Surface Crack Initiation and Crack Propagation of HANA-4 Cladding in an Iodine Environment

Nowadays, most nuclear power reactors are adopting a high burn-up to increase their fuel economy, where the refueling cycle of the fuel bundles is extended. As a fuel burn-up increases, the possibility of an iodine-induced stress-corrosion cracking (ISCC) is increased. KAERI had developed six kinds of advanced Zr alloy claddings named HANA (High performance Alloy for Nuclear Application) in order to meet the global demand for an extension of the fuel discharge burn-up to more than 70 GWd/MtU. In this high burn-up condition, the diameter of the cladding is decreased but the outer diameter of the fuel pellet is increased so that the PCI behavior will be very severely. Also, the concentration of iodine inside the fuel rod increased so that the possibility of both a mechanical and a chemical interaction which causes a SCC become more severe. So the PCI resistance of HANA cladding needs to increase. This study has been done to estimate the PCI behavior of HANA-4(Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr) cladding. The ISCC crack initiation and propagation mechanism of the HANA-4 cladding was explained well by the grain-boundary pitting coalescence (GBPC) and the pitting-assisted slip cleavage (PASC) models. The effect of a heat treatment and the microstructure on a PCI resistance was also explained well by the GBPC and PASC models. The PCI resistance for HANA-4 was higher than that of Zircaloy-4