Precipitation of copper-rich clusters is a major cause of in-service hardening of reactor pressure vessel steels and has attracted much attention. Experimental studies of microstructural changes in alloys under various conditions have revealed similarities and differences. It has been established that under ageing the precipitate ensemble experiences normal nucleation, growth and Ostwald ripening, a distinguishing feature of which is the bcc-9R-3R-fcc transformations the precipitates undergo during growth. The main effect of electron irradiation is believed to be enhancement of the diffusion of copper and hence acceleration of the kinetics. In the case of neutron irradiation, however, there are many aspects that are not clear. One is that at temperatures less than about 300°C the precipitate size is observed to be very small (∼1-3 nm), i.e. the coarsening rate is very low. In this paper we study this phenomenon by computer simulations based on the “mean-field” approach for describing microstructural evolution.