Nuclear Engineering Division

Corrosion and Mechanics of Materials

Light Water Reactors

 

Air Oxidation Kinetics for Zr-based Alloys

An experimental program was conducted to generate data on the air oxidation kinetics of unirradiated Zircaloy-4 and Zirlo cladding with an oxide layer that is representative of the current inventory of spent fuel discharged after a medium or high level of fuel burnup. Several experiments were conducted in which ring specimens were exposed in a steam environment at several temperatures and for various time periods. Based on a detailed analysis of weight change and oxide thickness from these tests, a steam exposure time of 140 h and temperature of 550°C were selected to simulate the oxide layer (≈25-30 µm in thickness) on the cladding in a spent fuel pool.

Scanning electron photomicrographs of cross sections of steam-preoxidized Zircaloy-4, after oxidation in air for ≈300 h at 600°C

Scanning electron photomicrographs of cross sections of steam-preoxidized Zircaloy-4, after oxidation in air for ≈300 h at 600°C. Click on image to view larger image.

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 Zr-based alloys were also exposed in air for comparison of the oxidation behavior of the cladding with and without steam-preoxidation.

Scanning electron photomicrographs of cross sections of steam-preoxidized Zirlo, after oxidation in air for ≈300 h at 600°C

Scanning electron photomicrographs of cross sections of steam-preoxidized Zirlo, after oxidation in air for ≈300 h at 600°C. Click on image to view larger image.

Results showed that the initial oxide scale developed in steam was adherent and crack free in both alloys but tended to crack when exposed in air at 600°C. The thickness of the oxide developed during air exposure increased significantly with time, and the transverse cracks persisted over the entire thickness of the scale, as shown below in the microphotographs. The growth rate for oxide at 500°C in air was much less, and the scales exhibited virtually no cracks, even after an exposure time of 400 h at 500°C. The oxidation rates in air for steam-preoxidized specimens were minimal to negligible at 400 and 300°C after exposure times of 600 and ≈1000 h, respectively. The data showed that in the pre-breakaway region, oxygen ingress into Zirlo occurs at a higher rate than in Zircaloy-4. In the post breakaway region, the rates for Zirlo are somewhat lower than for Zircaloy-4 at ≤500°C; however, at ≥600°C, the rates for Zirlo are higher than for Zircaloy-4.

Weight change and oxide thickness, determined 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 (see figures above) 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 previously published correlations (Nureg1 and Powers). However, 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.

Corrosion and Mechanics of Materials: Light Water Reactors
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Last Modified: Thu, April 21, 2016 4:56 AM

 

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