Sources of nuclear energy are ATOMS, which are the smallest particles of an element that can exist. ATOMS contain protons and neutrons, which are held together by a strong nuclear force. Nuclear energy is released when these particles are split apart or fused together in a process called nuclear fission or nuclear fusion. Nuclear fission is the process of splitting ATOMS to release energy, while nuclear fusion is the process of combining ATOMS to release energy. Nuclear energy is used to generate electricity, power ships, and create weapons.