Shimomura and Mukouda [1] reported that a void could be formed by clustering of only vacancy. In fission-neutron-irradiated copper at 300°C to 7 x 10 17n/cm2 at Kyoto University Reactor (KUR), the number density of voids exceeds the number density of hydrogen and helium atoms that are generated by the transmutation reaction [2]. A copper foil irradiated in this experiment was prepared to be a very low content of residual gas atoms by melting in highly evacuated vacuum. It is reported by the present authors that vacancy clusters move as a cluster at high temperature and coalesce to a larger cluster [3, 4]. The objective of the present work is to show how a vacancy cluster can grow to a void by clustering of only vacancies at high temperature. This means that a void can be formed without an inclusion of gas atoms in vacancy clusters in neutron-irradiated copper at high temperature. Of course if gas atoms are included in a small vacancy cluster, a void formation is promoted significantly as reported on experiments of multi-ion beam irradiated copper [5]
