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• ReaxFF force field parameters are newly developed for Ni-Cr-Fe-H-O system. • The driving forces for atomistic SCC initiation were calculated. • The classic slip-dissolution model for SCC initiation is extended. Targeting atomistic initiation of stress corrosion cracking (SCC) of Alloy 690, reactive molecular dynamics simulations of SCC of polycrystalline Ni 60 Cr 30 Fe 10 alloys under water with dissolved oxygen at 900 K were performed employing newly developed reactive force field (ReaxFF) of Ni-Cr-Fe-H-O system. Observed atomistic mechanochemical mechanism involve dissolution-precipitation and diffusion-oxidation behaviors for corrosion, and cleavage-slipping-twinning behaviors for cracking initiation with its driving forces including loacl shear stress/strain and oxidation-enhanced atomistic stress. Roles of cold work, tensile strain and grain boundary (GB) structures in impacting these mechanochemical behaviors were explored based on calculated surface and GB energy, GB diffusivity and atomistic stress/strain. [ABSTRACT FROM AUTHOR]