Detection of voids in welded joints using ultrasonic inspection : Quality control of welded joints in copper canisters for purpose of permanent storage of used nuclear waste

Abstract: This thesis was done i cooperation with SKB Clab in Oskarshamn and studies use of sonic waves for detecting voids and irregularities in the weld joints of copper capsules used for long term storage of radioactive waste. Since these could pose material failure and thereby risk radioactive contamination of ground water it is very important to find means of quality control before storage.  During the welding procedure changes occur to the integrity of the material. The homogenous metal – in this case copper – is distorted and voids appear in and around the welded volume. A non-destructive inspection method is needed to make sure that the metal holds for the strains of long term storage. These strains are not completely known at the moment and therefore the goal of this thesis is mainly to add another tool of inspection for future studies. The tests are done using ultrasonic mapping of the welded volume. This is achieved by sending ultrasonic pulse through test samples – welded copper pieces – and recording its reflection. The recorded signals are gathered in data matrices and processed using several different signal processing methods in search of irregularities and voids. To enhance the understanding of the results a graphical user interface (GUI) is developed that allows users to visualize the results.  The welded pieces, the ultrasonic mapping and its resulting data sets were delivered to this thesis and the scope of the thesis is to develop the GUI and apply known signal processing methods to the data set.  It is shown that the irregularities do appear and that ultrasonic detection and use of the processing method is useful for quality control of the material. Further field studies are needed to identify maximum number, size and perhaps shapes of irregularities that can be within tolerance levels of the storage project.