Avaliação dos parâmetros de produção para otimização microestrutural de pastilhas de dióxido de urânio (UO2)

Abstract

Control of porosity and grain size is necessary to enghance the fuel pellet’s performance, specifically the ability to retain fission products. To optimize the microstructure of fuel without the use of sintering additives, UO 2 pellets were sintered for 3 hours and 4 hours at temperatures of 1500°C, 1600°C, and 1700°C. No significant variations in porosity were observed. The largest average grain size was measured in the 1700°C for 4-hour range, establishing this condition as optimal. This condition was applied to pellets with O/U ratios of 2,15 and 2,26, as well as pellets made from mixtures of UO 2 with U 3 O 8 powders obtained by oxidizing UO 2 at 400°C (green U 3 O 8) and 1300°C (sintered U 3 O 8 ) in air. The purpose is to evaluate the effect of these conditions on the development of the microstructure during the sintering process. The addition of 10% U 3 O 8 did not result in grain size increments. This observation applies to both green and sintered U 3 O 8 . Despite the significant difference in specific surface area between the two powders, this difference did not influence the final grain size. Both mixtures yielded compatible average grain sizes and grain size distributions. The results indicate that stoichiometry affected the development of the microstructure, promoting grain growth even in a reducing atmosphere. A O/U ratio of 2.26 resulted in an average grain size of 12.4 μm, while a O/U ratio of 2.15 led to 8.0 μm. Therefore, the sintering of UO 2 with a O/U ratio of 2.26, possibly due to the homogeneous arrangement of U3O8 within its structure and hyperstoichiometry, yielded the best experimental result in this work. The achieved grain size exceeded the minimum acceptable value in industrial terms for pure UO 2 pellets without the use of sintering additives.