Since a few years ago it has been well known1that reactor construction materials which are exposed to high neutron fluences, i.e. about 1022neutrons/cm2, will undergo density changes caused by creation and growth of voids. The void formation is accompanied by swelling of the material, an effect which can be as large as several per cent. However, it takes more than a year of irradiation time in a high flux reactor to reach the necessary neutron dose for the creation of such large void effects making systematic studies very time consuming. It is therefore convenient to simulate radiation damage, for instance by ion implantation, using the fact that the cross section for the ion-nucleus interaction is several orders of magnitude larger than that of the neutron-nucleus interaction. Thus a radiation damage comparable with that obtained in a high flux reactor can be produced after only a few hours or days of accelerator irradiation.