Nondestructive inspection of nozzle inner corner at boiling water reactor pressure vessels with ultrasound-flaw detection and crack depth determination

The demonstration of inservice inspection methods with ultrasound for the nozzle inner corner at BWR pressure vessels has shown that a detectability of cracks with a depth in the range of 5-10 mm is possible if optimal inspection parameters are chosen. The investigation concerning the choice of the optimal parameters have delivered the following results: (1) Probe parameters: The angle of refraction should be in the range of 700 and a probe frequency of 1 MHz should be used. The tilting angle can be chosen between 18 and 220. (2) Sensitivity: The calibration of the sensitivity can be carried out with the help of a 7 mm side-drilled hole in a distance corresponding to the maximum sound path for the inspection. (3) Scanning range of the probe: The probe shall be moved in a range of about 100 mm in order to cover the whole gap between minimal and maximal radial distance for an elliptical scanning. (4) The evaluation of indications can be done based on C-scan presentations. In doubtful cases the time-of-flight curves will deliver some useful information. The first hint on the defect size is delivered by the extension of the indication in a C-scan presentation. A dynamic extension can be evaluated and correlated to the real defect size if sufficient knowledge about the specific relationship is available. For critical findings a correct flaw sizing cannot be avoided. In those circumstances it seems to be necessary to apply techniques like the acoustic holography or others. (orig./HP)