Stress Corrosion Cracking of Zircaloy-2 Cladding in Iodine Vapor

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Abstract
Iodine stress corrosion cracking of Zircaloy-2 cladding under static tensile stress has been studied over the temperature range of 250~450°C. The lowest iodine concentration required to cause cracking was 0.15 mg iodine per cm3 free volume in test ampoule at 300°C, and 1.1 mg/cm' at 350°C. The minimum circumferential plastic strain to cause failure was about 0.4% for specimen tubes possessing high cracking susceptibility. Softer tubes tolerate more circumferential deformation before they failed. This is attributed to greater margin retained for work hardening, rather than to smaller residual tensile hoop stress. Cracking was also inhibited by the presence of oxygen in the iodine atmosphere, the threshold partial pressures being 28 torr at 300°C and 9 torr at 350°C. Scanning electron micrographs of the fractured surfaces revealed brittle transgranular fracture carrying river patterns on part of the cleavage facets.

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