Ultrasonic Testing Procedures for Volumetric & Surface Inspection of CANDU Pressure Tubes

Canada Deuterium Uranium (CANDU) reactors use heavywater moderated, heavywater cooled pressure tubes as the primary containment for the fuel bundles. The integrity of these tubes is paramount because any defect can lead to coolant leakage, fuel damage, or, in worstcase scenarios, a loss of coolant accident. Ultrasonic Testing (UT) is the primary nondestructive technique employed to detect volumetric and surface anomalies in these longlength, thinwalled tubes. This page outlines the essential concepts, standard procedures, equipment, and interpretation methods used in the industryapproved ultrasonic inspection of CANDU pressure tubes.

1. Fundamentals of Ultrasonic Testing

UT relies on the transmission of highfrequency sound waves (0.515MHz) into a material. When a wave encounters a discontinuitysuch as a crack, inclusion, or change in material propertiespart of the energy is reflected back to the transducer as an echo. The timeofflight (TOF) and amplitude of these echoes are processed to locate and size the defect.

1.1 Types of Waves Used

• Longitudinal (compressional) waves: Travel parallel to the direction of particle motion; highly sensitive to volumetric defects.
• Shear (transverse) waves: Particle motion is perpendicular to propagation; useful for detecting surface cracks and orientationspecific flaws.
• Oblique/angled waves: Generated by tilting the transducer; allow access to regions not reachable by normal incidence.
• Guided waves (Lamb & Rayleigh): Propagate along the tube wall; employed for longrange inspection of thinwalled sections.

1.2 Principle of PulseEcho Testing

In the pulseecho method, a single transducer alternately transmits a short ultrasonic pulse and receives the reflected echoes. The echo amplitude is proportional to the size of the reflecting feature, while the TOF measures distance from the transducer.

1.3 Calibration and Reference Standards

Accurate defect sizing requires calibration blocks that mimic the acoustic properties of the pressure tube material (Zircaloy2/4). Standard references contain sidedrilled holes, flat-bottom holes, and notches of known dimensions. Calibration must be performed before each inspection campaign, using the same probefrequency, coupling medium, and scan parameters as the actual test.

2. Volumetric Inspection (Scanning the Tube Wall Thickness)

2.1 Objective

Detect internal flaws such as axial cracks, intergranular corrosion, voids, and foreignmaterial inclusions that may compromise the tubes ability to sustain operating pressure.

2.2 Typical Procedure

1. Preparation: Clean the tube surface with a solvent and a nonabrasive pad to remove oxides, grime, or paint. Apply a uniform couplant (glycerinbased gel) to ensure acoustic transmission.
2. Probe Selection: Use a 510MHz focused or linescan probe with a small (<1mm) focal spot to achieve the required resolution for a tube wall thickness of ~4mm.
3. Scanning Method: Perform a longitudinal scan (0 incidence) while the probe traverses the tube axially. For areas with curvature, a phasedarray transducer can be employed to electronically steer the beam.
4. Data Acquisition: Record the echo amplitude and TOF for each axial position. Modern UT systems store data as Bscans (crosssectional images) that display amplitude versus distance along the tube.
5. Acceptance Criteria: Compare echo amplitudes against calibrated reference blocks. Flaws are accepted if the measured echo size exceeds the minimum detectable flaw size (MDFS) defined by the applicable standard (e.g., CSAN285.396).
6. Documentation: Generate a report containing scan maps, defect locations (relative to tube ends), and sizing calculations. All data are archived for trend analysis.

Figure 1 Representative Bscan showing a volumetric flaw located near the inner wall.

2.3 Special Techniques

• TimeofFlight Diffraction (TOFD): Uses two angled beams to locate cracks based on diffraction from crack tips; provides higher sizing accuracy for axial cracks.
• Focused PhasedArray: Enables multiangle scanning without moving the probe, reducing inspection time for long tubes.
• Immersion Testing: The tube is submerged in a water bath; water acts as the couplant and provides a stable acoustic path for highprecision measurements.

