To evaluate crack initiation from small radius flaws in Zr-2.5 Nb pressure tubes in operating CANDU reactors, a number of crack initiation tests under both monotonic and cyclic loading in notched tension specimens have been carried out at ambient temperature. Test specimens were machined with the tensile axis parallel to the circumferential or transverse direction in both as-fabricated and ex-service pressure tube materials. The test specimens were first pre-conditioned by subjecting them to a notch-tip creep stress relaxation cycle, and a subsequent hydride formation cycle whereby notch-tip hydrides are produced. Some specimens with no notch-tip hydride were also tested. Test specimens were instrumented with acoustic emission (AE) and DC potential drop (PD) monitoring systems for detection of crack initiation and growth. Test results indicated the following trends: (a) the existence of notch-tip hydride lowers the crack initiation stress, with the hydride effect being much stronger under monotonic loading than under cyclic loading; (b) the greater the hydrogen concentration, the lower the notch-tip hydride crack initiation stress under monotonic loading; (c) for a given alternating elastic peak stress, the smaller the root radius the greater the resistance to crack initiation; (d) as expected, the number of cycles to crack initiation is a strong function of the alternating elastic peak stress, for a given specimen geometry. An analysis of the test results and their impact on flaw evaluation methodology for Zr-2.5 Nb pressure tubes is described.

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