Cold work changes dislocation network density and has been found to have a significant effect on the swelling characteristics of metallic materials. The higher dislocation density restricts the nucleation and growth of voids. The effects of cold work and fluence reveal that the same amount of cold work pro­vides less effect at higher fluences since cold work enhances the incubation dose for swelling but does not diminish the swelling rate in the linear swelling region at higher doses. This is thought to be a result of the fact that the disloca­tion density decreases in cold worked material during irradiation, while that in an annealed material increases. This accounts for the decreasing effect at higher doses.

Effect of Grain Size

Guthrie et al. [14] have demonstrated the effectiveness of fine grain size in inhibit­ing void formation by studying a film of sputtered nickel with ~0.5 pm thickness.

Under neutron exposure, the film with the ultrafine grain size did not show evi­dence of voids, while a nickel foil with a grain size of 30 pm produced voids. Also, similar experiments carried out in finer grain size material have shown similar results. The reason for this behavior is that the grain boundaries present in greater numbers in a fine grain size material act as efficient sinks for point defects. Recently, people have tried to investigate various forms of nanocrystalline material to see whether they are more resistant to radiation effects. However, the issue has not been resolved decisively to date.