Eddy Current Assessment of Support Plate Structures Degradation in Nuclear Steam Generators

Abstract

Testing the structural integrity of steam generator (SG) tubing and SG internals is a key element of the fitness-for-service assessments to assure the safe and continuous operation of nuclear power plants. Recent eddy current testing (ET) inspections of CANDU nuclear power plants revealed degradation of some of the carbon steel tube support plate (TSP) structures, which was also confirmed by visual inspection. The phenomena was described as metal loss, caused by flow-accelerated corrosion of the carbon steel trefoil support plate, generating minor degradation up to complete loss of the ligaments. This loss of TSP ligaments results in lack of support for adjacent tubes making them more susceptible to fretting-wear damage and fatigue cracking. A method for signal analysis, based on the responses at low frequency of two types of ET probes, has been developed to assess the degree of degradation. The standard impedance mode bobbin probe is used to detect and classify degradation, based on stages of partial or complete breach of ligaments. The advanced transmit-receive array X-probe is used to quantify the degree of degradation in the land regions, in addition to helping discriminate it from external magnetite and copper deposits. The complementary information obtained from each of the two probes, combined with visual inspections, helps to assess the overall condition of the TSP. These procedures, applied to present and past ET inspections, provide the technical basis to determine the location and progression rate of degradation. This paper presents the ET analysis methodology along with inspection results showing examples of degradation distribution and progression based on ET signals and visual inspection images.