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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To determine the fracture properties of Zr-2.5Nb pressure tubes irradiated until the end of design life, cantilever beam, curved compact toughness, and transverse tensile samples were prepared from a typical pressure tube and irradiated in the high flux reactor OSIRIS at CEA, Saclay, France. Experiments were conducted on two batches of samples mounted in two irradiation inserts. Each insert held sixteen samples of each type of specimen. The first insert was irradiated to a fluence corresponding to approximately half of the design life in a CANDU3 reactor. The experimental results were reported in [1]. Samples in the second insert were irradiated for 10.5 years in OSIRIS and received a maximum neutron fluence of 2.61 × 1026 n/m2 (E > 1 MeV), being equivalent to 2.98 × 1026 n/m2 (E > 1 MeV) in a CANDU reactor, i.e., corresponding to ∼30 years operation in CANDU reactors at 80 % capacity factor. The present report describes the results of tensile, fracture toughness, and Delayed Hydride Cracking (DHC) tests and XRD microstructure analysis from the second batch of specimens. A continuous and gradual evolution in tensile, fracture, DHC properties, and dislocation densities is demonstrated without any evidence of a sudden change following the initial transitient at very low fluence. In the whole high fluence range, there is a very slow rate of increase in c-component dislocation density, strength, and DHC velocity and a slow reduction in elongation and Nb concentration in the β-phase. The a-type dislocation density and fracture toughness remain approximately constant. The results from the second insert of specimens confirm that, following the initial transient at very low fluence, there is little further change in the fracture properties of Zr-2.5Nb pressure tube material. Therefore, material properties behave in a stable and predicable manner to the end of a 30 years design life for CANDU reactor pressure tubes.

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