... is an isotope with 235 neutrons of the Uranium atom with the following characteristic:
When shooting at Uranium-235 with a 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. With the disintegration energy in the form of radiation (heat) from converting a little of the original mass (E=mc2) and 3 neutrons are released. 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.