Structure and properties of hafnium after a 9-year operation in the RBT-6 research reactor

Hafnium control rods in the form of plates 6 mm thick were examined after a 9-year operation in the RBT-6 research reactor in water coolant at 68–86 °С to a maximal fluence 7.5 × 1025 m−2 (Е < 0.625 eV) and 2.4 × 1025 m−2 (E > 0.8 MeV). The Hf crystalline lattice volume increased by 0.4% and its density decreased. The oxide film thickness did not exceed 1 μm. Linear-type a-dislocations with a concentration of (5 ± 2) × 1013 m−2 were found in the structure as well as fine precipitates, where iron and chromium were detected, in addition to hafnium. Hf strength changed insignificantly; the material preserved its plasticity at a level of 8–15%. The performed examinations allowed the prolongation of the control rod design operation for up to 20 years.