The statistic of microcraks initiation in Zy-4 cladding

CANDU fuel operation in nuclear power plant showed that the main cause for failure is the stress corrosion cracking (SCC) mechanism. The paper presents a statistical study of microcrack initiation at the inner surface of the fuel cladding. These microckracs represent the first step of the SCC process because act like a fracture initiation pattern and induce a stress raising at the tip of microcracks. Experiments were performed on Zy-4 alloy and the samples were cut from CANDU type fuel cladding. The C-ring experimental method was used to obtain different strain and stress values. The stressed rings were exposed to a corrosive environment based on iodine. The period of chemical corrosion, temperature and iodine concentration were maintained constant, only the strain and stress state was varied. The experimental results, consisting in statistical distribution size and number of microcracks, obtained from investigations by SEM method, revealed the type of chemical corrosion, the shape and propagation modes of cracks. The results obtained can be used to develop probabilistic models dedicated to prediction of the cracking process at the inner surface of the CANDU type fuel cladding. Also, these results showed a good correlation with deterministic models used to describe the SCC process. (authors)