Effects of the microstructure and alloying elements on the iodine-induced stress-corrosion cracking behavior of nuclear fuel claddings

A quantitative analysis of the iodine-induced stress-corrosion cracking (ISCC) process for a Zircaloy-4 cladding and a ZIRLO cladding was performed to support grain-boundary pitting coalescence (GBPC) and pitting-assisted slip cleavage (PASC) models for an ISCC behavior. It was focused on the effects of the microstructure on a grain-boundary pitting and the transgranular cracking phenomenon during a crack propagation step. Also, a microscopic analysis of the stress intensity applied to pits on a grain surface was performed to evaluate the cleavage crack propagation rate of the stress-relieved (SR) and re-crystallized (RX) grains. During the ISCC cracking, it was revealed that the grain shape and cleavage habit plane played important roles in a grain cracking, which resulted in an IG–TGc (TG cracking by cleavage) or a TGf (TG cracking by fluting)–TGc cracking mode. An IG–TGc cracking took place for the RX microstructure through a GB pitting, however, a TGf–TGc cracking did occur for the SR one which in turn increased its propagation rate. The increase of the pitting resistance at the grain-boundary played a critical role in the crack propagation rate of the ZIRLO cladding.