VI - Amorçage des fissures de corrosion sous contrainte par l’iode dans le Zircaloy-4

On the initiation stage of iodine induced stress corrosion cracking of zircaloy-4: link between surface parameters and bulk characteristics of the cracks

Abstract

The pellet-cladding interaction (PCI) failures in Pressurized Water Reactor fuel rods are attributed to iodine induced Stress Corrosion Cracking (SCC) of Zircaloy-4. In this document, we focused our study on the intergranular initiation stage of the cracking, in an iodine methanolic environment.

Previous studies on the subject have underlined some parameters controlling crack location on the surface of tensile test specimens. In order to specify whether these parameters control the development of the cracks into the bulk of the material too, we have conceived an original experiment, based on successive polishing steps of the edge of a tensile test specimen, allowing to make the link between surface parameters and bulk characteristics of the cracks.

So, we have been able to show that the size, the shape and the density of the surface cracks had no influence on their development, which remains homogeneous and intergranular. We have shown moreover that every surface crack develops. Besides, we have confirmed that the geometric orientation of the grain boundaries towards the direction of the macroscopic applied stress was the decisive parameter for the location and the development of SCC cracks into the bulk of the material.

At last, we have proposed some explanations concerning the mechanisms of the intergranular iodine induced cracking of Zircaloy.