Ultrasonic technique for imaging welds in copper

This paper discusses the use of ultrasonic array technique for imaging weld structure in copper. An ultrasonic technique for assessing the integrity of the circumferential electron beam (EB) weld between the lid and walls of the copper-lined canisters developed by SKB (Swedish Nuclear Fuels and Waste Management Co.) for encapsulation of nuclear spent fuel is presented in detail. Due to the radiation emitted by the nuclear waste encapsulated in the canisters the inspection must be carried out completely automatically. An ultrasonic array system consisting of a linear ultrasonic array made of piezoelectric composite material and digitally controlled multi-channel electronics has been chosen as a solution to this problem. The array system enables electronic focusing, and rapid electronic scanning eliminating the use of complicated mechanical scanner. The array system has been tested in the laboratory using realistic test blocks with EB welds. Test results in form of ultrasonic images are presented in the paper. An electronic technique for suppressing material noise caused by ultrasonic scattering from material grains is also presented.