Inside the nucleus there is a competition between two opposing forces:
At this scale, the attraction caused by the strong force is greater than the electrostatic repulsion between protons. This means the nucleus is held together.
The strong force has a very short range, so larger nuclei are not as strongly bound. This leads to an unstable nucleus that can undergo radioactive decay or nuclear fission.
Figure 1. A nucleus showing the strong nuclear attraction (blue) and electrostatic repulsion (red).