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Zirconium alloys are susceptible to a stable cracking process called delayed hydride cracking (DHC). DHC has two stages: (a) crack initiation that requires a minimum crack driving force (the threshold stress intensity factor, KIH) and (b) stable crack growth that is weakly dependent onKl,. The value of KIH is an important element in determining the tolerance of components to sharp flaws. The rate of cracking is used in estimating the action time for detecting propagating cracks before they become unstable. Hence, it is important for reactor operators to know how these properties change during service in reactors where the components are exposed to neutron irradiation at elevated temperatures. DHC properties were measured on a number of components, made from the two-phase alloy Zr-2.5Nb, irradiated at temperatures in the range of 250 to 290°C in fast neutron fluxes (E ≥ 1 MeV) between 1.6 × 1017 and 1.8 × 1018 n/m2 ∙ s to fluences between 0.01 × 1025 and 9.8 × 1025 n/m2. The neutron irradiation reduced KIH by about 20% and increased the velocity of cracking by a factor of about five. The increase in crack velocity was greatest with the lowest irradiation temperature. These changes in the crack velocity by neutron irradiation are explained in terms of the combined effects of irradiation hardening associated with increased <a>-type dislocation density, and β-phase decomposition. While the former process increases crack velocity, the latter process decreases it. The combined contribution is controlled by the irradiation temperature. X-ray diffraction analyses showed that the degree of β-phase decomposition was highest with an irradiation temperature of 290°C while <a>-type dislocation densities were highest with an irradiation temperature of 250°C.

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