We report the dislocation microstructures of annealed Zircaloy-2, cold-worked Zr-2.5 wt% Nb and extruded Excel (Zr-3.5 wt% Sn-0.8 wt% Nb-0.8 wt% Mo) neutron irradiated in EBR-II at 670-690 K to fluences of 1.7–6.3 × 10²⁵ n/m² (E>1 MeV) and also of cold-worked Zr-2.5 wt% Nb irradiated to 1.2 × 10²⁵ n/m² (E>1 MeV) at 550 K in Unit 4 of Pickering Nuclear Generating Station.

The irradiation damage produced in zirconium alloys by fast neutron irradiation in EBR-II at 657–675 K consists of a type dislocation loops and, in annealed Zircaloy, c component dislocations. Both a type and c component dislocations survive fast neutron irradiation to relatively high fluences (5.5 × 10²⁵ n/m² at 657 K). Cold-worked Zr-2.5 wt% Nb irradiated in Pickering-4 to 1.2 × 10²⁵ n/m² at 550 K contains fine a type loops and residual c component dislocations from cold-work. All zirconium alloys exhibiting high (non-saturating) irradiation growth rates contain c type dislocations and a type loops. The presence of a component dislocations is also inferred. This suggests a modified model of irradiation growth in which vacancies segregate to c component dislocations or faulted loops and interstitials to a type loops.