A Frank Read source is activated inside a closed grain under constant strain rate conditions. Dislocation loops are piling up on the boundary of the grain, taken here as a cylinder. The applied stress is monitored so that the constant shear strain is maintained during the whole simulation. The pile-up backstress tends to block the source motion. Consequently the stress decreases when the loop size increases. When the loop reaches the boundary, the stress has to increase again so that a new loop is nucleated.