Ultrasonic examination of dissimilar-metal welds in BWR and PWR plants

The difficulties of ultrasonic examination of dissimilar-metal welds in BWR and PWR plants are discussed. Improved examination techniques were developed and are described. The complex structure of dissimilar-metal welds and the use of highly attenuating austenitic weld metals and castings make conventional shear-wave examination ineffective. An ultrasonic test procedure using refracted longitudinal-wave transducers was developed for examining the dissimilar-metal welds in the nozzle-to-safe-end joints in BWR recirculation piping. The techniques were developed in the laboratory with mock-ups containing notches and intentional cracks. Four field trials of the procedures using manual and automated methods were conducted to verify the laboratory results. Signal-processing techniques were investigated, and a spatial signal-averaging method was found to be effective in improving signal-to-noise ratios. For welds less than about 38 mm (1.5 in) thick, transducer frequencies of 2 MHz could be used and were capable of reliably detecting cracks with depths about 20% of the wall thickness or greater. Thicker welds required the use of large-element 1 MHz transducers. The best flaw detection performance was obtained with large angles of refraction (600 or greater). (author)