Nuclear fission reaction, computer artwork. At left is a neutron (blue) about to collide with an uranium-235 nucleus (grey). Upon collision the neutron combines with the nucleus to form uranium- 236. This is a highly unstable element that spontaneously undergoes nuclear fission (splitting), producing a vast amount of energy (seen here as a blue and green explosion), the elements barium-141, krypton-92 and three neutrons. This is the reaction that occurred in the atomic bombs dropped by the Americans on the Japanese cities of Hiroshima and Nagasaki during World War Two.