Fission

We have seen how energy may be released if a vary large nucleus splits into two smaller nuclei.

Here is an important example;

Uranium 235 is naturally fissile but has a very long half life. If we can get it to absorb a neutron it would become Uranium 236 which is very fissile and splits very quickly.

The split is usually asymmetric (one piece is bigger than the other)

About 200MeV are released per fission event. Most of this is in the form of kinetic energy of the fragments. This may not seem a lot but If you calculate how much energy is released from 1 kg of Uranium 235 you get quite a large figure.

Because larger nuclei are more neutron rich we also get 2 or 3 neutrons released per fission. If at least one of these goes on to cause another fission event then a chain reaction will result.