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Steam Generator Tube Inspection

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Significance

Steam generator (SG) tubes in nuclear power plants are continuously exposed to harsh environmental conditions including high temperatures, pressures, fluid flow rates and material interactions resulting in various types of degradation mechanisms such as mechanical wear, stress corrosion cracking (SCC), pitting, and inter granular attack (IGA). These flaws typically result in tube thinning and development of multiple crack-like flaws, which can potentially result in contamination of fluids on the secondary side. Consequently steam generator tubes in nuclear power plants are inspected periodically for cracks or leaks. Historically, steam generator tube inspection has been a difficult problem. There have been numerous cases of unscheduled plant shutdowns in the past, which can typically cost over $500,000 a day. Hence there is a strong economic incentive to develop reliable nondestructive evaluation (NDE) methods for steam generator tube inspection. Eddy current (EC) inspection has proved to be both fast and effective in detecting and sizing most of the degradation mechanisms that occurred in the early generators. However, as the nation’s generators have aged over years, newer and much more subtle forms of degradation have appeared that require more intelligent application of eddy current tests.
A simulation model having capability to predict eddy current signals from realistic SG tube and defect geometries is useful as a training tool for engineering and NDE personnel. The model helps visualize field/flaw interactions and thereby optimize probe designs and inspection parameters. Further the model can be used as a test bed for studying signal formation that can be used in training pattern classification algorithms used in automated signal analysis. The large amount of data generated by eddy current SG tube inspection requires the use of automated flaw detection and characterization systems.

Objectives

- Develop a model for simulating eddy current inspection of steam generator tubes. The model predicts probe signals from realistic SG tube and defect geometries, coil configurations, and arbitrary shape cracks or sludge accumulations.
- Develop advanced signal processing algorithms that will automatically analyze eddy current data and identify and characterize degradation mechanisms in SG tubes.

Steam Generator tube

Approach

The computational model is based on finite element analysis and solves the Maxwell’s equation governing eddy current phenomenon. The model uses reduced magnetic vector potential formulation so that the probe coil is not required to be re-meshed during probe motion. An interactive user interface allows user to input, plant model, tube and support geometry, defect parameters, material properties and probe/inspection parameters.
Data analysis software was developed to provide rapid, consistent, and accurate analysis of the field data collected during SG tubing inspection. This software is now deployed by industry for SG tube data analysis. Software has capability to detect various damage mechanisms at different regions of SG tubes.

Steam Generator tube analysis