[en] The α decay of fission products formed in UO₂ nuclear fuel during in-reactor irradiation generates defects and leads to helium production. To predict the spent fuel evolution during the long term storage, the behavior of He has to be understood in correlation with the defects induced by in-reactor irradiation and post-irradiation α decay. Slow positron beam coupled with Doppler broadening spectrometer was used to investigate near surface vacancy-defects in different sets of sintered polished uranium oxide disks, annealed in H₂/Ar atmosphere at different temperatures in the 500 to 1700 deg C range. The low and high momentum annihilation fractions, S and W respectively, were measured in the near surface region of the disks as a function of positron energy. The S(W) plots show that at high energy, positrons detect the same annihilation state in all disks. However, at lower energy, different annihilation states are observed, depending on the annealing temperatures. Vacancy defects are found in as-polished sintered. Larger vacancy defects are detected after annealing in the temperature range 1000 to 1400 deg C. The interaction of hydrogen with these vacancy defects is discussed. (authors)