Flaws in Zr-2.5Nb alloy pressure tubes in CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. An overload occurs when the hydrided region at a flaw is loaded to a stress higher than that at which this region is formed. Flaw disposition requires justification that the hydrided region overload from normal reactor operating and transient loading conditions will not fracture the hydrided region, and will not initiate DHC. Some preliminary test results on the effect of hydrided region overload on DHC initiation were presented in Reference [1]. In the present work, several series of more systematically designed monotonically increasing load experiments were performed on specimens prepared from an unirradiated pressure tube with hydrided region formed at flaws with a root radius of 0.1 or 0.3 mm under different hydride formation stresses and thermal histories. Crack initiation in the overload tests was detected by the acoustic emission technique. Test results indicate that the resistance to overload fracture is dependent on a variety of parameters including hydride formation stress, thermal history, flaw geometry and hydrogen concentration.

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