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In this study, ex-ADU UO 2 powders are doped by small amounts of various corundum oxides such as Cr 2 O 3 , Ti 2 O 3 , Al 2 O 3 , V 2 O 3 . Doped UO 2 powders were pressed up 50% theoritical density (TD) and sintered under 90% Ar + 10% H 2 atmospheres at 1700 ˚C for duration of 4 hours. Pellet densities, longitudinal and shear wave ultrasonic velocities for each pellet were measured. Average grain diameter for each pellet was found by image analysis system. Elastic properties of pellets were determined from ultrasonic measurements.

ThO 2 powder was obtained by precipitating thorium oxalate from thorium nitrate solution using oxalic acid in the presence of ultrasound and then calcinating the precipitate. ThO 2 and 0.25 mol% V 2 O 5 , 0.25 mol% Ta 2 O 5 and 0.25 mol% Nb 2 O 5 separately mixed then pressed into cylindrical pellets. The green pellets were sintered at 1400 and 1700 o C under Ar-10% H 2 atmosphere. Sintered densities of pellets were determined and microstructural observations carried out. The results showed that Ta 2 O 5 and Nb 2 O 5 doped ThO 2 pellets had higher densities than V 2 O 5 doped ThO 2 pellets after sintering at 1400 and 1700 o C. Ta 2 O 5 and Nb 2 O 5 doped ThO 2 pellets reached a density of 97% TD after sintering at 1700 o C. Microstructural observations showed that V 2 O 5 doped thoria pellets after sintering at 1700 o C had average grain size of 15 µm while it was 32 and 30 µm for the Nb 2 O 5 and Ta 2 O 5 doped ThO 2 pellets, respectively.