  The formation of nano-sized, coherent, solute-rich clusters (NSRC) is known
to be an important factor degrading the macroscopic properties of steels under
irradiation. The mechanisms driving their formation are still debated. This
work focuses on low-Cu reactor pressure vessel (RPV) steels, where solute
species are generally not expected to precipitate. We rationalize the processes
taking place at the nanometre scale under irradiation, relying on the latest
theoretical and experimental evidence on atomic-level diffusion and transport
processes. These are compiled in a new model, based on the object kinetic Monte
Carlo (OKMC) technique. We evaluate the relevance of the underlying physical
assumptions by applying the model to a large variety of irradiation
experiments. Our model predictions are compared with new experimental data
obtained with atom probe tomography and small angle neutron scattering,
complemented with information from the literature. The results of this study
reveal that the role of immobilized self-interstitial atoms (SIA) loops
dominates the nucleation process of NSRC.
