Nuclear Fission

There are two different ways of getting energy from the nucleus of an atom - these are NUCLEAR FISSION and NUCLEAR FUSION.
The use of nuclear energy has become very important in our everyday lives, not just for the production of electricity but also for the social implications and the possible effect on the environment.

NUCLEAR FISSION

As you know the nucleus of an atom consists of protons and neutrons, held together by very large forces. In one type of uranium nucleus there are 92 protons and 143 neutrons - we call this uranium 235.

If we fire another neutron at this nucleus it becomes unstable - a bit like a wobbly jelly and can't hold itself together. In the end the nucleus splits in half and this is called NUCLEAR FISSION. The word fission just means splitting. A simplified diagram of nuclear fission is shown in Figure 2.

Nuclear fission animation

For the schoolphysics animation showing nuclear fission please click the symbol:

Chain reaction

The neutrons from the fission of one nucleus go on and may split other uranium nuclei if conditions are right. These fissions then produce more neutrons that split more nuclei and so on.

This is called a CHAIN REACTION. (Figure 3)

You only get a chain reaction if there is enough uranium 235 and it is the right shape otherwise too many neutrons will escape from the outside and the reaction will stop. The smallest amount of uranium needed to keep a chain reaction going is called the CRITICAL MASS.