Fracture initiation for axial cracks in pressure tube (PT) specimens of hydrided irradiated Zr-2.5Nb materials with split circumferential hydrides at room temperature is examined by conducting three-dimensional finite element analyses. With a strain-based fracture criterion with consideration of stress triaxiality, the location for the earliest crack initiation is determined near the middle of the axial crack front. For PT specimens with split circumferential hydrides, three types of strain concentration are observed in the ligaments ahead of the crack front. The computational results suggest that the internal pressure for crack initiation of hydrided irradiated PT specimens with many randomly distributed split circumferential hydrides needs only 55% to 70% of that for unhydrided irradiated PT specimens. The computational results can be used to explain the near 40% reduction of the fracture toughness at room temperature obtained from hydrided irradiated PT specimens when compared with that from unhydrided irradiated ones.

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