Positron annihilation spectroscopy (PAS) based on positron lifetime measurements using the conventional positron annihilation lifetime set-up and the Pulsed Low Energy Positron System (PLEPS) was applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels as well as for investigation of new materials foreseen for Generation-IV and fusion facilities. PAS results obtained in framework of several scientific projects are summarised and correlated to other techniques. Possibilities for experimental modelling of irradiation effects using ion implantation instead of neutron irradiation are discussed in the paper. The actually running program of micro structural study of Fe-Cr binary alloys is presented in detail. For the next effective study of radiation degradation of fusion as well as fission materials, the experimental micro structural study of model alloys in combination to computer simulations is recommended.