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

: Weiß, R.; Becker, R.; Lucht, B.; Mohr, F.; Hartwig, K.


Nuclear Engineering and Design 206 (2001), Nr.2-3, S.311-323
ISSN: 0029-5493
Fraunhofer IZFP ()
eddy current inspection; nuclear power plant; reactor pressure vessel

As part of the reinspection 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.