Uranium is the heaviest naturally occurring chemical element. All of its isotopes are radioactive, and it would have disappeared from our environment if two of these isotopes, uranium-238 and uranium-235, did not have a life span comparable to the age of the solar system.
Uranium-238 and uranium-235, along with their daughter isotopes, are the source of some of the radioactivity to which we are exposed. Uranium-235 is the only natural nucleus that is easily fissile. It is highly sought after, but it is rare, since natural uranium is 99.3% isotope 238 and only 0.70% isotope 235. After enrichment, uranium-235 is used as fuel in nuclear reactors and as an explosive in atomic bombs.
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