Nuclear energy is the energy that holds together the nuclei of atoms. Atoms are the simplest blocks that make up matter. Every atom has in its center a very small nucleus. Normally, nuclear energy is hidden inside the atoms. However, some atoms are radioactive and send off part of their nuclear energy as radiation. Radiation is given off from the nucleus of unstable isotopes of radioactive substances.
Nuclear energy can also be freed in two other ways: nuclear fusion and nuclear fission. Nuclear fusion is the combining of two atoms into one and nuclear fission is the splitting of an atom. Both ways make big amounts of energy. They sometimes take place in nature. Fusion is the source of heat in the sun. Fission is also used in nuclear power plants to make electricity. Both fusion and fission can be used in nuclear weapons.
Nuclear power generates a number of radioactive by-products, including tritium, cesium, krypton, neptunium and forms of iodine.

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