How do nuclear fission and fusion release energy, and how do they differ?
Nuclear fission of heavy nuclei and the chain reaction, the parts of a nuclear reactor, and nuclear fusion in stars.
A CCEA GCSE Physics answer on nuclear fission of heavy nuclei and the chain reaction, the role of fuel rods, control rods and the moderator in a reactor, and nuclear fusion in stars and why it needs extreme conditions.
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What this dot point is asking
CCEA wants you to describe nuclear fission of heavy nuclei and the chain reaction, outline the parts of a nuclear reactor, and describe nuclear fusion in stars and why it needs extreme conditions. The fission and fusion comparison is a frequent exam question.
The answer
Nuclear fission
The chain reaction
The nuclear reactor
In a reactor:
- Fuel rods contain the uranium (or plutonium) that undergoes fission.
- Control rods (often boron) absorb neutrons and are raised or lowered to control the rate of the chain reaction.
- The moderator (such as water or graphite) slows the neutrons so they are more likely to cause further fission.
- Thick shielding (concrete) absorbs radiation to protect workers.
The energy heats water to make steam, which drives turbines and generators.
Nuclear fusion
Worked example: comparing fission and fusion
Examples in context
Example 1. A nuclear power station. A controlled fission chain reaction heats water to steam, which spins turbines and generators. Control rods are lowered to slow the reaction or raised to speed it up, keeping the energy output steady.
Example 2. The Sun. In the Sun's core, hydrogen nuclei fuse into helium under enormous temperature and pressure, releasing the energy that reaches Earth as light and heat. Recreating these conditions on Earth is the goal of fusion research.
Try this
Q1. What is nuclear fission? [2 marks]
- Cue. The splitting of a large, heavy nucleus into smaller nuclei, releasing energy and neutrons.
Q2. What is the job of the control rods in a reactor? [1 mark]
- Cue. They absorb neutrons to control the rate of the chain reaction.
Q3. Why does fusion require very high temperatures and pressures? [2 marks]
- Cue. To make the positive nuclei move fast enough and get close enough to overcome their repulsion.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe what happens in nuclear fission of a uranium-235 nucleus, and explain how a chain reaction can occur.Show worked answer →
In fission a uranium-235 nucleus absorbs a slow-moving neutron and becomes unstable. It splits into two smaller (daughter) nuclei, releasing energy and two or three more neutrons.
The released neutrons can be absorbed by other uranium-235 nuclei, causing them to split too. This is a chain reaction, which can grow if each fission triggers further fissions.
Markers reward: nucleus absorbs a neutron and splits into smaller nuclei releasing energy and neutrons; released neutrons cause further fissions, giving a chain reaction.
CCEA style4 marksExplain the difference between nuclear fission and nuclear fusion, and state why fusion requires very high temperatures and pressures.Show worked answer →
Fission is the splitting of a large, heavy nucleus into smaller ones, releasing energy. Fusion is the joining of two light nuclei (such as hydrogen) into a larger nucleus, also releasing energy.
Fusion needs very high temperatures and pressures so the positively charged nuclei move fast enough and get close enough to overcome their electrostatic repulsion and join together.
Markers reward: fission splits heavy nuclei, fusion joins light nuclei; both release energy; and fusion needs extreme conditions to overcome the repulsion between positive nuclei.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)