Performance of copper overpack for repository canisters

The final disposal of the canisters containing spent nuclear fuel involves long term storage in a deep geological repository. The canisters include a 50 mm overpack (top layer) of oxygen free phosphorus doped (OFP) copper for corrosion protection. In storage the canister is subjected to some heating by the residual activity of the contents, requiring reliable prediction of safe creep life from the available rupture and strain properties of the overpack that includes electron beam (EB) or friction stir welds (FSW) for sealing. Assessments of creep strain and rupture at relevant service conditions are inevitably susceptible to any bias in the applied material models, the underlying material data and predictive tools. FSW appears to generally produce stronger welds, although significant creep weakening has been indicated in case of small scale root defects. In EB welds large grains and characteristic patterns of solidification result in some anisotropic mismatch to reduce creep strength and increase strain localisation of welds. In this paper FSW and EB cross weld test results are assessed and compared for creep response. The models have been converted to comply with the requirements for in-house Finite Element Assessment (FEA) code and used for simulating the FSW behaviour. The simulated strain response is compared with corresponding measurements in long term creep testing. The results of the simulations and material models are discussed in the view of targeted life span of the canister overpack. Observations are presented regarding the expected stress, strain and multi-axial constraint in the welded structure.