6.44P Explain the difference between nuclear fusion and nuclear fission

The idea

Fusion and fission are opposite processes. In fusion, two light nuclei, such as hydrogen isotopes, combine to form a larger nucleus, for example helium; the product has slightly less mass than the reactants and the difference is released as energy. In fission, a heavy nucleus such as uranium-235 absorbs a neutron and splits into two smaller nuclei, also releasing energy and further neutrons. Fusion requires extremely high temperature and pressure to overcome repulsion between nuclei, which is why it powers stars, whereas fission can occur in a reactor at much lower temperatures when a heavy nucleus is struck by a neutron. Both release energy from a small change in mass, but the direction of the change in nuclear size is opposite.

What earns the marks

• States that fusion combines light nuclei into a larger nucleus.
• States that fission splits a heavy nucleus into smaller nuclei.
• Notes that both processes release energy linked to a change in mass.
• Contrasts the conditions, recognising that fusion needs very high temperature and pressure while fission can occur in a reactor.
• Identifies fusion as the process in stars and fission as the process used in nuclear power reactors.

Common mistakes and how to put them right

• Reverses the two processes.

  Put it right: Keep fusion as joining light nuclei and fission as splitting a heavy nucleus.

• Says only one process releases energy.

  Put it right: Note that both fusion and fission release energy.

• Claims fission powers the Sun.

  Put it right: Identify fusion as the star's energy source and fission as the reactor process.

• Treats the two as identical because both release energy.

  Put it right: Point out the opposite change in nuclear size and the different conditions needed.

Exam tips

• Compare the two processes side by side in your thinking: what joins or splits, and what conditions are needed.
• Choose the option that correctly pairs fusion with joining and fission with splitting.
• Check any option that says both release energy, because that statement is correct and may be the key.
• Link fusion to stars and fission to reactors to keep the two contexts distinct.