This work extends the object kinetic Monte Carlo model for neutron irradiation-induced nanostructure evolution in Fe–C binary alloys, introducing the effects of substitutional solutes like Mn and Ni. The objective is to develop a model able to describe the nanostructural evolution of both vacancy and self-interstitial atom (SIA) defect cluster populations in Fe(C)MnNi neutron-irradiated alloys at the operational temperature of light water reactors, by simulating specific reference experiments. To do this, the effects of the substitutional solutes of interest are introduced, under simplifying assumptions, using a ‘‘grey alloy’’ scheme. Mn and Ni solute atoms are not explicitly present in the model, but their effect is introduced by modifying the parameters that govern the mobility of the defect clusters. In particular, the reduction of the mobility of point-defect clusters as a consequence of the presence of solutes proved to be key to explain the experimentally observed disappearance of detectable defect clusters with increasing solute content. Solute concentration is explicitly taken into account in the model as a variable determining the slowing down of self-interstitial clusters; small vacancy clusters are assumed to be significantly slowed down by the presence of solutes, while for clusters bigger than 10 vacancies complete immobility is postulated.