Amount of iodine responsible for I-SCC of Zircaloy-4 in PCI-conditions: recoil-implanted and thermally released iodine Original Research Article

Pellet cladding interaction (PCI) phenomenon can lead to cladding failure by iodine induced stress corrosion cracking (I-SCC) of Zircaloy-4 during power transients. The aggressive species is present as gaseous iodine thermally released in the gap, and as recoil implanted iodine in the cladding. The aim of this work is to determine the respective roles of gaseous and implanted iodine on the SCC phenomenon. Quantities of iodine involved during a power transient in PCI conditions are then evaluated for a two PWR cycle irradiated fuel rod. An iodine recoil-implant profile in the cladding is first established. This calculation, in addition to some complementary SCC tests presented elsewhere, shows that implanted iodine seems to have no influence on the SCC susceptibility of the material. On the other hand, a detailed calculation of the surface concentration of free iodine in the inter-pellet area and in the radial cracks of the pellet confirms that, upon power transients, enough gaseous iodine is released to induce cladding SCC failures. Thus, thermally released iodine appears to be the chemical active species responsible for SCC.