Advances in the understanding of the mechanisms of iodine induced SCC cracking in zirconium alloys

In pressurized water reactors (PWR) the fuel rod cladding is the first barrier against the spread of fission products. It is therefore essential to guarantee its use in the reactor. Sometimes the production of electricity requires that certain power plants operate in 'network monitoring'. The fuel introduced into nuclear power reactors can then undergo sometimes significant power variations. Following a severe reactor power transient, clad failure can occur through a stress corrosion phenomenon (SCC), under the combined action of mechanical stresses and gaseous fission products generated by the fuel pellets. Among those iodine plays a major role, for it may induce SCC in zircaloy. In the early ages of water cooled reactors (PWRs, BWRs or CANDU), series of similar failures took place following sharp startups. Today power increase rates as well as instantaneous local power levels are limited. Indeed, it is well know that cladding failure by iodine induced stress corrosion cracking (I-SCC) may occur under pellet cladding interactions (PCI) conditions during power transients in PWRs. In this paper we review the advances in the understanding of these SCC cracking mechanisms of the fuel rod cladding that would then allow better control of the integrity of the clad during the more severe demands related to the operating conditions of the PWRs. (author)