Nuclear Fusion

Nuclear fusion occurs when two smaller, lighter nuclei join to form a single larger and heavier nucleus. Stars use nuclear fusion reactions to produce energy.

For example, under high temperature and pressure, two hydrogen nuclei can fuse together to form the nucleus of a helium isotope. A neutron will be released in the process and a large amount of energy.

A diagram illustrating a fusion reaction. In the centre, a bright orange sphere represents the fusion process. From the left, there are molecules labelled "Hydrogen-2" and "Hydrogen-3" converging towards the centre. As a result of the fusion, on the right, a molecule labelled "Helium (He)" and a separate particle labelled "Neutron (n)" emerge. Additionally, an arrow labelled "ENERGY" points outward, emphasising the release of energy during the fusion. The visual demonstrates the combination of hydrogen isotopes to produce helium, a neutron, and energy.

Elements heavier than hydrogen are formed over time when lighter elements undergo nuclear fusion.

Advantages and Disadvantages of Nuclear Fusion


Nuclear fusion produces a lot of energy because some of the mass of the original nuclei gets converted to energy, rather than being transferred to the new nucleus. This means that the mass of the final nucleus will be slightly less than the combined mass of the original nuclei.

Only a small amount is converted to energy in each reaction. However, when you combine the high number of reactions taking place, this produces a large amount of energy.

  • The Sun releases approximately $3.8 \times 10^{26}$ joules of energy every second

Another benefit of nuclear fusion is that it does not produce any radioactive waste. The process also requires hydrogen fuel, which can be made fairly easily.


An issue with nuclear fusion is that it only takes place at very high temperatures and pressures. This means that we cannot currently carry out sustained nuclear fusion reactions on Earth. However, there is a lot of research aiming to discover how we can use nuclear fusion at some point in the future.

Nuclear fusion requires such high temperatures because the process involves merging positively charged nuclei. As nuclei are positively charged, they will naturally repel each other. This means the nuclei must travel at a fast speed and fuse very quickly to overcome the electromagnetic repulsion.

Higher temperatures result in higher kinetic energy. So, to travel at such a high speed, the nuclei must travel in a hot gas or plasma that is at least 100,000,000 degrees Celsius (°C).

Comparing Nuclear Fusion to Nuclear Fission

Try not to confuse nuclear fusion with nuclear fission.

Nuclear fusion is the joining of lighter nuclei to form heavier nuclei, which releases a large amount of energy in the process. Nuclear fusion takes place in the stars at very high temperatures and pressures.

Whereas, nuclear fission is when unstable nuclei split into smaller nuclei. This process releases a lot of energy but not as much as nuclear fusion. We use nuclear fission on Earth to generate electricity. This is because it can take place at much lower temperatures and pressures than nuclear fusion.

A comparative diagram titled "FISSION VS FUSION". On the left, under "FISSION", a large atom with an explosive background splits into two smaller atoms, depicted by downward arrows, accompanied by the caption "SPLITS a larger atom into 2 or more smaller ones". On the right, under "FUSION", two smaller atoms with energetic trails combine to form a larger atom, shown by an upward arrow, with the caption "JOINS 2 or more lighter atoms into a larger one".