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Theoretical concepts of the positron annihilation process in structural materials of nuclear reactors, taking into account the peculiarities of their electronic structure, have been developed. The Tao - Eldrup model, which allows to analytically calculate the lifetime of a positron in a spherically symmetric potential well, has been modified for the case of a potential well of finite height, in order to expand the limits of the model's application. The dependence of the positron lifetime on the height and width of the potential well, which occurs at the point defects, was determined. The results obtained within the framework of the modified model provide important information for the analysis of positron lifetime spectra in irradiated materials and data for the verification of quantitative calculations by the method of density functional theory. [ABSTRACT FROM AUTHOR]