Hydrides effect on the SCC initiation on zircaloy-4 claddings

The main cause of failure of CANDU-type fuel bundles during their operation is the Stress Corrosion Cracking (SCC). This mechanism appears in presence of the simultaneous action of a corrosive agent and of mechanical stress. The presence of precipitated zirconium hydride platelets can affect the propagation of the SCC cracks. For this reason, we studied the influence of content of hydrogen on the initiation and the propagation of SCC crack. To do this study, some C-rings samples with different content of hydrogen (100-500ppm) were exposed in a corrosive environment containing iodine (10(-4)g/cm(2)) at 320 degrees C. The stress state, the concentration of the corrosive environment and the temperature were maintained constant. For comparison, in the same conditions some as-received samples were exposed. To relieve the morphology of the precipitates of the zirconium hydride the samples were been investigated using metallographic method, before and after thermal treatments. While before the experiment a circumferential orientation of the hydrides precipitates was observed, after the thermal treatments a reorientation of hydrides in a radial direction was been remarked. Investigation by SEM method revealed the propagation modes of cracks in the case of the as-received samples and it was put in evidence the existence of the microcracks on the inner surface of tested samples.