Effect of mixed powder homogeneity on the UO2-Gd2O3 nuclear fuel sintering behavior

Gadolinium oxide homogeneously mixed with uranium dioxide nuclear fuel is used as burnable poison in modern nuclear reactors. Direct incorporation of gadolinium to the fuel is important to provide longer fuel cycles and to optimize the fuel utilization by reactivity compensation and power distribution adjustment. According to the published data, the results on sintering UO2-Gd2O3 compacts are contradictory and apparently very sensitive on the mixed powder production route. The incorporation of Gd2O3 powder to the UO2 powder by the most attractive commercial method of dry mechanical blending leads to technological difficulties for attending to the specified density of sintered fuel pellets. Otherwise, good sintered densities are obtained when the co-precipitation route is adopted to prepare the mixed fuel powder. This work presents the results in sintering UO2-Gd2O3 compacts prepared by mechanical dry blending of both ceramic powders and by two co-precipitation methods. The results showed that the final sintered density and the gadolinium distribution homogeneity observed in the sintered fuel pellets are directly related to the homogeneity of the initial powder mixture. The powders prepared by the different methods presented different sintering behavior. Possible mechanisms are proposed to explain the divergent sintering behaviour. (orig.)