When it decays, the uranium nucleus contains 235 nucleons. It decays to approximately two equal halves, each having between 95 and 125 nucleons. There exists a great variety of ways the nucleus disintegrates, and thus the fission products include many different elements. The most common are iodine and cesium. The other possible elements include barium, krypton, xenon, strontium, zirconium, and tellurium. Many of these elements are radioactive and undergo radioactive decay to yet other elements.

Individual fission products of ²³⁵U, ²³⁹Pu and ²³³U atomic nucleus fission caused by thermal neutrons.

Uranium ²³⁸U may capture a neutron and then by a couple of beta decays transmute to plutonium ²³⁹Pu which is fissionable and decays in turn, producing elements with atomic numbers between 100 and 135. These elements also include a whole range of other radioactive isotopes that also decay.

In a single ton of spent nuclear fuel there are 45 kg of fission products.

Apart from fission and other radioactive changes taking place in a nuclear reactor, there is also neutron capture. Elements enriched in this way often undergo subsequent beta decays. By neutron capture and subsequent beta decay, ²³⁹Pu may be transmuted all the way to americium ²⁴¹Am.

Schematic depiction of fission products contained in spent fuel.

After four years inside a nuclear reactor, the fuel pellets contain almost all the elements of the periodic table. Spent nuclear fuel is 95.5% uranium (of which 95% is ²³⁸U and 0.5% ²³⁵U), 0.9% plutonium, 3.1% stable fission products (lanthanides and others), 0.1% actinides (neptunium, americium), 0.2% short half-life fission products (cesium and strontium), and 0.2% long half-life fission products (zirconium, iodine, and technetium).

Fresh fissionable material contains up to 1,300 radionuclides of various types, the majority of them with a very short half-life.