Detecting the Oxidation of Zircaloy Claddings by Infrared Interference

As the expected life of dry cask storage installations increases, it becomes increasingly desirable to monitor the state and performance of the cask internals to ensure that they continue to safely contain the radioactive materials in the fuel. One aspect of this task is the monitoring of oxidation of the cladding. With this consideration in mind, Zircaloy-4 (Zr-4) cladding samples were exposed to air at 500 degrees C for various duration times to create thin corrosion oxide layers on the surface. The surfaces of the oxidized samples were then systematically scanned by Fourier Transform Infrared (FT-IR) spectroscopy to achieve the infrared (IR) interference spectra and study the relationship between the optical interference and the various thicknesses of the oxide layers. The profiles of the oxide layers were verified througth cross-sectional examination by Scanning Electron Microscopy. The IR interference patterns varied with oxide layer thickness, enabling the determination of oxide layer thickness of values, including half micron thick. Further analysis demonstrated that the interference oscillation period and the oscillation amplitude decreased with increasing oxide layer thickness. Combined with a physical model that describes the optical interference, the interference spectra were directly correlated to the oxide layer thickness quantitatively. The study provides the basis for an accurate, nondestructive and sensitive method to monitor the degree of zirconium-based cladding corrosion due to oxidation.