A study on the oxidation characteristic of UO2-Gd2O3 pellet for recycling of burnable absorber pellet scrap

The development of recycling process of defective (U,Gd)O2 scrap is one of the important subject in this project. Among the several burnable absorbers, Gd has a very large neutron absorption cross-section. Therefore, gadolinia bearing UO2 fuel, (U,Gd)O2, has been widely used as a burnable absorber in light water reactors. During the pellet fabrication process, fairly amount of defective (U,Gd)O2 pellets are produced and it is necessary to recycle the scraps. Generally, the defective scraps are powdered through the oxidation in air in the temperature range of 450 to 550 deg C and then mixed with co-milled powder, and further processed to fabricate (U,Gd)O2 pellets. In addition, the sintered pellet properties are closely depend on the powder property of oxidized M3O8 powder. Therefore, the careful investigate of oxidation kinetics and related powder property of (U,Gd)O2 is very important. The oxidation behavior of UO2-6wt% Gd2O3 and UO2-12wt% Gd2O3 has been studied in the temperature range from 350 to 700 deg C using TGA and XRD techniques in air. UO2 was necessarily oxidized to U3O8 regardless of oxidation temperature and its weight gain was 4wt%. However, (U,Gd)O2 exhibit a different oxidation behavior ; The final phase and saturated weight gain depends on oxidation temperature. The saturated weight gain increases with oxidation temperature up to 500deg C and thereafter decreases with temperature. In addition, the amount of weight gain obtained at 500 deg C was smaller in UO2-12wt% Gd2O3 than in UO2-6wt% Gd2O3 and the final phase at the saturated weight gain was M3O8 in UO2-6wt% Gd2O3 but the mixture of M4O9 and M3O8 in UO2-12wt% Gd2O3. It is supposed that Gd substitution for U decreases the equilibrium O/M ratio and thereby enhance the stability of M4O9 type cubic phase