Fiber optic probe for in-situ inspection of surface cracks in steam generator tubes

Abstract

The automatic inspection of cracks in the inner side of steam generator tubes is routinely carried out using eddy currents techniques. These methods are mature and have quite high performance but the achievable resolution is relatively low (approximately 1 mm) and present drawbacks related to the reliability of the inspection of the areas of tubes at the neighborhood of the supporting plates. Several years ago we have proposed a solution to these limitations based on a new fiberoptic reflectometric technique that has already been theoretically and experimentally analyzed using a first laboratory prototype. In this work we describe a second generation prototype that is integrated in a whole inspection system that could perform an inspection in the field. The results obtained over cylindrical tubes with inner diameter 16.8 mm (corresponding to real steam generator tubes) with cracks created by electrical disintegration and by stress- corrosion and employing conditions close to real operation confirm our previous conclusions: both circumferential and axial cracks of widths as small as 30 micrometer can be detected and measured and typical resolution capability of 10 line pairs/mm can be obtained.