[en] The neutron irradiation embrittlement of reactor pressure vessel (RPV) steels needs to be properly predicted and managed for the safe long-term operation of light water reactors. To investigate the effects of the solute elements Ni, Mn and Si on the irradiation hardening at high dose levels, pertinent to long-term operation, in RPV materials without Cu, ion irradiation for Fe-1.0Ni-0.2Si, Fe-1.0Ni-1.4Mn and Fe-1.0Ni-1.4Mn-0.2Si alloys was carried out at 290degC up to 5.0 dpa by Fe-ions with the energy of 2.8 MeV. Nano-indentation techniques were applied to evaluate the ion irradiation hardening. The results show that Mn significantly accelerates the irradiation hardening and addition of Si reduces the irradiation hardening. The depth dependence of irradiation hardening was discussed, and the irradiation hardening was the most significant at the indentation depth of about 120 nm in all alloys. The hardening behavior in deeper region was slightly different from that observed at about 120 nm depth, which may be caused by the interactions between solutes and defects. (author)