The presence of voids in irradiated materials can be primarily measured from transmission electron microcopy studies. Only this kind of examination can pro­vide information on the void size and density. The extent of swelling is generally measured by Archimedes principle by immersing the irradiated material in a fluid of known physical density and compared with the original volume (V), that is, it is measured in terms of the ratio of swelling-induced volume change (Д V/V) to the

original volume. For narrow void distribution, the term is given by the following relation:

DV 4 _3 , N

— = 3 pR 3Nv, (6.2)

where R is the average void radius and Nv is the number of voids per cm3.