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

Pellet cladding interaction (PCI) phenomenon can lead to cladding failure b y iodine induced stress corrosion cracking (I-SCC) of Zircaloy-4 during pow er transients. The aggressive species is present as gaseous iodine thermall y 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 implan ted iodine on the SCC phenomenon. Quantities of iodine involved during a po wer transient in PCI conditions are then evaluated for a two PWR cycle irra diated 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 c alculation of the surface concentration of free iodine in the inter-pellet area and in the radial cracks of the pellet confirms that, upon power trans ients, enough gaseous iodine is released to induce cladding SCC failures. T hus, thermally released iodine appears to be the chemical active species re sponsible for SCC. (C) 1998 Elsevier Science S.A. All rights reserved.