Diskussion möglicher mechanismen von PCI-Defekten

Abstract

To investigate the I-SCC behaviour of Zry-tubing a standard test procedure on the basis of internally pressurized creep specimens (I-SCC standard test), and a split ring test (I-SCC laboratory test) were developed. The critical iodine concentration for SCC was determined to be approximately 10 −6g I cm 2 of specimen surface. A thermodynamic evaluation reveals that SCC only occurs in the region of I concentration and temperature where stable zirconium iodides in the condensed phase are to be expected. This hypothesis was also proven by using the reaction products from iodine with zirconium instead of elementary iodine to produce SCC. The tests performed this way show that prereacted zirconium iodides generate the same type of brittle fracturing as fresh iodine does. A distinct minimum is found in the uniform elongation versus strain rate. Altogether, our out-pile investigations clearly indicate that I-SCC of Zry-cladding needs both, enough I to provide a sensitive I chemistry and a critical strain rate or stress in the I affected area. After a prolonged exposure at low power ramp of sufficent height generates tensile stresses in the cladding leading to local plastic deformations. After a certain delay time the fission product release shows a peak leading to an additional temperature rise in the fuel by decreasing the thermal gap conductance between fuel and cladding (thermal feed-back). I-SCC cracking is now possible if enough I is released to provide a critical I chemistry at a time interval longer than the incubation plus propagation period and where simultaneously the mechanical load is in the critical range.