At short distances between the nucleons at roughly 3 femtometers or so, the strong nuclear force between two nucleons begins to overcome the electrostatic repulsion (if both or one nucleon is a proton). The strong force then becomes it's strongest at roughly 1.5 femtometers, hence meaning that is the typical seperation between two nucleons. At even shorter distances than that the force then starts to become a repulsive force at roughly 0.2 femtometers or so, this is to prevent the nucleons from colliding into a singlarity.