Nuclear energy is the energy that holds together the nucleus of atoms. Atoms are the most simple 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 light atoms into a heavier one and nuclear fission is the splitting of a heavy 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.
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Nuclear power generates a number of radioactive by-products, including tritium, cesium, krypton, neptunium and forms of iodine some of which can be dangerous for millions of years. 
Nuclear energy production and use has been a controversial topic over the years. Nuclear energy can supply a very large amount of electrical power for use in cities, but the nuclear materials that make the electricity can be very dangerous for thousands or millions of years after they have been used. Countries have made decisions to use nuclear power based on their current energy demands as well as environmental conservation demands. Countries could venture into nuclear energy production to help meet the rising energy demand, to help conserve the environment by avoiding air pollution, and also as a long lasting replacement for the depleting fossil fuels sources of energy, but would have to ensure the ongoing safety of production and waste storage. There have been upgrades to reactor design to avoid nuclear energy accidents like those of Chernobyl and Fukushima, but accidents still continue to occur and ageing reactors are a safety concern. Countries using the nuclear energy production process to make dangerous weapons of mass destruction is a related concern.