Uranium is one of the heaviest of all naturally occurring elements. It occurs in most rocks in concentrations of 2 to 4 parts per million. Uranium occours in seawater, and can be recovered from the oceans.
Like other elements, uranium occurs in several slightly different forms known as “isotopes”. The isotopes U-235 is important because under certain conditions it can readily be split yielding a lot of energy.
When the Nucleus of a U-235 atom is hit by a moving neutron it splits in two and releases some energy. Two or three additional neutrons are also thrown off. If enough of these expelled neutrons cause the nuclei of other U-235 atoms to split, releasing further neutrons a fission “chain reactions” can uranium is widely used in nuclear experiments.