The nuclear reaction (n, p) contributes to the displacement damage and generates a significant amount of hydrogen in metal lattices. One way of reducing the influence of H on the properties of reactor materials is changing the transport rate by trapping at defects. The hydrogen atoms are trapped by the same open-structure defects that trap positrons. The trapping of H at vacancies can be detected through positron interactions with open volume defects. By model calculations based on the homogeneous electron gas theory developed by Kohn and Sham, the behavior of micro-voids containing H can be predicted. The results from model calculations of positron lifetime as a function of temperature show a correlation between the micro-void size in nickel, hydrogen releases and values of positron lifetime.