Definition: A radioactive metallic element, atomic no. 92, atomic wt. 238.0289, occurring mainly in pitchblende and notable for its two isotopes: 238U and 235U (99.2745% and 0.720%, respectively, the rest being made up by 234U), 235U being the first substance ever shown capable of supporting a self-sustaining chain reaction.
[G. myth. character, Uranus]