Nuclear fission, nuclear fusion and power

​​In a nutshell

Nuclear fission is the splitting of a large unstable nucleus into smaller nuclei. This is used in nuclear power stations as large amounts of energy are released. Nuclear fusion is the joining together of two nuclei to form a single larger nucleus, which releases large amounts of energy.

Nuclear fission

Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei. This releases a large amount of energy. 

Most of the time this process is not spontaneous and is caused by firing a slow-moving neutron at the original nucleus.

Example

Uranium-$$235$$​ is unstable and radioactive due to its size and the large number of neutrons compared to protons. It has a long half-life of $$700$$​ million years. 

Firing a neutron at uranium-$$235$$ causes it to become uranium-$$236$$. This isotope is violently unstable and so it undergoes nuclear fission and splits into two large fragments called 'daughter nuclei'. It also releases two or three neutrons and produces high energy photons.

The nuclear equation for the above process is

​$${^{235}_{92}U}+n\to{^{236}_{92}U}\to{^{144}_{56}Ba}+{^{90}_{36}Kr}+2n+\gamma$$​​

Fission reactants (on the left of a nuclear equation) are converted into fission products (on the right of a nuclear equation). 

Note: The symbol $$n$$ is used to represent neutrons. Neutrons have a mass number of $$1$$ which must be considered when balancing the nuclear equations.

The process of fission results in two or three neutrons being released. If the neutrons move slow enough, they can be absorbed by another large nuclei and cause it to undergo nuclear fission as well. This continuation of nuclear fission is called a chain reaction.

Nuclear power

A nuclear reactor uses chain reactions of nuclear fission to produce energy. 

The amount of energy produced is controlled by control rods. These can be lowered or raised to absorb the neutrons that are released. If the control rods absorb more neutrons, it slows down the chain reaction which produces less energy.

The energy produced by fission reactions heats water to produce steam. This steam causes turbines to rotate which turns the generator to generate electricity.

Uncontrolled reactions produce lots of energy. Enough energy being released would result in an explosion. This is how nuclear bombs work.

Nuclear fusion

Nuclear fusion occurs when two similar-sized smaller nuclei combine at high speed to form a larger nuclei.

The larger nuclei does not have as much mass as the two lighter nuclei. The excess mass will be converted into energy and is released as radiation.

Example

A proton can fuse together with deuterium (a hydrogen isotope) to form helium and a gamma ray. The nuclear equation for this process is

$${^{1}_{1}H}+{^{2}_{1}H}\to{^{3}_{2}He}+\gamma$$​​

This process is seen in stars and is how they produce energy.

The process of nuclear fusion requires extremely high temperatures and pressures to maintain because the reactants need to have a lot of energy to fuse together.