Nuclear energy occurs when nuclear bonds split or fuse.
Explanation:
When nuclear bonds or atoms are split in two or more smaller subunits i.e nuclei it releases by products also is termed nuclear fission. The reaction takes place in unstable heavy atoms as uranium. Nuclear binding energy is the energy present in heavy metal.
In nuclear fusion two atomic nuclei fuse to make heavy nuclei in which either energy is released or absorbed.
Electricity is generated by using fusion process. Sun's energy is by fusion process.
Nuclear energy is resultant of nuclear reactions between the atomic nuclei.
Nuclear energy is useful in radioactive radiations (reactors and turbines), to generate electricity, agriculture, medicine and industries etc.