A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO2 disks implanted with different 1 MeV He-3 fluences and annealed in ArH2 or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 x 10(14) to 2 x 10(16) He-3 cm(-2), the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 degrees C under ArH2. This evolution is found to be dependent on the He-3 fluence implanted in the sintered UO2 disks. For the lowest fluence of 2 x 1014 3 He cm(-2), the S(M plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 x 1015 to 2 x 1016 3 He cm(-2)) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 degrees C. Measurements performed in implanted UO2 disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed. (c) 2005 Elsevier B.V. All rights reserved.