Localization of plastic deformation in copper canisters for spent nuclear fuel

In Finland and Sweden the spent nuclear fuel will be deposited in a deep repository in copper corrosion barrier canisters surrounding cast iron inserts. The 50 mm thick copper canisters will we sealed using either electron beam welding (EB) or friction stir welding (FSW) to join the tubes and the lids/bottoms. The canisters will deform in the repository conditions, e.g., due to the hydrostatic pressure. The deformation will localize to different discontinuities, such as defects and microstructural heterogeneity. This study compared the localization of plastic deformation in EB and FSW welds as well as in the base materials (both forged and extruded) using optical strain measurement methods. The results show that in the base materials the deformation occurs very uniformly over the entire gauge length. In FSW welds the deformation localizes in the weld at a line of entrapped oxide particles. In EB welds the deformation localizes to the large grains in the middle of the weld or at the steep grain size gradient between the weld and the tube material. Tensile strength is lowest in the EB welds (175 MPa as compared to 200 MPa or higher for the other samples). Elongation to fracture of the FSW welds is similar to those of the base materials, but for the EB welds it is significantly lower (appr. 40 % compared to 65 %). (orig.)