Abstract

(This is the abstract of a paper that will be presented at the 7th ECNDT in Copenhagen. The article presented here introduces the same work)
Ultrasonic arrays offer a number of advantages in NDE applications, the most important is electronic beamforming and scanning enabling generation of the desired field distributions and defect insonifica-tion from different directions resulting in improved defect detection, and characterization.
The applicability of linear array technique for inspection of copper lined canis-ters for nuclear waste fuel has been investigated recently at Uppsala University.
The objective of this project is development of ultrasonic array technique for assessing the integrity of the circumferential electron beam (EB) weld between the lid and walls of copper lined canisters developed by SKB (Swedish Nuclear Fuels and Waste Management Co.) for encapsulation of nuclear waste.
Due to the radiation emitted by the nuclear waste encapsulated in the canisters the inspection must be carried out completely automatically. In order to achieve a very high level of confidence in the NDT results of copper canisters, the inspection system should be capable of inspecting 100% of cimcurferential weld zone in thick section copper. An ultrasonic array system consisting of a linear ultrasonic array, made of piezoelectric composite material, and a computer controlled multi-channel electronics is proposed as a solution to this problem.
Modern array technique which has become recently commercially available for NDT applications has a great potential in this application. The array sys-tem chosen for this application enables electronic focusing and rapid electronic scanning eliminating the use of complicated mechanical scanner. Secondly, it is capable of reducing the influence of grain noise by using a combination of focused sound field and signal processing. Thirdly, it is characterized by inherent flexibility enabling 100% inspection of the weld zone even if the weld is not parallel to the surface.
Our present research activity in this project addresses two issues: the development of software tools for efficient beam-forming, and a laboratory verification of the performance of ultrasonic array system on real EB welded canister samples. A special attention is paid to ultrasonic grain noise scattered by the internal copper structure. A method for grain noise suppression by electronic filtering B-scan images is verified in this application. Here we present some recent results obtained during the inspection of EB welded copper test blocks containing both artificial and natural defects.