Ultrasonic Array Technique for the Inspection of Copper Lined Canisters for Nuclear Waste Fuel

Ultrasonic array technique for assessing the integrity of the circumferential electron beam weld between the lid and walls of copper canisters is presented in the paper. The results of laboratory verification of the performance of the ultrasonic array system on real canister samples are presented. A method for suppression of grain noise scattered from the internal copper structure based on digital filtering B-scan images is verified in this application. Introduction 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 insonification from different directions resulting in improved defect detection, and characterization. The applicability of linear array technique for inspection of copper lined canisters 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 to the NDT results of the copper canisters asessment, 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 was 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 system 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 an inherent flexibility enabling 100% inspection of the weld zone even if the weld is not parallel to the upper 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. Experiments Equipment. All measurements have been performed using 128 channel ultrasonic array system Allin from NDT Systems, France. The Allin System was connected to a 3 MHz linear array from Imasonic. The array was made of piezoelectric composite material and had 64 elements with dimensions 1 by 33.5 mm. The array was fast focused in one plane (cylindrical focus with focal distance 190 mm in water) and was electronically focused in the other plane. The Allin enabled focusing both in the emission and reception and electronic scanning using an aperture up to 32 elements. Besides scanning and beamforming, the Allin performed also data acquisition and steering of the mechanical scanner used for guiding the array. The Allin enabled storing all the measured data in real-time, which means that all RF signals included in the acquired B-scans can be stored. Thus when inspecting a 2D surface, a sequence of Bscans is stored forming ”data cube”. The stored data can then be processed and presented offline and the desired C-scans can be calculated. This feature has been used extensively during the experiments. Array SCANNER with 3 free axes Linear X, Y, Z Astus Expliotation