Zr-2.5Nb pressure tubes used in CANDU reactors are susceptible to hydride cracking-induced crack initiation mechanisms, a.k.a delayed hydride cracking or DHC and overload crack initiation. For the integrity assessment of CANDU pressure tubes, it is required to evaluate the likelihood of DHC and overload crack initiation from serviceinduced flaws and to determine whether the pressure tubes are still fit for continued service. The main focus of this proposal is to evaluate the effect of in-situ flux effect on the structure and fracture properties of zirconium hydrides. This is over and above the post-irradiation effects caused by changes in matrix properties such as increase in yield stress. This proposed experimental work will be performed on hydrided Zr-2.5Nb pressure tube material. Specimens prepared from the hydrided material will be subjected to simulated irradiation in an accelerator. Notch-tip hydrides will be formed in selected specimens and overload experiments performed afterwards to determine the hydride fracture stress. Electron microscopy will be performed before and after simulated irradiation to characterize microstructural changes and changes in hydride structure. Findings from this project will advance the understanding of hydride fracture behavior in reactor operating conditions and provide support for the structural integrity evaluation of CANDU pressure tubes.