U-bend Support Structure Verification and Mapping

Abstract

Support of the steam generator tube bundle in the U-bend or square bend by antivibration bars (AVB) or other structures is important for safe and efficient operation of the steam generators. Several industry events where the inadequate tube support was identified resulted in unplanned plant shutdowns and extensive tube repairs. Not controlling tube-to-AVB clearance is known as one of the most important contributors to the tube wear resulting from the fretting between the tube and the AVB. Depending on the steam generator design, there may be several tens of thousands of tube-to-AVB intersections so any verification and evaluation of AVB signals require automated screening tool. Automated software normally used for the data analysis can be very efficiently used as the screening tool to determine exact position of each AVB within the U-bend thus providing the most important information which is the depth of insertion. Data obtained by the bobbin probe during regular eddy current examination of tubing can also be used to identify possible axial misalignment between two AVBs supporting the tube at the same designed location and to a lesser level to provide the qualitative information about the tube-to-AVB clearance. Rotating probe can provide more information as it is the surface riding technique that has the spatial resolution so it is capable to quantify individual AVB signal unlike the bobbin probe that integrates impact of both AVBs into one signal. The highest precision particularly when measuring the tube-to-AVB clearance is obtained by the ultrasonic testing. This paper deals with the development and application of eddy current and ultrasonic techniques intended to verify the locations of U-bend support structures, to identify and quantify possible anomalies and discrepancies between the designed and “as built” conditions. It focuses on the application of AREVA’s automated data analysis tool (AIDA) and delineates possible benefits to the utilities when having the full information of the U-bend supports.