What are nuclear fission and fusion, and how do they release energy?
Nuclear fission and the chain reaction in a reactor, nuclear fusion in stars, and the difference between them.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on nuclear energy, covering nuclear fission and the chain reaction in a reactor, nuclear fusion in stars, and the difference between fission and fusion.
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What this dot point is asking
WJEC Double Award Unit 6 wants you to describe nuclear fission and the chain reaction, nuclear fusion in stars, and the difference between them.
Nuclear fission
The energy released is used in nuclear power stations to heat water, make steam and drive a turbine and generator. Fission is the process used in current nuclear reactors.
The chain reaction
If a chain reaction is not controlled, it releases energy extremely quickly (as in a nuclear bomb), which is why reactors are carefully controlled.
Nuclear fusion
Fusion needs extremely high temperatures and pressures (found in the cores of stars) to force the nuclei close enough to join, which is why it is very hard to use on Earth as a power source.
Advantages and disadvantages of nuclear power
Nuclear power (from fission) has clear advantages and disadvantages, often weighed up in exam questions. Advantages: it produces a large amount of energy from a small amount of fuel, and it releases no carbon dioxide while running, so it does not add to climate change. Disadvantages: it produces dangerous radioactive waste that must be stored safely for a very long time, there is a risk of a serious accident, and the power stations are expensive to build and decommission. Being able to give a balanced view of nuclear power is a common evaluation question.
Why fusion would be a good energy source
Although it is hard to achieve, fusion would be an excellent future energy source if it could be controlled on Earth. Its fuel (forms of hydrogen) is plentiful, it releases even more energy than fission, and it produces little radioactive waste. The challenge is reaching and containing the extremely high temperatures needed for the nuclei to join. Scientists are working on fusion reactors, and recognising fusion's potential advantages over fission - cleaner and with abundant fuel - is a common forward-looking exam point.
Comparing fission and fusion
Try this
Q1. What is nuclear fusion? [1 mark]
- Cue. The joining of two small nuclei to make a larger one.
Q2. What process powers the Sun? [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 style4 marksDescribe what happens in nuclear fission and how a chain reaction occurs.Show worked answer →
A Unit 6 describe question worth 4 marks. Reward: in nuclear fission, a large unstable nucleus (such as uranium) is hit by a neutron and splits into two smaller nuclei (1), releasing energy and more neutrons (1); these released neutrons can hit other nuclei, causing them to split too (1); this is a chain reaction, which is controlled in a reactor (1). Markers credit the splitting after absorbing a neutron, the release of energy and neutrons, and the chain reaction. A common error is to confuse fission (splitting) with fusion (joining).
WJEC style3 marksExplain the difference between nuclear fission and nuclear fusion.Show worked answer →
A Unit 6 explain question. Reward: fission is the splitting of a large nucleus into smaller ones (1); fusion is the joining of two small nuclei (such as hydrogen) to make a larger one (1); both release energy, and fusion is the process that powers stars (the Sun) (1). Markers credit splitting large for fission, joining small for fusion, and fusion powering stars. A common error is to mix up which process splits and which joins.
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