Special sintering features in U O2 active powders

In order to obtain large grains of U O2, we prepared active powder in special conditions of precipitation, by variation of the uranium concentration in ammonium uranate solution and of the precipitation time, i.e. : 50, 100, 200 g U/l, and 15, 30, and 60 minutes. Their behaviour in sintering was compared to a U O2 powder evolution, obtained by standard technology. All powders were hyperstoichiometric, with O/U > 2.1. The pressed powders have been sintered in H2 and He + H2, at different temperatures between 1400-1700o C. The average grain sizes for active powders are twice larger than for standard U O2. The main characteristics is the high rate of grain growth during the first two hours at 1700o C which strongly decreases in time. The grain boundaries became almost flat after 6 hours and the grains grow very slowly. The average grain size is in direct correlation with the uranium concentration in AU solution and with precipitation time. These two parameters determine a large density of source defects and a large grain boundary thickness which preserve during the subsequent processes until sintering. The grain growth is based on grain boundary diffusivity. The mechanism is not effected by the helium addition, but the grain growth rate is higher in this atmosphere. The activation energy of grain growth, 30± 7 K cal/mol is characteristic for hyperstoichiometric powders. (Author)