Friction stir welding of copper and its NDT – From novelty to highly-developed!

Ronneteg, Ulf; SKB Swedish Nuclear Fuel and Waste Management Co.; Sweden

Ronneteg, U.; SKB Swedish Nuclear Fuel and Waste Management Co.; Sweden

ID: ECNDT-0156-2018
Download: PDF
Session: Nuclear Industry 3
Room: H2
Date: 2018-06-13
Time: 16:20 - 16:40

SKB develop the KBS-3 method for final storage of the spent nuclear fuel. The method consists of three protective barriers, a canister, bentonite clay and the bedrock. The canisters in which the fuel is encapsulated have a diameter of 1 metre and a length of 5 metres. The canister consists of a cast iron insert surrounded by a 5 centimetre thick shell of copper, sealed by friction stir welding (FSW). The canisters are disposed in the bedrock at a depth of about 500 meters surrounded by bentonite clay. In order to assess the long-term safety, an extensive quality control programme is applied to the canisters before deposit, in which the use of non-destructive testing (NDT) is vital. In the 1990s SKB developed a full-scale electron beam welding system for the copper canister. In parallel, the FSW technology started to grow and initial trials were done on thin copper samples. This paper will describe the iterative development process in which the welding process and the applied NDT techniques are developed jointly, and the resulting advanced welding process control and NDT techniques. Initially the knowledge of the welding process, the welded material and the possible defects were limited. By testing the limits for welding parameters, defects have been created which then been used to develop NDT techniques. The NDT techniques have then been used as input to identify the development needs in the welding process and also to verify the actions taken to optimise the process. Conversely, the good knowledge of the welding parameters and its effect on the weld quality, has given the opportunity to, by purpose, create defects for further development and verification of the NDT techniques. This has resulted in a reliable automatic welding process and reliable NDT techniques; applying digital radiography, phased array ultrasonics and eddy current arrays.