Abstract
It is planned that doped copper overpacks will be utilized in the spent nuclear fuel repositories in Finland and in Sweden. The assessment of long-term integrity of the material is a matter of importance. Grain structure variations, segregation and any possible manufacturing defects in microstructure are relevant in terms of susceptibility to creep and damage from the loading evolution imposed by its operating environment. This work focuses on studying the microstructure level length-scale dependent deformation behavior of the material, of particular significance with respect to accumulation of plasticity over the extensive operational period of the overpacks. The reduced micromorphic crystal plasticity model, which is similar to strain gradient models, is used in this investigation. Firstly, the model’s size dependent plasticity effects are evaluated. Secondly, different microstructural aggregates presenting overpack sections are analyzed. Grain size dependent hardening responses, i.e., Hall-Petch like behavior, can be achieved with the enhanced hardening associated with the micromorphic model at polycrystalline level. It was found that the nominally large grain size in the base material of the overpack shows lower strain hardening potential than the fine grained region of the welded microstructure with stronger strain gradient related hardening effects. Size dependent regularization of strain localization networks is indicated as a desired characteristic of the model. The findings can be utilized to provide an improved basis for modeling the viscoplastic deformation behavior of the studied copper alloy and to assess the microstructural origins of any integrity concerns explicitly by way of full field modeling.
Highlights
Regarding the spent nuclear fuel repository in Finland, the copper overpack of the canister will be an important barrier against radioactive discharge from the repository
The major difference in this work is that we investigate the model behavior for polycrystalline copper, while the referred-to previous studies focused on the regularization of local slip bands as well as on the model’s characteristics and effects of micromorphic parameters
Various length-scale related hardening responses can be achieved with the reduced micromorphic model which is akin to strain gradient models
Summary
Regarding the spent nuclear fuel repository in Finland, the copper overpack of the canister will be an important barrier against radioactive discharge from the repository. The overpack is surrounded by bentonite blocks that swell due to the ground water. As the hydrostatic and bentonite swelling pressure developed the overpack will be pressed against the insert, affecting the stress state in the overpack. High purity oxygen-free phosphorus-doped (OFP) copper is the intended material for the overpack to reduce the amount of Cu2O inclusions. The phosphorous is added to further reduce the inclusion since as the hydrogen diffuses through the copper it can react with the Cu2O inclusions, forming H2O steam bubbles at the grain boundaries. The overpack will be internally heated up to 100 ◦C by residual nuclear fuel activity and time-dependent deformation and damage mechanisms (creep, stress relaxation and cyclic loads) are active during its operational lifetime
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