The formation of Cu-rich precipitates and defects in reactor pressure vessel steels(RPVS) has been considered as a key factor to induce embrittlement

and been investigated extensively.In this study

scanning electron microscope(SEM) and the positron annihilation spectroscopy were used to study the effect of thermal cycles on the Cu-rich precipitates and defects.It was found show that the defects in Fe-1.5wt%Cu alloy recovered after 2 h high-vacuum annealing at 1 173 K

while part of the Cu atoms aggregated to form nano-particles

which attracted and captured thermal positrons.The SEM images showed that density of the Cu nano-particles reduced with increasing annealing temperatures

demonstrated by changes in the W parameters.It was also found that the alloy still contained a small amount of Cu nano-particles even at annealing temperature of 1 473 K.