3. Surface Inspection (Detecting SurfaceBreaking Cracks)

3.1 Objective

Identify surfacebreaking or nearsurface defects such as stress corrosion cracks, fatigue cracks, and machining scratches that could serve as crack initiation sites.

3.2 Procedure Overview

1. Probe Choice: A highfrequency (1015MHz) shearwave or dualfrequency probe provides the necessary resolution for surface defect detection.
2. Angle of Incidence: The probe is tilted to an angle between 30 and 45 (commonly 45) to generate shear waves that graze the tube surface.
3. Scanning Path: The probe traverses the tube circumferentially (around the pipe) and axially (lengthwise) to cover the entire surface.
4. Coupling: A thin layer of gel is used; excessive couplant can mask small surface cracks, so a minimal yet continuous film is recommended.
5. Signal Processing: Edgedetection algorithms enhance the contrast of defect echoes against the background noise.
6. Acceptance Criteria: The smallest detectable surface crack (MDFS) is typically set at 0.2mm length for CANDU tubes; any echo exceeding this threshold must be recorded.

Figure 2 Surfacescan image showing a stresscorrosion crack on the outer wall.

3.3 SurfaceWave Techniques

Guided wave methods (e.g., Rayleigh waves) can also be used for rapid inspection of large surface areas. The technique launches a lowfrequency wave that travels along the tube wall, reflecting from any surface discontinuities. Signaltonoise improvement is achieved by averaging multiple passes.

4. Inspection Planning and Safety

4.1 Access Points

Pressure tubes are inspected through the fuel channel head and the tube support plates. Temporary removal of end fittings is required to attach the transducer fixtures. All work must follow the CANDU plants radiation safety procedures.

4.2 Personnel Qualification

Inspectors must hold a Level II or Level III ultrasonic certification according to the International Society of Automation (ISA) standards, plus specific training on CANDU tube geometry and safety protocols.

4.3 Environmental Controls

• Temperature: Ultrasonic velocity changes with temperature; the tube surface temperature must be recorded and compensation applied.
• Vibration: The scanning platform must be isolated from plant machinery to avoid false echoes.
• Radiation: Use remotehandling fixtures where possible; shielded enclosures protect operators and equipment.

5. Data Interpretation and Reporting

5.1 Signal Evaluation

Each echo is evaluated for its amplitude, shape, and TOF. A true defect echo displays a sharp rise, consistent amplitude, and geometry that matches the calibrated reference. Spurious echoes (e.g., from grain boundaries) are identified by their irregular, lowamplitude appearance.

5.2 Sizing Methodology

Defect size is calculated using the Ascan amplitudetosize relationship established during calibration. For cracks, TOF between the leading and trailing edges of the echo provides an estimate of crack length.

5.3 Reporting Format

Standard reports include:

• Inspection date, location, and personnel details.
• Equipment list (transducer type, frequency, coupling medium).
• Calibration records and reference block data.
• Defect map (axial and circumferential coordinates).
• Size estimations with associated confidence limits.
• Recommendations (e.g., monitoring, repair, or tube removal).

6. Maintenance of Inspection Equipment

Regular maintenance of UT equipment ensures consistent performance:

• Transducers: Inspect for delamination, cracked backing, or wear of the protective cap.
• Couplant: Replace expired gel and avoid contamination with oil or water.
• Calibration blocks: Store in a temperaturecontrolled environment; verify integrity annually.
• Data acquisition system: Perform software updates and backup data after each inspection.

All maintenance actions should be logged in accordance with the plants quality management system.

7. References & Standards

8. Conclusion

Ultrasonic testing remains the cornerstone technique for ensuring the structural integrity of CANDU pressure tubes. By combining volumetric scans with surfaceinspection methods, inspectors can detect a wide variety of defects, from deepseated inclusions to superficial cracks. Adherence to calibrated procedures, rigorous personnel training, and meticulous documentation guarantees that the inspection results are reliable and meet the stringent safety requirements of nuclear power operations.