Nuclear energy, also known as atomic energy, is divided into two categories: nuclear fusion energy and nuclear fission energy, and is a form of primary energy. Nuclear energy is the energy released from atomic nuclei through nuclear reactions. The discovery of nuclear energy is a great achievement in human history, and it can be traced back to the end of the 19th century when a British physicist, Thomson, discovered the existence of electrons, which led to the gradual discovery of atomic nuclei. 1. Nuclear energy (or atomic energy) is the energy released from atomic nuclei through nuclear reactions, in accordance with Albert Einstein's mass-energy equation E=mc2, where E=energy, m=mass, and c=the speed of light. Nuclear energy can be released through three types of nuclear reactions: nuclear fission, where heavier atomic nuclei split and release energy; 2. Nuclear fusion, where lighter atomic nuclei combine to release energy.