Abstract: Early-stage formation of Cu-rich nano-clusters in thermal aging nuclear reactor model pressure vessel steel quenched and tempered was investigated by atom probe tomography(APT).After the initial heat treatment at 880 ℃ for 0.5 h and water quenching,the materials were tempered for 10 h at 660 ℃ and air cooled.Cu-rich nano-clusters were observed in the samples aged at 400 ℃ for 1 000 h.The change in composition of the steel in the early-stage of precipitation of Cu-rich nano-clusters was studied by the maximum separation envelope method(MSEM).Cu nanoclusters are observed to form from high nickel regions.The Cu concentration increases and the Ni concentration decreases at the central cores with increasing Cu atoms congregation.Ni and Mn atoms aggregation on the exterior side of the cluster/matrix interface is also evident.Based on experimental results,the reason that Ni and P can increase the sensitivity to neutron irradiation embrittlement of the nuclear reactor pressure vessel steel is discussed.