About 400 pressure-tubes made of cold-worked Zr-2.5Nb are used in a CANDU power reactor to contain the nuclear fuel and the heat transport fluid, heavy water, at between 250 to 310°C at a pressure of about 10 MPa. The fracture toughness of these tubes is needed to determine if they are fit for service. We have measured the fracture toughness by bursting 450 mm long tube sections and using 17 mm curved compact toughness (C(T)) specimens, employing the J-integral resistance curve method in both cases. The burst test is expensive but it simulates the stress state under which the pressure tubes operate, while the small specimen method is more economical although the interpretation of data is more difficult.
The results indicate that the fracture toughness decreases rapidly during the first few years of operation, but when the fluence reaches about 3 × 1025 n/m2 (E > 1 MeV) the decrease in toughness slows down. The post-irradiation toughness is not affected by either the initial mechanical properties or the difference in irradiation temperatures at the inlet and outlet of the fuel channel.
Although the results from small specimens and the burst tests show the same trend for the decrease of toughness with fluence, the JR curves obtained from the C(T) specimens are more conservative. Reasons for the differences are discussed. The evolution of microstructure during irradiation was also studied in an attempt to correlate the change to fracture toughness.