[en] The effect of nickel alloying element on the precipitation of Cu-rich clusters in RPV model steel has been investigated by means of atom probe tomography. 40 kg ingots of RPV model steel were prepared by vacuum induction furnace melting with two different Ni contents (0.79 wt.% and 1.52 wt.%) and with higher Cu content (0.5 wt.%). The specimens with 50 x 40 mm² in 4 mm thickness were heat treated by 880 degree C/20 min water quenching, 660 degree C/10 h tempering and 400 degree C aging for different time from 100-2000 h. The results show that the increase of nickel content in RPV model steels will promote the precipitation of the Cu-rich clusters. The contents of Ni and Mn were detected in the Cu-rich clusters at the early stage. The Ni-rich clusters containing Cu and Mn were detected, and they could act as the nucleation sites for the precipitation of Cu-rich clusters. Mn-rich clusters could also be detected. Mn-rich clusters are not the nucleation sites for the precipitation of Cu-rich clusters, but the segregation of Cu atoms in those Mn-rich clusters containing much higher Ni could be found. Ni-rich clusters will act as the nucleation sites during the precipitation of Cu-rich clusters, therefore the increase of nickel content in RPV steels could promote the precipitation of Cu-rich clusters. This is the essential reason that the presence of nickel in PRV steels increases its sensitivity to neutron irradiation embrittlement. (authors)