Low Frequency Eddy Current Inspection for Surface and Subsurface Defects of the RPV-Cladding by a New DSP Controlled Eddy Current System

In Germany some older nuclear power plants are close to their designed end-of-life. Under the aspects of possible radiation damage in core-near id-surface areas of the pressure vessel cadding, it is from interest to have reliable nondestructive testing (NDT) techniques for detecting, classifying and sizing of surface-breaking cracks, cracks and inclusions in the cladding as well as subclad cracks, growing from the interface into the base material. So far the weld microstructure is sensitive for neutron degredation, i.e. by high Cu-content, the demand for safety assessment by NDT is much stringent. In the case of cladding thickness much larger than the specified, the cladding itself causes a noticeable contribution to a shift in the fracture appearence transition temperature (FATT) to lower temperatures, so far the integrity of the cladding, i.e. the defect-free state, is guaranted. In addition to a NDT technique for defect inspection also thickness measurements are necessary. The contr ibution discusses the development of an optimized eddy current technique (ECT) based on a specialized yoketype absolute coil transmitter and differential coil receiver. The working frequencies are in the range lower than 500 Hz and the combination of informations in a multifrequency approach in order to enhance signal to noise ratio and to suppress disturbing parameters is applied. An electronic deveice with high dynamic range has been developed. Results of the qualification procedure of the inspection technique are presented, documenting the high reliability of the system confirmed at specialized test and calibration blocks, as well as inservice inspection results are discussed. On the basis of the presented ECT, combined with an optimized ultrasonic testing (UT), the inspected pressure vessel again is in safe service