Three coordinated research programs were undertaken on the fracture toughness of Zr-2.5Nb pressure tubes to determine relationships between irradiated and unirradiated values, the effect of long-term irradiation, and the causes of the variation in toughness. The present paper describes results from these programs and their implications.
It is shown that a correlation exists between the toughness of the pressure tube material before and after irradiation to a fast-neutron fluence of 1 × 1026 n ∙ m-2 (E > 1 MeV). Further, for the majority of tubes, degradation occurs early in the lifetime of a CANDU reactor; that is, at fluences <2 × 1024 n ∙ m-2. Limited results on relatively low-toughness materials indicate that the flux does not have a major effect on the irradiated toughness and that saturation is complete by about 1.6 × 1025 n ̇ m-2.
The results are discussed in the light of detailed fractographic and microstructural (X-ray diffraction, transmission electron microscopy) studies carried out on the irradiated and unirradiated pressure tube material. It is shown that in the presence of void nucleating sites, such as microsegregated species (chlorine and carbide) and particles (phosphides and carbides), very large reductions in toughness occur with irradiation. In the absence of such species, there is little deterioration in toughness, and the high crack-tip strains (toughness) achieved with unirradiated material can be maintained after irradiation.