Thermochemical evaluation of PCI failures in LWR fuel pins

In searching for the reasons behind the PCI failures of LWR fuel pins two questions have obviously remained unanswered: (a) what is the iodine potential necessary to cause SCC of zircaloy, and (b) is this iodine potential available in a fuel pin. To answer these two questions, a consistent set of thermochemical data for the Zr-I system was created, the results of laboratory tests of iodine-induced SCC of zircaloy were evaluated, and, finally, equilibrium calculations for the fuel-fission-product system were performed to determine the temperature and oxygen potential required to produce an iodine potential high enough to cause SCC of zircaloy. The conclusion of this study is that SCC of zircaloy can be caused by an iodine potential equal to or greater than that needed to form ZrI with metallic zirconium. This iodine potential is available in an LWR fuel pin at oxygen potentials corresponding to stoichiometric fuel. The carrier of the potential, i.e. the attacking species, is CsI. (orig.)