Delayed hydride cracking in irradiated zircaloy cladding

Slow stable crack growth by a mechanism identified as a form of delayed hydride cracking has been studied on irradiated Zircaloy cladding. The background to the investigation was the formation of long axial cracks in defected fuel rods. Post-irradiation examination of such fuel rods has indicated that precipitation and subsequent cracking of hydrides at the tips of the long cracks has played an important role in the crack growth process. The present investigation conducted on irradiated cladding with hydrogen concentrations above about 500 ppm has demonstrated that a hydrogen-induced crack growth process can occur in such material. In the laboratory it was necessary to subject the sample to an overtemperature cycle in order to initiate crack growth after fatigue precracking. It was also observed that an incubation time on the order of 20 h was necessary before crack growth started. The crack growth rates were strongly dependent on the applied stress intensity factor K in a narrow range above a threshold value K-1H, which was about 10 MPa root m, Stage I. The growth rate then reached a plateau value when it was independent of K. Stage II. This plateau value was about 10(-6) m/s at 300 degrees C and about 2 x 10(-7) m/s at 200 degrees C. This temperature dependence is consistent with a mechanism based on stress-induced diffusion of hydrogen at the stress concentration of the crack rip. Metallographic and fractographic observations suggest that the details of the mechanism can be best described as a localized reduction of fracture toughness due to reorientation of hydrides so that they become perpendicular to the applied stress in the region of the crack tip. This is somewhat in contrast to previous DHC mechanisms in which longer-range diffusion of hydrogen to one large hydride at the crack tip is usually modeled. The difference is that in the present case the hydride content is higher and therefore more hydrides are present.