Nuclear fission — AQA GCSE Physics
Test yourself on Nuclear fission with AQA GCSE practice questions.
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Nuclear fission explained
If more than one neutron from each fission goes on to cause a further fission, the number of fissions doubles or triples with every generation.
Read the full explanation
Because each generation lasts a tiny fraction of a second, a huge number of nuclei split in an extremely short time and all the energy is released at once, which is the explosion of a nuclear weapon. The same runaway behaviour in a power station, where the fuel is not suitable for a bomb-type explosion, shows itself as a meltdown, the core overheating and damaging the reactor, as at Chernobyl; at Fukushima the meltdown followed a loss of cooling after the reactors had already shut down, not a runaway chain reaction. A reactor avoids this by absorbing the surplus neutrons in control rods so that on average only one neutron per fission continues the chain.
Your focus
- Give one consequence of an uncontrolled chain reaction and explain how it arises from the neutron numbers.
- Explain why a controlled and an uncontrolled chain reaction release the same energy over very different timescales.
- Contrast the neutron economy of a nuclear weapon with that of a reactor at steady output.
Nuclear fission exam tips
Marking Points
- one mark for each fission causing more than one further fission, so the number of fissions grows rapidly
- one mark for a named consequence of an uncontrolled chain reaction, such as a nuclear explosion, a nuclear bomb or a meltdown
- one mark for the energy being released in a very short time rather than steadily
- one mark for the contrast with a reactor, where control rods absorb surplus neutrons to hold the rate steady
Examiner Tips
- 💡Naming an accident such as Chernobyl is accepted as a consequence, so it is a safe one-mark answer.
- 💡Use the words 'uncontrolled' and 'controlled' explicitly when contrasting a weapon with a reactor.
- 💡Anchor the explanation on the number of neutrons that go on to cause fission, which is the physics being tested.
Common Mistakes
- describing the explosion as a chemical one, like burning or an ordinary bomb
- writing about fusion when the question is about an uncontrolled fission chain reaction
- answering 'it is very dangerous' instead of naming a consequence
- saying the explosion happens because the uranium gets hot rather than because the fission rate multiplies
- claiming a UK power station reactor could detonate exactly like a nuclear weapon