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Eddycurrent.com

Eddycurrent.com

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Condition Monitoring for Steam Generator Inspections

Eddy Current Inspection (ECT) Overview

Eddy current testing (ECT) is a nondestructive examination (NDE) technique used widely in the inspection of steam generator (SG) tubing in nuclear power plants. Steam generator tubing is a critical barrier between the radioactive primary side of a nuclear reactor and the secondary side, where steam is generated for turbine operation. Detecting degradation in these tubes is essential for ensuring plant safety and preventing costly shutdowns.

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  • Key Principles:

    • ECT uses electromagnetic induction to identify flaws, such as cracks, corrosion, wear, pitting, or tube support degradation.

    • Probes with coils generate an alternating current, inducing eddy currents in the tube material. Flaws disrupt the flow of eddy currents, which can be measured and analyzed.

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  • Common Probe Types:

    • Bobbin Probe: For general wall thinning, wear, and axial flaws.

    • Rotating Probes (e.g., Array or Pancake Coils): For detecting complex flaws like circumferential cracks or small pits.

    • Array Probes: Provide higher resolution and faster coverage by using multiple coils.

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  • Advantages:

    • Real-time data acquisition and analysis.

    • Ability to inspect non-accessible areas like bends or tight geometries.

    • High sensitivity to small flaws with appropriate calibration.

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NEI 97-06: Steam Generator Program

NEI 97-06, Steam Generator Program Guidelines, was developed by the Nuclear Energy Institute to standardize the approach to managing steam generators in U.S. nuclear power plants. This guideline outlines best practices to ensure the structural and functional integrity of SG tubes over their service life.

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Purpose

To establish a proactive management strategy for SG tubes that balances:

  • Operational reliability.

  • Regulatory compliance.

  • Cost-efficiency.

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Key Elements

  1. Performance Criteria:

    • Structural integrity: Tubes must meet stress criteria for normal operation, transients, and postulated accidents.

    • Leakage integrity: No tube leakage exceeding specified limits during normal or postulated accident conditions.

    • Operational assessments: Evaluating inspection results to predict degradation progression until the next inspection.

  2. Inspection Requirements:

    • Frequency: Inspections are typically conducted every outage or based on risk-informed criteria (e.g., every 18-24 months).

    • Techniques: Eddy current testing is the primary method, often supplemented by secondary methods such as ultrasonic testing or visual inspections.

    • Coverage: 100% of in-service tubes or risk-based sampling, depending on SG age and degradation history.

  3. Tube Repair and Plugging Criteria:

    • Repairable tubes are those with degradation beyond defined limits (e.g., wear exceeding 40% of wall thickness).

    • Plugging ensures degraded tubes are removed from service, preserving SG integrity and preventing potential tube rupture.

  4. Degradation Management:

    • Common degradation mechanisms include stress corrosion cracking (SCC), pitting, fretting wear, and general corrosion.

    • Programs address specific degradation modes through material upgrades (e.g., thermally treated Alloy 690), water chemistry control, and continuous monitoring.

  5. Reporting and Documentation:

    • Licensees must submit inspection results and operational assessments to the NRC, demonstrating compliance with NEI 97-06 and plant-specific technical specifications.

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Integration of ECT and NEI 97-06

Eddy current inspections directly support the NEI 97-06 objectives:

  • Detecting early-stage degradation ensures proactive repairs or plugging before integrity is compromised.

  • High-resolution imaging (e.g., C-scans from array probes) enables precise flaw characterization and reduces uncertainties in structural assessments.

  • Automated data analysis streamlines compliance with inspection frequency and documentation requirements.

Challenges and Innovations

  1. Challenges:

    • Inspection of tightly packed U-bends in SGs.

    • Interpretation of complex signals from mixed-mode degradation (e.g., SCC overlapping with wear).

    • Reducing radiological exposure during inspections.

  2. Innovations:

    • AI-driven automated analysis to reduce human error in signal interpretation.

    • Improved probes for detecting flaws in challenging geometries.

    • Risk-informed inspection intervals supported by probabilistic assessments.

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