For nuclear fission you
need material that will undergo nuclear fission when bombarded by neutrons such as
uranium or plutonium and this is called fissile material.
To split all the atoms in a
sample the neutrons from one fission must go on to split other atoms and so on throughout
the sample. This is known as a chain reaction.
To keep a chain reaction going you
need a certain minimum amount of this fissile material. This minimum amount is called the
critical mass, and is thought to be about 10 kg. If you have less mass than this then too many
neutrons escape from the surface to maintain the reaction. You also need a source of
neutrons called the initiator.
In a bomb the uranium is divided into two parts, both
less than the critical mass. They are placed at opposite ends of a tube. The device also
contains a neutron source. Everything is fine as long as the two masses of fissile material are
kept apart.
To explode the bomb one of these pieces of uranium is fired into the
other using some conventional explosive. Suddenly there is a mass of uranium (or
plutonium) greater than the critical mass, a chain reaction starts and the bomb
explodes.