6.37P Recall that nuclear reactions, including fission, fusion and…

The idea

Nuclear reactions change the nuclei of atoms and release energy that can be harnessed. Fission splits a large nucleus such as U-235 into smaller nuclei, releasing energy and neutrons; this happens in nuclear power stations. Fusion joins light nuclei such as hydrogen isotopes to form a heavier one, releasing far more energy per kilogram, as in the Sun. Radioactive decay is the spontaneous change of an unstable nucleus emitting radiation, releasing energy continuously without any input. All three convert nuclear mass into energy, so they are energy sources. A question may ask which process powers the Sun (fusion) or which occurs in a reactor (fission).

What earns the marks

• Identifies fission, fusion and radioactive decay as nuclear reactions that release energy.
• States that fission splits a large nucleus such as U-235 and is used in nuclear power stations.
• States that fusion joins light nuclei and is the energy source of the Sun.
• Recognises that radioactive decay releases energy spontaneously from unstable nuclei.

Common mistakes and how to put them right

• Confuses fission with fusion.

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

• Claims radioactive decay needs a fuel supply.

  Put it right: Recognise that decay releases energy spontaneously.

• Assumes only fission releases energy.

  Put it right: Fusion and decay also release energy.

Exam tips

• Link each process to a familiar example: fission in reactors, fusion in the Sun, decay in a radioactive source.
• Read the question carefully to see which specific process is being asked about.
• Remember all three are nuclear reactions that release energy, not chemical burning.