What is the difference between nuclear fission and nuclear fusion?
Nuclear fission as the splitting of a large unstable nucleus, the chain reaction in a reactor and its control, nuclear fusion as the joining of two light nuclei, and why fusion needs extremely high temperatures and pressures.
A focused answer to OCR Gateway GCSE Physics A topic P6 on nuclear fission and fusion, covering fission as the splitting of a large nucleus, the chain reaction and its control in a reactor, fusion as the joining of light nuclei, and why fusion requires extremely high temperatures and pressures.
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What this topic is asking
OCR wants you to describe nuclear fission as the splitting of a large nucleus, explain the chain reaction and how it is controlled, describe nuclear fusion as the joining of light nuclei, and explain why fusion requires extremely high temperatures and pressures. This is part of topic P6.2 of the OCR Gateway Physics A (J249) specification.
Nuclear fission
The energy released in fission comes from the loss of mass as the nucleus splits, and is far greater per atom than the energy from any chemical reaction such as burning fuel.
The chain reaction and its control
If the chain reaction is left uncontrolled, the number of fissions grows rapidly and releases energy explosively (the principle of a nuclear weapon). In a power station the control rods are lowered to slow the reaction and raised to speed it up, keeping the energy output steady and safe.
Nuclear fusion
Why fusion needs high temperatures and pressures
Nuclei are positively charged, so two nuclei brought close together repel each other strongly (electrostatic repulsion). To fuse, they must collide hard enough to overcome this repulsion. This needs extremely high temperatures, so the nuclei move very fast, and extremely high pressures (or densities), so collisions are frequent and the nuclei get close enough. These conditions exist in the core of the Sun but are very hard to create and contain on Earth, which is why fusion is not yet used to generate electricity.
Try this
Q1. State what is meant by a chain reaction in nuclear fission. [2 marks]
- Cue. Neutrons released by one fission are absorbed by other nuclei, making them split and release more neutrons, so the reaction sustains itself.
Q2. State the role of the control rods in a nuclear reactor. [1 mark]
- Cue. They absorb surplus neutrons to keep the chain reaction steady (one neutron per fission goes on to cause the next).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDescribe the process of nuclear fission, and explain how a chain reaction occurs in a nuclear reactor.Show worked answer →
A P6 question worth four marks. Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235) into two smaller nuclei, usually after the nucleus absorbs a neutron (1 mark). The process releases energy and two or three more neutrons (1 mark). These released neutrons can be absorbed by other nuclei, causing them to split and release more neutrons, so the reaction sustains itself: this is a chain reaction (2 marks for the released neutrons triggering further fissions). Markers reward the splitting of a large nucleus after absorbing a neutron, the release of energy and neutrons, and the chain of further fissions. A common error is to describe fusion (joining) instead of fission (splitting).
OCR 20214 marksExplain what nuclear fusion is, and explain why nuclear fusion only happens at extremely high temperatures and pressures.Show worked answer →
A P6 question worth four marks. Nuclear fusion is the joining (fusing) of two light nuclei (such as hydrogen nuclei) to form a heavier nucleus, releasing a large amount of energy (2 marks for the joining of light nuclei releasing energy). It needs extremely high temperatures and pressures because the nuclei are positively charged and repel each other (electrostatic repulsion); only at very high temperature do they move fast enough, and at high pressure close enough, to overcome this repulsion and fuse (2 marks for the repulsion needing high temperature and pressure to overcome). Markers reward fusion as joining light nuclei releasing energy, and the need to overcome the repulsion between positive nuclei. A common error is to confuse fusion with fission.
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