Uranium Core
Natural uranium contains about 0.7% U235 -- the active
nucleus in fission reactors. Mined uranium is enriched to about
3% U235, and carefully packaged into fuel elements.
An individual fuel element is used for about 3 years in a modern
reactor. Following its removal from the reactor, fuel elements are
stored near the reactor, as the (mostly political) problems of long term
storage have not yet been solved. A used fuel element is highly radioactive
even though most of the the uranium fuel has been used up because the products
of the reaction are themselves radioactive.