Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
6.39P Explain the principle of a controlled nuclear chain reaction
Full specification wording
Topic 6 – Radioactivity: 6.39P Explain the principle of a controlled nuclear chain reaction
A chain reaction happens when each fission event releases neutrons that go on to trigger further fission events, so the number of fissions can grow rapidly.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 35 of 42
Study notes
From the checked teaching notes for this specification point.
The idea
A chain reaction happens when each fission event releases neutrons that go on to trigger further fission events, so the number of fissions can grow rapidly. In uranium-235, absorbing a slow neutron splits a nucleus into two smaller nuclei and releases two or three neutrons plus energy. The principle of control is to keep the number of further fissions per fission at about one, so the rate stays steady rather than multiplying. If on average more than one released neutron causes another fission, the rate rises; if fewer than one does, the reaction dies down. Control therefore means managing how many neutrons are absorbed or lost before they can cause fission, not stopping fission altogether.
What earns the marks
- States that fission of a nucleus releases neutrons which can trigger further fission events.
- Explains that a controlled chain reaction keeps the average number of further fissions per fission at about one.
- Describes that absorbing or losing neutrons reduces the number available to cause further fission.
- Identifies that the rate of fission rises when more than one released neutron causes another fission and falls when fewer than one does.
Common mistakes and how to put them right
Confuses control with stopping the reaction completely.
Put it right: Describe control as keeping the rate steady at about one further fission per fission.
Assumes every released neutron causes another fission.
Put it right: State that some neutrons escape or are absorbed so the average is managed.
Treats the chain reaction as a chemical burning process.
Put it right: Describe it as nuclear fission releasing neutrons and energy.
Exam tips
- Link the word 'controlled' to the balance of neutrons causing further fission, not to switching the reactor off.
- Use the idea of an average of about one further fission per fission to explain a steady rate.
- Contrast a rising rate with a falling rate by referring to more or fewer neutrons causing fission.
Try the lesson
The start of the Topic 6 – Radioactivity: 6.39P Explain the principle of a controlled nuclear chain reaction lesson, made from checked teaching notes for this specification point.
Have a go first
What is meant by a chain reaction in a nuclear reactor?
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Topic 6 – Radioactivity: 6.39P Explain the principle of a controlled nuclear chain reaction
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge