Development of Fracture Mechanics Method to Evaluate Iodine Stress Corrosion Cracking of Zirconium Alloys

Iodine stress corrosion cracking (ISCC) is a leading candidate failure mechanism to explain brittle cracking of zirconium alloy fuel claddings during rapid power changes in LWR plants. Known as pellet-clad interaction (PCI), details of this failure are not well understood, with limited data at temperatures exceeding 315 deg. C. This study developed an experimental method to produce and measure iodine SCC of zirconium alloys using both bolt-loaded and ring-loaded short double cantilever beam (SDCB) specimens. Method development included derivation of K and compliance coefficients for SDCB specimens, sizing a load ring, and fabricating and establishing environmental control in zirconium alloy test vessels. Vacuum creep experiments did not reveal void formation or crack propagation, but load relaxation in both specimen and ring was significant above 482 deg. C. Brittle fracture features characterized by varying levels of transgranular facets with fluting and smooth intergranular facets were observed under all test conditions. Intergranular cracking was more prevalent as iodine concentrations increased and as stress intensity factor values decreased. Bolt-loaded specimens were completely relaxed at the end of the test, showing the importance of ring-loading to sustain load as cracks advanced. Ring-loaded crack growth rates approached 1.8 μm/s. (authors)