| 235U is unique in its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. In fact, U-235 is the
only fissile isotope found in nature. It was discovered in 1935 by Arthur Jeffrey Dempster. 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.
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Notes
Notable Isotopes 
232U [140 neutrons]
Abundance: synthetic