The main aim of scientific research is development of theoretical ideas with respect to the positron annihilation process in materials taking into account the peculiarities of its electronic structure. Implementation of applied calculations to experiments with positron lifetime spectroscopy in structural materials for nuclear reactors to study its defect structure. Research concerns the current issue of the influence of radiation on various types of metals, including structural materials of nuclear and thermonuclear reactors. Unambiguous explanation of the radiation effect on the material is quite a difficult task, because after irradiation it changes its properties. The method of positron annihilation spectroscopy provides unique opportunities for studying the electronic structure of irradiated material and determining the concentrations of point and extended defects, because the features of the electronic structure change the lifetime of the positron in the material.