Aged degradations of structural materials of boiling water reactors by stress corrosion cracking (SCC) have been frequently reported. SCC is the results of the synergistic interaction of mechanical stress and corrosive environment, and the investigation of this phenomenon has been an important issue. Although many kinds of studies for SCC have been carried out, we have not clarified the fundamental mechanisms of SCC initiation and propagation yet. In the recent experimental studies, nano-scale observation around crack tips using transmission electron microscopy have shown three characteristics of SCC of nuclear structural materials as follows; the size of crack tip is nanometer order, the opening crack is filled with the oxides, and oxygen atoms exist in the grain boundary beyond the crack tips. The second and third ones show that the corrosive environment is mainly influenced on the SCC propagation behavior. Furthermore, electron back scatter diffraction pattern analyses have shown that about 10–20% of plastic strain exists around the crack tips and crack sides. The existence of oxygen atoms along grain boundaries and plastic strains around grain boundaries could be related to the crack propagation mechanism of SCC. In this study, in order to observe the influence of oxygen atoms on the SCC propagation behavior, the two-dimensional SCC propagation model considering diffusion of oxygen atoms along grain boundaries is developed. In this model, the stress distribution of polycrystalline system is obtained by the crystal plasticity theory, and the concentration of oxygen atoms depending on stress localization around cracks is calculated using the diffusion equation of oxygen atoms considering the stress gradient. The density distribution of oxygen atoms is adopted for the threshold of the crack propagation. Relation between the threshold of crack propagation as a viewpoint of density of oxygen atoms along grain boundaries and the geometry of SCC is discussed in this paper.

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