A microfabricated ECT probe featuring internal distance standards for micro-level oxide film thickness measurement

Background Fuel claddings in nuclear reactors undergo progressive oxidation during operation, resulting in the formation of oxide films on the outer surface of the cladding tubes. Excessive oxide growth degrades thermal conductivity and may accelerate corrosion, thereby compromising fuel assembly safety. Accurate quantification of oxide film thickness is therefore essential for condition assessment and life management of nuclear fuel rods. Objective The objective of this study is to develop and validate a microfabricated eddy-current probe for oxide-film-thickness measurement on zirconium-alloy fuel cladding. By utilizing precisely controlled coil spacing realized through semiconductor microfabrication technologies, the proposed probe aims to provide a reliable hardware platform for self-calibration-based thickness measurement without relying on external calibration standards. Methods The probe is realized through a series of microelectromechanical fabrication processes, enabling precise construction of multilayer coils with microscale spacing. Such a microfabricated architecture provides highly accurate and repeatable coil positioning, allowing the oxide film thickness to be determined from the differential responses of coils located at known separations. Results The fabricated coils exhibit a copper thickness of 1 μm, a line width of 32 μm, and a turn spacing of 8 μm, demonstrating the capability of microfabrication techniques to achieve dimensions that are difficult to realize using conventional winding methods. The relationship between oxide film thickness and induced voltages was investigated experimentally using Zr-4 cladding tubes. Experimental results show that the proposed probe can measure oxide film thickness with an average error below 5 μm. CONCLUSION: The proposed method achieves accurate oxide thickness measurement without requiring prior calibration, offering a promising solution for in-service inspection of nuclear fuel claddings.