Environment induced cracking of zircaloy-2 fuel cladding tube

The influence of iodine vapour on the stress corrosion cracking (SCC) behaviour of zircaloy-2 fuel cladding tubes has been studied. SCC tests have been carried out by internal pressurisation technique in the temperature range of 300-400degC, with applied stress varying from 16-37 kg/mm2. The iodine concentration was varied from 1 mg/cm2 to 10 mg/om2 of the exposed surface of zircaloy tubes. Time-of-failure of the stress-relieved tubes was found to be strongly dependent on iodine concentration, temperature and applied hoop stress. The time-to-failure was found to increase from 6 minutes to 10 hours as the iodine concentration was decreased from 10 mg/cm2 to 1 mg/cm2. Similarly with 1 mg/cm2 iodine, the time-to-failure was increased by about 20 times by lowering the temperature from 400degC to 300degC. It has been observed that for the tube to fail by SCC, applied hoop stress should be around the yield stress of the material. Experiments under similar conditions were also carried out with graphite coated (inside surface only - CANLUB) zirocaloy-2 tubes. Results indicated that graphite coated tubes are more resistant to SCC as compared to the bare tubes. In order to find the SCC behaviour of the zircaloy-2 tubes under power ramp condition, SCC tests were also carried out in which the temperature was increased suddenly from 300degC to 500degC. It was found that the life time of the graphite coated tubes is about 1.5 times longer than that of the bare tubes. SEM examination showed the presence of corrosion products at the inside surface of tubes failed by SCC. The mechanism of failure appears to be one of mechanical fracture assisted by chemical attack. In the presence of iodine, preoxidised tubes had increased the life time, but the hydrided tubes failed within a very short time. Presence of cadmium has been observed to be harmful. (author)