Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

Delayed hydride cracking (DHC) tests were conducted on CANDU1 Zr–2.5Nb tubes containing hydrogen with a microstructure of elongated α-Zr and semi-continuous β-Zr at temperatures ranging from 100 to 300 °C. DHC velocity (DHCV) was found to be around two times higher in the axial direction than that in the radial direction even on the same cracking plane. According to Kim’s DHC model that DHCV at temperatures lower than 300 °C is governed mainly by hydrogen diffusion and to a lesser extent by the hydrogen concentration gradient at the crack tip, we suggest that the enhanced DHCV in the axial direction arises from a faster hydrogen diffusion in the axial direction with a semi-continuous distribution of β-Zr. Evidence to this suggestion is provided by Levi’s experiment that Zr–2.5Nb tubes or plates with a fully discontinuous β-Zr due to annealing at 400 °C for 1000 h or 650 °C for 9 h has no anisotropic DHCV with the orientation. Hence, we conclude that Kim’s DHC model is plausible. An anisotropic DHCV of irradiated CANDU Zr–2.5Nb tubes is discussed based on their microstructural evolution with the neutron fluence and operating temperatures.