In order to determine if the formation of vacancy defects in pressure vessel steels during neutron irradiation is partly controlled by the chemical composition, we have studied the effects of electron irradiations on the following model alloys : Fe, Fe-0, l%Cu, Fe-0,l%Cu-0,004%C, Fe-0, l%Cu-0,010%C, Fe-0, l%Cu-0, 5%Mo, and Fe-0, l%Cu-l, 2%Mn. The irradiations have been carried out at 288°C and 150°C with fluences up to 4 1023 e-.m-2. After irradiation, some samples underwent 30 min annealing treatments at temperatures ranging up to 380°C.
The mobility of vacancies and their ability to form clusters have been studied by the positron annihilation technique. We have noticed that the clustering of single vacancies in iron is favoured by the presence of molybdenum or carbon. The observed defects are small and are made of 10 to 13 vacancies. In the same conditions, copper and manganese do not seem to have any effect on the formation of vacancy clusters. We have also observed that the recovery of the vacancy-type defects occurs at temperatures close to that of pressure vessel of PWR (288°C). These temperatures depend also on the nature of the alloying elements.