Ultrasonic Investigations on Copper Canister Welds in Preparation for the Storage of Spent Nuclear Fuel in a Deep Repository

Abstract. The Swedish KBS-3 design for the disposal of spent fuel is based on its encapsulation cast iron cylinders that have an outer 30 – 50 mm thick shield of copper and deposition in crystalline rock embedded in bentonite at a depth of about 500 m. The KBS-3 system is based on a multi barrier system where the canister is the primary barrier. The cast iron insert gives the canister the necessary strength and the outer shell that gives corrosion protection is made of oxygen free copper. SKB’s efforts are based on a stepwise program for the implementation of deep geological disposal of spent fuel including concurrent activities in the areas of deep repositoryand encapsulation technology. Major milestones in the program are application for construction and building of an encapsulation plant at 2006 and a deep repository at 2008. According to the program initial operation for the system will start at 2017. The critical part of the encapsulation of spent fuel is the sealing of the canister which is done by welding the copper lid to the cylindrical part of the canister. Two welding techniques have been developed in parallel, Electron Beam Welding (EBW) and Friction Stir Welding (FSW). Mid 2005 SKB decided that FSW is the preferred sealing technique. Determining the reliability of non-destructive testing (NDT) for FSW and EBW welds were investigated by BAM during 2004–2005. Since the welding methods are based on different principles, the NDT methods that were developed by SKB were tailored to the particular welding methods. The aim of the investigations were to determine the reliability of the NDT in terms of probability of detection (POD) and size estimation accuracy for the various kinds of discontinuities that could occur. Ultrasonic investigations were executed as one reference measurement during the validation of the inspection procedures developed by SKB. Results were presented as CScans in order to show the extension of different discontinuities in the welds. The applied sound-fields of the used phasedarrayequipment were determined by means of modelling calculations. Echo heights for different reflector geometries were calculated for comparison with measured data or for the generation of POD curves within a second working group.