The analysis of the mechanism of formation of radiation-induced clusters in RPV steels has been done.

Radiation-induced clusters are formed after the mechanism based on the inverse Kirkendall effect in two stages.

At post-dynamic stage a flow of vacancies moving to the center of the cascade entrains Cu atoms contained and forms a stable atom-vacancies cluster.

At the 2nd stage Cu, Ni, Mn, Si atoms forming complexes with vacancies diffuse into a cluster driving out Fe and Cr atoms from the cluster.

The cluster sizes remain unchanged.

Part of the vacancies clusters form stable vacancies-impurity clusters.

Part of vacancies clusters are transformed into dislocation loops dropping of matrix supersaturation with vacancies.