Properties of native vacancy defects and induced by irradiation and implantation in uranium dioxide determined using positron annihilation spectroscopy

The behaviour of uranium dioxide under irradiation has been extensively studied by using different irradiation conditions and characterization techniques. However experimental studies are still necessary to determine the fundamental properties of vacancy defects that can be induced by irradiation. In this thesis, positron annihilation spectroscopy has been used to study the properties in UO2 (nature, size, charge state, migration and agglomeration under thermal treatment) of both native vacancy defects and induced by irradiation with different particles (3He, e-, α). Positon lifetime has been measured in the bulk of sintered disks of UO2, and the positron-electron momentum distributions (Doppler broadening) in the first micrometer from the surface of the disks. These measures vary with the positon annihilation state and are characteristics of the nature of the present defects (vacancies or ions). Annihilation characteristics of polished UO2 disks annealed at 1700 deg C during 24 hours under a ArH2 wet atmosphere are equal to those of the UO2 lattice. After irradiation with different particles (3He, e-, alpha). defects induced by uranium atoms displacement have been detected; they are uranium mono-vacancies Vu and/or vacancy complexes implying Vu, like the divacancy Vu-Vo or the Schottky defect Vo-Vu-Vo. Their annihilation characteristics have been determined. Study of these vacancy defects evolution as a function of the temperature and the atmosphere allowed to highlight several annealing stages leading to vacancy clusters formation and/or vacancy complexes with hydrogen or helium. (author)