2007:12 Development of a constitutive model for the plastic deformation and creep of copper and its use in the estimate of the creep life of the copper canister

The copper-iron canister for disposal of nuclear waste in the Swedish Programme has a design life exceeding 100,000 years. Whilst the operating temperature and operating stress are modest, the very long design life does require that the likely creep performance of the canister should be investigated. Many studies have been carried out by SKB but these have all involved very short duration tests at relatively high stresses.

Reported creep tests on oxygen free (OF) copper have demonstrated that copper can have extremely low creep ductility. However with the addition of about 50 ppm phosphorus to the copper it appears as if the creep brittleness problem is avoided. OFP copper has consequently been chosen as the canister material. One of the requirements for the copper used in the waste canister is a creep ductility of 10 % [SKB TR-06-09]. The information presented so far by SKB has not demonstrated conclusively that the OFP copper actually have this ductility at the canister operating temperatures.

A previously developed model for the plastic deformation and creep of copper (included as an Appendix to the present report) has been used as the basis for a discussion on the possibility of brittle creep fracture of the copper canister during long term storage of nuclear waste in this project.