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Crack nucleation mechanisms and nucleation conditions are still poorly understood. Experimental evidence suggests that crack nucleation mainly occurs when growing twins, slip or persistent slip bands collide with grain boundaries. We therefore started to investigate these processes by atomistic simulations. As a first step, we present here simulations of crack nucleation at the ∑9(221) grain boundary in tungsten resulting from the interaction with dislocations. Symmetric twin boundaries and most other high-angle grain boundaries are relatively stable and act as barriers to dislocation motion. When a dislocation is absorbed by such grain boundaries, a stress concentration develops at the absorbing site and a crack is nucleated there. Crack nucleation is investigated under different loading conditions. Our results show that crack nucleation is aided significantly by additional dislocation activity. The relevant mechanisms are significantly more complex than captured by simple pile-up models.