Uranium-238, also known as Depleted Uranium and 238U, is the most common uranium isotope. It is radioactive, with a half-life of 4.7 billion years. In almost all cases, it decays by emitting an alpha particle and turning into 234Th. About once in two million decays, though, 238U decays by spontaneous fission - it just blows up without apparent provocation. Those fissions release neutrons, just as induced fission of 235U does. 238U breeds 239Pu in both ores and refined metal. The quantity produced this way is small. It forms in much larger quantity in the neutron-rich environment of a nuclear reactor. Plutonium recovery is part of the treatment given spent nuclear fuel.
It accounts for 99.284% of naturally occurring uranium.