Effects of neutron flux on irradiation-induced hardening and defects in RPV steels studied by positron annihilation spectroscopy

Neutron-flux effects on irradiation-induced hardening and microstructures in a reactor pressure vessel steel were studied. An A533B-type steel containing no Cu was neutron-irradiated with fluxes of 1 x 10(14) n/cm(2)/s (high-flux) or 1 x 10(12) n/cm(2)/s (low-flux) to the same fluence of approximately 3 x 10(19) n/cm(2), and the same temperature of approximately 290 degrees C. The recovery behavior of irradiation-induced defects and irradiation-hardening, Delta Hv, was investigated by post-irradiation isochronal annealing from 275 to 450 degrees C. In both the high- and low-flux cases, the recovery behavior of Delta Hv and the average positron lifetime, tau(ave), corresponded well to the annealing, suggesting that defects in which positrons are trapped are the origin of irradiation-hardening. The values of (Delta Hv)under bar and tau(ave) in the high-flux sample started to recover at around 350 degrees C, while those in the low-flux sample started to recover at around 400 degrees C. Thus, in the high-flux sample, unstable defects transiently existing at low temperature but annealed out at around 350 degrees C, are indicated. Such defects are suggested to be defect-(Mn, Ni, Si) complexes, where the nature of the defect is that of a mono-vacancy and/or dislocation loops. (C) 2020 Elsevier B.V. All rights reserved.