1) The positive energy of its protons.
2) The binding energy holding the nucleus together.
3) Radioactive energy emitted during nuclear decay
There are a number of energy types associated with the nucleus of an atom, including those listed above.
All nuclei contain protons, which immediately provide them with a positive charge that maintains an electron shell or cloud around them. Balancing the numbers of protons and electrons in an atom results in chemical reactions and compounds.
The protons in the nucleus do not want to stay together due to their like positive charges. They are held together by a stronger force called the binding energy of the atom.
If the atom contains a large number of protons, it may become unstable due to natural or outside forces working on it. If the instability becomes critical, the atom may start to decay and particles may break away from the nucleus in a radioactive reaction.