Angular correlation positron annihilation spectroscopy applied in investigation of neutron irradiated RPV-steels

Angular correlation positron annihilation spectroscopy (AC PAS) has been used for the investigation of reactor pressure vessel (RPV) specimens prepared for the ``Extended surveillance specimen program'' in the 3rd and 4th units of NPP Jaslovsk\'e Bohunice (Slovakia). PAS spectra showed that the changes associated with the effects of neutron irradiation can be well detected. The samples from RPV base material (15Kh2MFA) and weld (Sv 10KhMFT) were measured by angular correlation positron annihilation spectrometer before and after irradiation. Samples have been irradiated in VVER-440 reactor (units 3 and 4 in Bohunice) by neutron fluence from 7.8\times 10^{23} m ^{-2} up to 2.5\times 10^{24} m ^{-2} . Results of PAS measurements are presented and discussed in detail.