A chain reaction occurs when uranium-235 undergoes nuclear fission in a nuclear reactor. Describe how a chain reaction is produced and explain how control rods prevent the chain reaction from becoming uncontrolled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a nuclear reactor, uranium-235 is used as the fissile fuel. Without careful management, the fission process could escalate rapidly.
Model answer (4 marks)
A uranium‑235 nucleus captures a thermal neutron and splits into two smaller nuclei.
The split releases 2–3 neutrons, which can then strike other U‑235 nuclei, causing further fission.
Control rods made of a neutron‑absorbing material (e.g. boron, cadmium) are inserted into the core.
They absorb excess neutrons, reducing the number that can induce further fission, so that only about one neutron per fission continues the chain, keeping the reaction steady.
The split releases 2–3 neutrons, which can then strike other U‑235 nuclei, causing further fission.
Control rods made of a neutron‑absorbing material (e.g. boron, cadmium) are inserted into the core.
They absorb excess neutrons, reducing the number that can induce further fission, so that only about one neutron per fission continues the chain, keeping the reaction steady.
Examiner tips
- Use the term ‘thermal neutron’ and ‘neutron absorption’. Show the chain reaction loop and the role of control rods. Mention that only one neutron per fission is needed for a steady state.
Common mistakes
- Confusing fast neutrons with thermal neutrons. Saying control rods *stop* the reaction instead of *moderate* the neutron population. Omitting the fact that 2–3 neutrons are released per fission.
Mark scheme (4 marks)
- A uranium-235 nucleus absorbs a (slow-moving / thermal) neutron and splits into two smaller (daughter) nuclei
- The fission event releases 2 or 3 neutrons (which go on to strike other uranium-235 nuclei)
- Control rods absorb neutrons (reducing the number available to induce further fission)
- Control rods ensure only one neutron per fission event goes on to induce further fission, keeping the reaction steady / controlled
Key terms in this question
chain reaction · nuclear fission · control rods · uranium-235
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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