How do nuclear fission and fusion release energy, and how is fission used in a reactor?
Nuclear decay equations, nuclear fission and the chain reaction, the nuclear reactor, and nuclear fusion.
A focused answer to WJEC GCSE Physics topic 2.9 on nuclear decay and nuclear energy, covering nuclear decay equations, nuclear fission and the chain reaction, how a nuclear reactor is controlled, nuclear fusion, and how fission and fusion differ.
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What this topic is asking
WJEC wants you to write simple nuclear decay equations, describe nuclear fission and the chain reaction, explain how a reactor is controlled, and describe nuclear fusion. This is topic 2.9 Nuclear decay and nuclear energy in Unit 2 of WJEC GCSE Physics (3420).
Nuclear decay equations
Balancing is the key skill in equation questions: write the mass numbers (top) and atomic numbers (bottom) for everything, then make sure both totals match across the arrow. Because the atomic number changes, the decaying atom turns into a different element, which is why radioactive decay can never be stopped or sped up by chemical means: it is a change in the nucleus itself, not in the electrons.
Nuclear fission and the chain reaction
The nuclear reactor and nuclear fusion
Fusion is the energy source of every star, including the Sun, where hydrogen nuclei fuse into helium (see The life cycle of stars). The difficulty on Earth is that nuclei are positively charged and repel one another, so only at star-like temperatures and pressures do they move fast enough to collide and fuse. This is why, despite its huge promise of clean energy, fusion power is still being developed while fission already runs in power stations.
Try this
Q1. State what is released, besides energy, when a uranium nucleus undergoes fission. [1 mark]
- Cue. Two or three neutrons (and two smaller nuclei).
Q2. Name the process that releases energy by joining small nuclei together. [1 mark]
- Cue. Nuclear fusion.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20204 marksDescribe what happens in a nuclear fission chain reaction.Show worked answer →
A topic 2.9 Describe question. A slow-moving neutron is absorbed by a large unstable nucleus such as uranium-235 (1 mark), which becomes unstable and splits into two smaller nuclei, releasing energy and two or three more neutrons (1 mark). These neutrons go on to split further nuclei (1 mark), so the reaction grows: this is the chain reaction (1 mark). Markers reward the neutron absorbed, the splitting with energy and neutrons released, and the chain. A common error is to confuse it with fusion.
WJEC 20223 marksExplain the difference between nuclear fission and nuclear fusion.Show worked answer →
A topic 2.9 Explain question. Fission is the splitting of a large nucleus (such as uranium) into smaller nuclei, releasing energy (1 mark). Fusion is the joining of two small nuclei (such as hydrogen) to form a larger nucleus, releasing energy (1 mark). Fusion needs extremely high temperatures and pressures and is the process that powers stars, while fission is used in nuclear power stations (1 mark). Markers reward splitting for fission, joining for fusion, and a valid context. A common error is to swap the two definitions.
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)