A metallographic and fractographic study of outside-in cracking caused by power ramp tests

The failure mechanism experienced by high burnup segment rods in power ramp tests was studied using metallographic and fractographic observations on rods which had been ramp-tested after four and five cycles of base-irradiation in a BWR. The observations revealed that cracking was initiated outside the cladding tubes and penetrated inwards. The process started an axial split with cracking of radial hydrides that were formed during the power ramp test, followed by propagation caused by step-by-step cracking of hydrides at a crack tip. The metallographic observation showed high hydride content in the Zr-layer facing pellet–pellet interfaces. This high hydride content seemed important for the formation of radial hydrides outside the tubes. Hydrogen at the crack tip in the propagation process was produced by oxidation of the crack surface by diffused water vapor and might also be critical to the mechanism as well as hydrogen in the cladding tubes.