Sensitivity analysis of FEA simulations and the effect of instability factors in stress relaxation on the life assessment of the final disposal canister made of copper (POHELY)

Oxygen-free phosphorus-doped (OFP) copper is a prime candidate outer shell material to protect the spent fuel canisters against corrosion in the underground repository. Because the canister and its copper shell are subjected to internal heating by residual activity of the contents, it is possible that with sufficient loads or strains from e.g. thermal, handling or other sources shell will creep to some extent. In particular, the shell may experience stress relaxation, when such loading is first peaking and then followed by steadier periods under constraint. The stress relaxation and subsequent creep behavior can be expected to depend significantly on the degree of bentonite buffer saturation and the affiliated temperature profile during the early stages of canister life. Since these early stages, typically below 1000 years of operation, set the conditions for long term operation, they also significantly influence the expected life of the canister over affiliated periods of time, up to 100 000 years. In this work the stress relaxation behavior of OFP is studied by experimental research and stress relaxation modelling. This is considered necessary before extending the modelled relaxation behavior to multi-axial cases and finite element analysis (FEA) of the canister (overpack) details. The FEA constitutive model is derived on the basis of the experimental relaxation results. It is compared to the logistic creep strain prediction (LCSP) model for OFP copper, which has been demonstrated to describe long term creep life accurately. One of the main targets of this work is to present how the more refined treatment of relaxation behavior influences canister life and improves the lifing procedure overall. Analysis cases are presented demonstrating the significance of including a purpose build relaxation model.