Nuclear reactions

When the nucleus of an atom changes new nuclei are formed and energy is released. This can happen in two ways and is called a nuclear reaction.

Nuclear fission

Nuclear fission is the process by which energy is released when a large atom is hit by a neutron, becomes unstable and splits into two or more smaller pieces plus two or three neutrons.

A neutron crashes into a target nucleus, which then splits into individual neutrons and fission products. The fission products are smaller nuclei that resemble the initial target nucleus. — Figure caption,
A neutron causes fission of a target nucleus

When this happens, some of the mass of the atom is 'lost' – it has been converted directly into energy. This energy is in the form of heat which can be harnessed and used to generate electricity in a nuclear power station.

Chain reactions

The neutrons produced by the reaction can then go on to further fission reactions.

A neutron crashes into a uranium nucleus which then emits two neutrons. These neutrons each smash into two more uranium nuclei, which then each emit two neutrons, which smash into four further uranium nuclei, which each emit two neutrons. The collisions are labelled first, second, third and fourth generation. — Figure caption,
A chain of fission reactions

At each stage the reaction increases in speed, increasing the rate at which energy is released. If the reaction rate continued to increase, a huge quantity of energy would be released in a very short time – a nuclear explosion.

In a nuclear reactor, the chain reaction is controlled by using Boron control rods. These absorb neutrons and can limit the number of neutrons hitting atoms and so will limit the energy produced.

A neutron crashes into a uranium nucleus which then emits two neutrons. One is absorbed and the other crashes into another uranium nucleus, which emits two neutrons, one of which is absorbed, the other crashes into another uranium nucleus.

Nuclear fusion

Nuclear fusion is the process by which energy can be released when two smaller nuclei fuse together to form a larger nucleus.

Fusion of a trilium H3 nucleus (two neutrons and one proton) and a deuterium H2 nucleus (one neutron and one proton), resulting in a helium He4 nucleus (two neutrons and two protons) and a single neutron.

This is the same process that occurs within stars such as our Sun that produces heat and light.

Revise: Nuclear radiationNuclear reactions