[en] Full text of publication follows: The understanding of the behaviour of fission nuclear fuel under irradiation is of first importance to foresee the state of the fuel in reactors and also if it is used as a nuclear waste storage matrix. Uranium dioxide is a major component of nuclear fission fuel and is used as a model material to study the behaviour of the nuclear fuel. The behaviour of UO₂ under irradiation has been extensively studied by using different techniques such as Channeling Rutherford Backscattering, RX diffraction and so on (see reference [1] for a review), but only very few studies have been focused on the direct observation of point defects and the determination of their properties. Positron annihilation spectroscopy is a useful and powerful method to study the vacancy defects. In this work, we have used positron annihilation lifetime spectrometry (PALS) to characterize the vacancy defects induced by irradiation in UO₂ and to determine their thermal properties. Vacancy defects induced in UO₂ disks by irradiation with electrons and alpha particles have been characterized by using ²²Na based positron lifetime spectroscopy (PALS). Several UO₂ disks have been irradiated in different conditions: with electrons at two energies, 1 MeV and 2.5 MeV, with alpha particles at 45 MeV. These results indicate that the vacancy defects detected are formed only when displacement in the U sub-lattice occurs. The annihilation characteristics have been measured in the irradiated UO₂ disks after annealing at temperature in the range from 400 to 1300 deg. C during 1 hour in Ar/H₂ atmosphere. Their evolution shows that the vacancy defects distribution evolves with temperature. One annealing stage appears at about 800 deg. C. We will discuss the phenomena that could explain these annealing stage. [1] Hj. Matzke, Radiat. Eff. 64, 3 (1982); Nucl. Instr. Meth. B 32, 455 (1988). [2] W.J. Weber, Journal Nucl. Mater. 114 (1983) 213. (authors)