Positron annihilation study of neutron irradiated pure Fe and Fe-Cu binary alloys

The hardening and embrittlement of RPV steels is of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage. Fe-Cu binary alloys are often used to mimic the behaviour of such steels. Their study allows identifying some of the defects responsible of the hardening, especially when compared to pure iron or C-micro-alloyed iron. Owing to their self-seeking and selective trapping, positrons are used to determine the nature of these defects. Recently, at SCK•CEN, a new PAS setup has been built, calibrated and optimized to measure the CDB and LT of neutron irradiated materials. This set-up has been used to measure n-irradiated Fe and Fe-Cu alloys. It is found that the clustering of Cu take place at the very early stages of irradiation using the CDB while LT measurement are showing much more vacancy clustering for low Cu alloys than in the higher ones. Increasing the neutron dose up to 1.3 1020 n/m2, allows the follow up of the kinetic of Cu and V-clustering especially in Fe-Cu alloys. It is found that both copper and carbon decrease the size of vacancy-cluster, when added to iron.