Cladding specimens were obtained from two fuel rods irradiated in the Big Rock Point Reactor to a burnup of ∼8 gigawatt days per ton. Both claddings had a uniform, thick (∼4 µm) zirconium oxide layer on the inner surface. The significant difference between the two rods was the degree of fission-gas release (0.2 versus 14.3 percent). The high-gas-release rod contained a significantly greater amount of fission-product deposits, and, in some areas, breaks in the inner-surface oxide were observed. The cladding speciments, with the fuel removed, were subjected to stress-rupture tests in an iodine environment using internal gas pressurization to evaluate their stress corrosion cracking (SCC) susceptibility. All tests were conducted at an initial iodine concentration of 0.6 mg/cm2 and a temperature of 325°C. Specimens from the high-gas-release rod exhibited signifciantly increased susceptibility to iodine SCC, with a threshold stress level of ∼200 MPa as compared with ∼280 MPa in the specimens from the low-gas-release rod. The results suggest that the inner-surface oxide provides a barrier to iodine penetration. Hence, the thick unbreached oxide on the low-gas-release rod provided greater resistance to iodine SCC than the breached oxide on the high-gas-release rod.

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