The material generally used in a nuclear fission reactor is uranium metal or uranium dioxide, and sometimes plutonium dioxide is added. Natural uranium exists as mainly two isotopes - fissile uranium-235 (235U92 - 0.7%), which is capable of undergoing fission, and fertile uranium-238 (238U92 - 99.3%), which is capable of absorbing neutrons to form a fissile material. Plutonium (239Pu94) is not found naturally, but is produced artificially in a reactor from uranium-238. Most modern day reactors use fission of uranium-235, or a combination of uranium-235 and plutonium to provide energy. [Dickson, N. (2002)]

In a nuclear reactor, uranium-235 is bombarded with slow neutrons much less than 1 electron volt (a thermal neutron), to induce fission. In a common reaction, a nucleus of uranium-235 captures a neutron and then undergoes a fission event, splitting to form two lighter nuclei (such as Krypton and Barium), releasing two or three neutrons of about 1 million electron...