Final answer:
As two nuclei come very close, the strong nuclear force leads to a reduction in potential energy, thus increasing stability. Upon reaching a certain proximity, repulsion between positively-charged nuclei causes a sharp increase in potential energy, reducing stability.
Step-by-step explanation:
When two nuclei move very close together, at distances less than the size of the nucleus, the strong nuclear force comes into effect. Initially, this attractive force is stronger than the repulsive force, causing the potential energy of the system to decrease. This reduction in potential energy indicates an increase in the stability of the system. As the distance further decreases and the nuclei move closer to each other, they reach a point where the potential energy is at its lowest, indicating a highly stable system.
However, if the nuclei continue to move even closer, the repulsive force between the two positively-charged nuclei starts to dominate. This repulsion is formidable and results in a sharp increase in potential energy, destabilizing the system and causing the potential energy to rise sharply.