The microstructure, grain size and grain boundary (GB) structure of copper in canisters for encapsulation of spent nuclear fuel have been characterized. Coincident site lattice (CSL) GBs were found to be more common than random high angle GBs. Apart from Σ3, the Σ9 and Σ27 GBs show a higher frequency of occurrence than the other CSL GBs. This effect is more pronounced for the material in the canister lid that is slightly deformed. The high fraction of CSL boundaries of 60–65% is of the same order as for grain boundary engineered material. One critical property for the canister is the creep ductility that is improved by the addition of phosphorus (P) to the copper (Cu-OFP). The structure and segregation energies for P onto CSL boundaries have been determined with quantum mechanical calculations. In comparison to a previous study where literature data for the frequency of occurrence of CSL GBs was used, the absolute values of the segregation energies and the occupancy of P at GBs are significantly increased when using data for the canister copper. The presence of P reduces the amount of creep cavitation, which controls the ductility during brittle creep rupture. The creation of cavities and creep ductility are predicted. The computed creep ductility is slightly higher than in a previous study mainly due to the frequent occurrence of the Σ9 GB, which leads to strong segregation of P. The influence of grain size on the creep ductility has also been analysed. A large reduction in ductility with grain size is found for Cu without P which agrees with observations. For Cu-OFP a reduction in creep ductility for grain sizes up to 300 µm is also predicted, but no further reduction is obtained for still larger grain sizes.