Some German nuclear power plants are approaching the end of their designed life
spans. Considering the possibility of radiation damage to the RPV material in close
proximity to the core, reliable nondestructive testing techniques for the detection,
classification, and sizing of surface and sub-surface cracks, as well as sub-clad cracking
with extensions into the base material, are of immense interest. It has been shown that
weld microstructure is particularly sensitive to neutron degradation (primarily due to
high copper content), and thus very stringent NDE safety assessments are imperative.
In addition to inspecting clad integrity, clad thickness measurements also become
essential. This paper discusses the development of optimized, high dynamic-range,
eddy current techniques utilizing a specialized yoke-type absolute coil transmitter and
differential coil receiver. The eddy current working frequencies are below 500 Hz, and
the combination of eddy current data with data acquired using a multi-frequency
approach allows for signal-to-noise ratio enhancements and filtering of disturbing
signals. Results are presented from the qualification process, which documented the
reliability of the system as confirmed with specific test and calibration blocks, and
during inservice inspections. And finally, because of the presented eddy current
techniques in combination with optimized ultrasonic testing techniques, two older
German reactors were able to achieve a high level of safety and confidence, and were
once again put on-line.