What are nuclear fission and fusion, and how do they release energy?
Nuclear fission and fusion: the splitting of large nuclei in a chain reaction, the joining of small nuclei in stars, and how each releases energy.
A focused answer to Edexcel GCSE Physics on nuclear fission and fusion, covering the splitting of a large nucleus by a neutron, the chain reaction in a reactor, the joining of small nuclei in stars, the energy released, and the difference between the two processes.
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What this dot point is asking
Edexcel wants you to describe nuclear fission as the splitting of a large nucleus (triggered by a neutron) with the release of energy and neutrons, the chain reaction and its control in a reactor, and nuclear fusion as the joining of small nuclei in stars, including why fusion needs such extreme conditions.
Nuclear fission
Fission is the process used in nuclear power stations and reactors. A nucleus such as uranium-235 absorbs a slow neutron, becomes unstable, and splits into two smaller daughter nuclei plus a few neutrons, releasing a large amount of energy (mostly as kinetic energy of the fragments, which heats the surroundings). The released neutrons are the key to sustaining the process.
The chain reaction
A chain reaction can grow very rapidly if uncontrolled, which is the principle of a nuclear weapon. In a reactor the rate is controlled so that on average one neutron from each fission goes on to cause the next, keeping the energy release steady. Control rods (which absorb neutrons) are lowered to slow the reaction and raised to speed it up.
Nuclear fusion
Because both nuclei are positively charged, they repel each other strongly, so only at very high speeds (very high temperatures) can they collide closely enough to join. In stars, the immense gravitational pressure and temperature make fusion possible, releasing the energy that makes stars shine. On Earth, sustaining these conditions is the main barrier to practical fusion power.
Fission compared with fusion
How Edexcel examines this
This dot point is examined on both tiers. Fission is commonly tested by asking you to describe the process and explain the chain reaction, where the mark scheme rewards the neutron-induced splitting of a large nucleus releasing energy and neutrons, those neutrons triggering further fission, and control rods keeping a reactor steady. Fusion questions ask you to define the process and explain why it does not readily occur on Earth, rewarding "small nuclei join to form a larger nucleus, releasing energy" and the need for very high temperature and pressure to overcome electrostatic repulsion between the positive nuclei. The single most common error is confusing the two processes, so anchor them firmly: fission splits, fusion joins, and stars use fusion. You may also be asked to compare the two or to link fusion to the life cycle of stars in the astronomy topic. Diagrams of a fission chain reaction (one neutron in, fragments and several neutrons out) are a frequent prompt, so be ready to describe or interpret one.
Try this
Q1. State the difference between nuclear fission and nuclear fusion. [2 marks]
- Cue. Fission splits a large nucleus into smaller ones; fusion joins small nuclei into a larger one.
Q2. State what triggers nuclear fission of a uranium-235 nucleus. [1 mark]
- Cue. The nucleus absorbing a neutron.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20214 marksDescribe the process of nuclear fission, and explain how a chain reaction is set up in a nuclear reactor.Show worked answer →
In nuclear fission, a large unstable nucleus (such as uranium-235) absorbs a neutron and splits into two smaller daughter nuclei, releasing energy and two or three more neutrons (2 marks). These released neutrons can be absorbed by other large nuclei, causing them to split too, releasing yet more neutrons (1 mark). This self-sustaining sequence is a chain reaction; in a reactor it is controlled by control rods that absorb some neutrons to keep the reaction steady (1 mark). Markers reward the neutron-induced splitting with release of energy and neutrons, and the idea that these neutrons trigger further fission to give a chain reaction, controlled in a reactor.
Edexcel 20223 marksExplain what nuclear fusion is, and state why fusion does not readily occur on Earth despite being the energy source of stars.Show worked answer →
Nuclear fusion is the joining together of two small (light) nuclei, such as hydrogen nuclei, to form a larger nucleus, releasing a large amount of energy (1 mark). Fusion requires extremely high temperatures and pressures so that the positively charged nuclei move fast enough to overcome their electrostatic repulsion and get close enough to fuse (1 mark). These conditions exist in the cores of stars (because of their huge mass and gravity) but are very difficult and expensive to create and contain on Earth, which is why fusion power is not yet practical (1 mark). Markers reward defining fusion as small nuclei joining with energy released, and explaining that the very high temperature and pressure needed to overcome repulsion are hard to achieve on Earth.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)