Improving sinterability of UO2-Gd2O3 fuel pellets by high-energy mixing

Abstract

The optimization of nuclear fuels for use in pressurized water reactors can be achieved by increasing the burnup cycle. One way to achieve these goals is to anticipate an initial excess of reactivity in the reactor design and incorporate so-called burnable poisons directly into the fuel to control the neutron population. Due to its neutronic properties, gadolinium is one of the main elements incorporated in the form of oxide (Gd2O3). The dry mechanical mixing of UO2 powder and Gd2O3 is the commercially most attractive method due to its simplicity. However, this method leads to challenges in obtaining sintered pellets with high sintered density for use in nuclear reactors due to the Kirkendall effect. In this work, 10 wt% of Gd2O3 powder was added to UO2 powder through dry mechanical mixing in low- (Turbula®) and high-energy (SPEX®) mixers. The powders were homogenized in turbula mixer for 1 h and 80 h. The mixing time did not improve the sintered density, which were about 91.5 % of theoretical density (TD). Homogenizations in the SPEX mixer were carried out at 15, 30, and 60 min. The highest sintered densities in this case were 95.8 % and 96.3 % TD achieved with 30 min and 60 min, respectively. Mixing the powders through a process involving more intense agitation allows for the breaking of Gd2O3 clusters without significant changes in the properties of UO2, thus improving the sinterability of the UO2- Gd2O3 fuel pellet.