Material integrity of welded copper overpack: Annual report 2014

The research programme on the creep issues of the nuclear waste disposal canister copper material has continued with long-running uniaxial and multiaxial experiments as well as relaxation testing to support the material modelling activities, which produce appropriate material models to be used in the FE analysis, which will be used to assess the stresses and strains in the critical locations of the canister. The relaxation testing is ongoing and it has proven necessary to use a servo-mechanical testing machine in order to generate good quality test data. The initial FE prediction of strain rates calculated by the LCSP creep model and a relaxation model suggests a clear difference, but this needs to be verified after a better relaxation model is defined based on much more data. The most important result of this year has been the observation of cracking in the oxide particle zone at the friction stir weld root in the longest running CT test, which was terminated after 50 000 hours for sectioning and metallographic investigations. The oxide particles have nucleated creep cavities which have eventually lead to cracking of the oxide particle zone in spite of the low stress state 8 mm ahead of the notch tip. This observation is in line with the earlier results from uniaxial creep tests with a radial sample from the weld root. The weakness only manifests itself in long-term testing because the cavity nucleation and growth process is a slow process. Because of the low strength of the oxide particle zone welding in air should be avoided and vacuum or inert gas should be used instead.