Explain what happens inside a nuclear reactor to produce energy, including how the chain reaction is controlled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear power stations generate electricity using energy released from the nuclei of uranium atoms.
Model answer (5 marks)
Uranium nuclei in the fuel rods absorb neutrons and become highly excited, causing them to split (fission) into two smaller nuclei.
The fission process releases a large amount of energy and several free neutrons.
These neutrons can go on to strike other uranium nuclei, sustaining a self‑sustaining chain reaction.
Control rods made of neutron‑absorbing material (e.g. boron or cadmium) are inserted into or withdrawn from the core to absorb excess neutrons and regulate the rate of fission.
The energy released heats water in the reactor core; the hot water produces steam that drives a turbine, generating electricity.
The fission process releases a large amount of energy and several free neutrons.
These neutrons can go on to strike other uranium nuclei, sustaining a self‑sustaining chain reaction.
Control rods made of neutron‑absorbing material (e.g. boron or cadmium) are inserted into or withdrawn from the core to absorb excess neutrons and regulate the rate of fission.
The energy released heats water in the reactor core; the hot water produces steam that drives a turbine, generating electricity.
Examiner tips
- Use the exact terms: "neutron absorption", "fission", "chain reaction", "control rods", "steam turbine". Show the causal sequence: absorption → fission → neutron release → chain reaction → control → heat → electricity. Include the role of control rods explicitly to earn the control point.
Common mistakes
- Mixing up the order of events (e.g. saying neutrons are released before fission). Forgetting to mention that control rods absorb neutrons. Using vague terms like "splitting" without specifying fission or daughter nuclei.
Mark scheme (5 marks)
- Uranium nucleus absorbs a neutron / is struck by a neutron
- The nucleus undergoes fission / splits into two smaller nuclei (daughter nuclei)
- More neutrons are released which can go on to cause further fissions / trigger a chain reaction
- Control rods absorb neutrons to control the rate of fission / chain reaction
- Energy released (from fission) is used to heat water / produce steam to drive a turbine / generate electricity
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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