Air Oxidation Kinetics for Zr-Based Alloys (NUREG/CR-6846)

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Publication Information

Manuscript Completed: March 2004
Date Published: June 2004

Prepared by:
K. Natesan, W.K. Soppet

Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439

S. Basu, NRC Project Manager

NRC Job Code Y6696

Prepared for:
Division of Sytems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

An experimental program was conducted to generate data on the air oxidation kinetics of unirradiated cladding of Zr-based alloys (such as Zircaloy-4, Zirlo, and M5) with an oxide layer that is representative of the current inventory of spent fuel discharged after a medium or high level of fuel burnup. The oxide layer that forms on the cladding while in the spent fuel pool was simulated by a preoxidation step in a steam environment for 140 h at 550°C for Zircaloy-4 and Zirlo and for 427 h at 550°C for M5. This resulted in an oxide thickness in the range of 25-30 µm for all three alloys. The steam-preoxidized specimens were subsequently oxidized in air at temperatures in the range of 300-900°C. Oxidation tests in air emphasized temperatures in the range of 300-600°C, which is representative of cladding heatup in the event of a partial or full draining of spent fuel pool coolant. The maximum air oxidation times ranged between 300 h at 600°C and ≈1000 h at 300°C. Weight change and oxide thickness measurements were made on the specimens exposed at various times to establish the kinetics of the scaling process as a function of temperature. Bare capsules of the three alloys were also exposed in air for comparison of the oxidation behavior of the alloys with and without steam preoxidation. Limited tests were conducted at 400 and 600°C to evaluate the oxidation performance of Zircaloy-4 in a low-oxygen high-nitrogen environment. Isothermal oxidation tests were also conducted with tube specimens of steam-preoxidized Zircaloy-4 with internal pressures in the range of 50-400 psig at 600 to 900°C. Extensive metallography was used on the tested specimens to examine the oxide scale development, pin-hole rupture morphology, and oxide cracking propensity. The data indicate that the oxide thickness is somewhat larger in the tests conducted with high internal pressure than that obtained in the absence of it, especially at 600 and 700°C. The increase is attributed to microcracks in the oxide (and associated oxidation) that result from increased deformation in the specimen due to pressurization. Weight change and oxide thickness data, generated in the present program, were used to develop correlations to depict the air oxidation behavior of the alloys as a function of time and temperature. The results showed that the correlation, developed for Zircaloy-4 from the oxidation data generated in the current project, is in fair agreement with that based on Nureg1 and Powers. The predictions based on Nureg2 and CODEX correlations for Zircaloy-4 are several orders of magnitude lower than those based on current work, especially at lower temperatures. A comparison of the oxidation data for Zirlo with those for Zircaloy-4 showed that the post-breakaway rates are somewhat lower for Zirlo at T≤500°C; however, the rates are higher for Zirlo than for Zircaloy-4 at T ≥600°C. The oxidation rates (in the postbreakaway region) for M5 are consistently lower than for other two alloys at 500 and 600°C. Oxide scale thickness data, developed on the three materials during air exposure, are used to evaluate the time- and temperaturedependence of oxidation of cladding of typical wall thickness.