MICROSTRUCTURAL ASPECTS OF NEUTRON EMBRITTLEMENT OF REACTOR PRESSURE-VESSEL STEELS - A VIEW FROM POSITRON-ANNIHILATION SPECTROSCOPY

A comprehensive review of positron annihilation studies of Cr-Mo-V reactor pressure vessel (RPV) steels (Soviet type 15Kh2MFA) in unirradiated and neutron irradiated states is presented. The influences of lattice defects, impurity atom distribution, irradiation temperature, flux and fluence of fast neutrons on positron annihilation parameters, especially during isochronal annealing, are discussed in terms of the positron trapping model. In contrast to the literature, where irradiation-enhanced Cu precipitates and solute coated microvoids are thought to be major defect types responsible for strengthening and hence embrittling of RPV steels, we suggest irradiation-induced precipitates, i.e. probably carbides, to play this role. Possibilities to probe this model are suggested.