In certain materials under neutron irradiation, there is no way of turning off the helium production as a means ofcontrolling void formation. So, in materials where helium is produced, helium bubbles are formed and contribute to the swelling behavior. Some details of helium production have been discussed in Section 6.2.3. In reality, void formation takes place under conditions of damage rate, temperature, and sink density at which the vacancy supersaturation is not enough to nucleate voids. This observation led Cawthorne and Fulton [18] to suggest that helium atoms in the metal stabilize the small void nuclei and prevent the collapse of embryo voids into vacancy loops. Helium remains intimately involved in the formation of voids as nucleation process turns heterogeneous. So, voids are essentially partially or

fully vacuum. As the radiation dose increases, more helium is produced to fill up the voids fully and then the features are rather called “bubbles.” However, one important distinction should be remembered — voids do not need helium gas to grow, but bubbles need gas to grow. The morphology of the bubbles tends to be more spherical compared to voids that assume faceted shape.