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Usually referred to as just u235. It is an isotope of uranium that differs from the element's other common isotope, u238, by its ability to cause a rapidly expanding fission chain reaction. A uranium nucleus that absorbs a neutron splits into two lighter nuclei; this is called nuclear fission. It releases either two or three neutrons which continue the reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.  One atom of U-235 generates 200 MeV = 3.2 × 10-11 J, i.e. 18 TJ/mol = 77 TJ/kg.  Only around 0.72% of all natural uranium is uranium-235, the rest being mostly uranium-238. This concentration is insufficient for a self sustaining reaction; enrichment, which just means separating out the uranium-238, must take place to get a usable concentration of uranium-235. For an explosion, approximately 90% purity is required.

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