Spontaneous Fission

For heavy elements (greater than mass 232) the nucleus can split into two smaller elements and a few spare neutrons. This is called spontaneous fission. For example Uranium 235 can decay into Barium-141 and Krypton 92

With fission the atom can split in very many different ways although the ratio of the atomic masses tend to be a 40%:60% split. For Uranium this leads to the formation of an element with atomic mass around 80-110 and another element with atomic mass 125-155. Therefore unlike alpha and beta decay, fission produces a variety of elements after the decay. These include isotopes of Strontium, Iodine and Caesium.

For Uranium it is very rare that it will undergo spontaneous fission – it is much more likely to decay via α decay.

Induced Fission

There are ways of inducing fission. This is usually done using neutrons.

When a neutron hits the nucleus of certain isotopes such as Uranium 235 one of the things that it can do is to can cause the Uranium to undergo fission. As mentioned before this produces some more neutrons. These neutrons can then go on to hit other Uranium 235 nuclei which then undergo fission and produces more neutrons. This is the chain reaction.

Not all isotopes undergo this self sustaining fission – Uranium 235 does but Uranium 238 does not.

To get a chain reaction you have to have a large enough lump of uranium 235 (because some neutrons will be lost to the outside world at the surface. The size of this lump is the critical mass. The Uranium 235 must also be of sufficient purity.

Wikipedia