... is an isotope with 235 neutrons of the Uranium atom with the following characteristic:
When shooting at Uranium-235 with a slow neutron, it initially sticks to the atom and the very unstable Uranium-236 isotope results. This isotope quickly disintegrates into two new atoms, Krypton and Barium. This process is called induced fission. The disintegration releases energy from converting a little of the original mass (E=mc2) in the form of radiation (heat) along with 3 neutrons. These neutrons create new Uranium-236 isotopes, which also disintegrate and again release energy and neutrons...
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... a chain reaction which releases huge amounts of energy extremly fast if not controlled.