation of large pores, which are difficult to remove in the final stage of sintering. Considering this mechanism as responsible for the poor sintering behavior of UO₂-Gd₂O₃ fuel prepared by the mechanical blending method, it was possible to propose, discuss and, in certain cases, preliminarily test feasible adjustments in fabrication procedures that would minimize, or even totally compensate, the negative effects of pore formation due to the Kirkendall effect. This work presents these considerations. (author)