Qualification of the LF-eddy current technique for the inspection of stainless steel cladding and applications on the reactor pressure vessel

As part of the re-inspection of the reactor pressure vessel of the nuclear power plant, the low-frequency-eddy current technique was implemented during the 1995 outage. Since then, this inspection technique and the testing equipment have seen steady further development. Therefore, optimization of the entire testing system, including qualification based on the 1995 results, was conducted. The eddy current testing system was designed as a ten-channel test system with sensors having separate transmitter and receiver coils. The first qualification of the testing technique and sensors was performed using a single-channel system; a second qualification was then carried out using the new testing electronics. The sensor design allows for a simultaneous detection of surface and subsurface flaws. This assumes that testing is performed simultaneously using four frequencies. Data analysis and evaluation are performed using a digital multi-frequency regression analysis technique The detection limits determined using this technique led to the definition of the following recording limits for testing in which the required signal-to-noise ratio of 6 dB was reliably observed.

• Detection of surface connected longitudinal and transverse flaws:

• notch, 3 mm deep and 10 mm long, for weave bead cladding;

• notch, 2 mm deep and 20 mm long, for strip weld cladding.

• Detection of embedded planar longitudinal and transverse flaws:

• ligament of 7 mm for 8 mm clad thickness and 3 mm;

• ligament for 4 mm clad thickness, notch starting at the carbon steel base material with a length of 20 mm.

• Detection of embedded volumetric longitudinal and transverse flaws:

• 3 mm diameter side-drilled hole (SDH) for 8 mm clad thickness; ligament, 4 mm. For 4 mm clad thickness: diameter, 2 mm SDH; ligament, 2 mm. All SDHs are 55 mm deep.

Article Outline

1. Problem

2. Objective

3. Execution and results

3.1. Test instrument and electronics

3.2. Performance demonstration (qualification)

3.3. Summary of results and assessment of the qualification

3.4. Flaws open to the surface

3.5. Planar flaws in the cladding and sub-clad flaws

3.6. Volumetric flaws in the clad

3.7. Additional evaluations

4. Qualification results

5. Results from the 1999 outage

1. Problem

The reactor pressure vessel is equipped with a stainless steel (austenitic) cladding for corrosion protection. This cladding can only protect if no flaws are present at the surface or in the volume. The verification of the integrity of the cladding is currently conducted using state-of-the-art ultrasonic testing. Ultrasonic testing has an excellent capacity of proof for these types of flaws, but it generally cannot distinguish between flaws at the clad surface, in the clad volume, or at the clad-to-base material interface. Using the low-frequency (LF)-eddy current technique, these differences can be documented. For this reason, the LF-eddy current technique was developed and also supported by those who employ diverse testing technology in addition to ultrasonic testing for this type of testing.

2. Objective

The goal of the qualification described in this paper was the optimization and verification of the test procedure and test equipment based on the test systems currently used and, in addition, implementation of the results achieved with the newly built WS98 test electronics, a ten-channel eddy current testing system. The completion of the tasks should be performed in accordance with the ENIQ qualification guidelines. Following the successful qualification, the test system will be utilized during the 1999 reactor pressure vessel outage at the Stade nuclear power plant (KKS). The project started in August 1998, leaving approximately 6 months for the set-up of the equipment, system performance demonstration (qualification), and to compile the required documentation.

3. Execution and results

The following essential parameters for the qualification of the testing technique were determined by the test situation:

• sensor size of, maximum, 40 mm×40 mm×30 mm (L×W×H) for NF-absolute sensors;

• sensor size of, maximum, 60 mm×30 mm×30 mm for T/R sensors;

• frequency range, 0.5–20 kHz;

• effective coil width, ≥10 mm (6 dB drop);

• gain (amplification), up to 100 dB;

• long-term stability of the test instrument and electronics.

3.1. Test instrument and electronics

The eddy current instrument is designed for single-channel or multi-channel automated testing of the surface areas of piping systems, pressure vessels, and forgings for both mobile testing services in the field and also for use in stationary facilities in the area of manufacturing testing or inservice inspections.The instrument can easily be adapted to the requirements of the respective test situation due to its modular design. This is accomplished by increasing the testing electronics to the necessary number of sensor and/or frequency channels.The design of the eddy current electronics and the data flow can be seen in Fig. 1.