Microstructural evolution in medium copper low alloy steels irradiated in a pressurized water reactor and a material test reactor

A533B steels containing 0.12% and 0.16% Cu were irradiated to 3x1023 and 6x1023 n/m2 (E>1 MeV) at 290 deg. C in a pressurized water reactor (PWR) and a material test reactor (MTR). Microstructural changes were examined using atom probe, small angle neutron scattering, field emission gun scanning transmission electron microscopy and post-irradiation annealing (PIA) coupled with positron annihilation (PA) and hardness testing (Hv). Cu rich precipitates had a Cu enriched core with surrounding Ni, Mn and Si rich region and the atomic composition was Fe-(7-16)Cu-(2-8)Mn-(0-4)Ni-(0-4)Si. The size and number density of Cu rich precipitates and the residual Cu concentration in matrix were almost saturated at above 3x1023 n/m2. Low flux irradiation in PWR produced slightly larger precipitates of a lower density with a higher Cu concentration in the precipitates. PIA (PA and Hv) examination showed that vacancy type matrix defects after PWR irradiation were more stable and more effective for hardening than those after MTR irradiation