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The cause of material variety in the nodular corrosion susceptibility was investigated through measurements of the distributions of alloying element concentrations. A Zircaloy-2 specimen surface area of 5 by 5 mm was divided into a 500 by 500 square array of 10 by 10 μm spots, and the concentrations of alloying elements, tin (Sn), iron (Fe), chromium (Cr), and nickel (Ni), were measured on each spot and examined statistically.
No significant differences in the variance of tin concentrations, which were relatively uniform, were observed between specimens tested. However, the respective variances of iron, chromium, and nickel concentrations were larger than that of tin and differed between specimens. A specimen having a larger variance showed heavier nodular corrosion in steam corrosion tests. The measured low concentration spots were randomly distributed on the specimen surface and their frequency correlated with the nodule number density suggesting that a local area deficient in these elements could be one site for nodular corrosion.
Both (α + β)- and β-quenching homogenized the concentrations of iron, chromium, and nickel; hence these treatments improved the nodular corrosion resistance.
Zircaloy, nodular corrosion, alloying elements, steam tests, heat treatment, boiling water reactors
Senior researcher, Nippon Nuclear Fuel Development Company, Ltd., Ibaraki-ken,