Influence of recoil-implanted and thermally released iodine on I-SCC of Zircaloy-4 in PCI-conditions: chemical aspects

The pellet cladding interaction (PCI) phenomenon can lead to cladding failu re by iodine induced stress corrosion cracking (I-SCC) during power transie nts. In these situations, the aggressive species is present as both, recoil implanted iodine in the cladding, and gaseous iodine thermally released in the gap. The aim of this work is to determine the respective roles of impl anted and gaseous iodine in the SCC phenomenon. Two types of SCC tests have thus been performed. In the first one, zirconium and iodine recoil implant ed tensile test specimens were used, with implantation profiles typical of those existing in a cladding under PCI conditions either on the dose or on the induced damage standpoint. These tests have shown that recoil implanted iodine has no chemical effect on the development of the SCC cracks. The se cond type of tests was performed on reference tensile test specimens at 350 degrees C with iodine released either, in oxygen containing atmospheres or , at increasing temperatures. The iodine efficiency for cracks initiation w as found to be stronger when no oxygen is available for repassivation and w hen iodine is released at higher temperature. These two conditions being fu lfilled during PCI loading, since no gaseous oxygen is available in the fue l-to-clad gap, and since iodine is released through the pellet radical crac ks at high temperature, thermally released iodine can be considered as the chemical active species responsible for SCC. (C) 1999 Elsevier Science B.V. All rights reserved.