The microstructural evolution of a pressure vessel steel (C shell base metal) was studied after a long-term irradiation (13 years) in the surveillance program of a CHOOZ A reactor. The positron lifetime technique was used to determine if the neutron-induced damage is partially composed of voids. Analysis of spectra measured on irradiated samples (fluence = 14 × 10¹⁹ n∙cm^-2, E > 1 MeV) reveals that the neutron irradiation has not induced the formation of voids containing more than about 50 vacancies (that is, voids with a radius higher than about 0.5 nm). On the other hand, some results suggest the presence after irradiation of a low density of very small voids (less than 10 vacancies) probably located in precipitates.