Nuclear Fission
Build confidence in Nuclear Fission for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: nuclear fission splits a large unstable nucleus into smaller ones, releasing energy and neutrons that can start a chain reaction.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium. This is a Triple-only topic.
Spontaneous fission is rare. Usually the nucleus must first absorb a neutron. It then splits into two smaller nuclei of roughly equal size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction.
The emitted neutrons can be absorbed by other nuclei, causing further fission. This is a chain reaction. In a nuclear reactor the chain reaction is controlled so that energy is released at a steady rate, using control rods that absorb surplus neutrons. An uncontrolled chain reaction produces a nuclear explosion.
Revision notes
The fission process
A large unstable nucleus, usually uranium or plutonium, absorbs a neutron and becomes more unstable.
It splits into two smaller nuclei of roughly equal size, and emits two or three neutrons plus gamma rays. Energy is released. Spontaneous fission without absorbing a neutron is rare.
Chain reactions
The neutrons emitted can be absorbed by other nuclei, causing them to undergo fission too.
Each of those releases more neutrons, so the reaction spreads. This is a chain reaction, and it is what allows a sustained release of energy.
Control
In a nuclear reactor the chain reaction is controlled so energy is released at a steady rate.
Control rods absorb surplus neutrons, so exactly one neutron from each fission goes on to cause another. An uncontrolled chain reaction releases energy explosively.
Key points
- Fission is the splitting of a large unstable nucleus.
- The nucleus usually absorbs a neutron first.
- It splits into two roughly equal nuclei.
- Two or three neutrons and gamma rays are emitted.
- Emitted neutrons can cause further fission.
- Control rods absorb surplus neutrons.
Worked examples
Example 1
Describe what happens during nuclear fission. [3 marks]
Model answer
A large unstable nucleus absorbs a neutron (1 mark). It splits into two smaller nuclei of roughly equal size (1 mark), emitting two or three neutrons and gamma rays, and releasing energy (1 mark).
Example 2
Explain what is meant by a chain reaction. [2 marks]
Model answer
The neutrons emitted by one fission are absorbed by other nuclei (1 mark), causing them to undergo fission and release further neutrons, so the reaction continues and spreads (1 mark).
Example 3
Explain how control rods are used in a nuclear reactor. [2 marks]
Model answer
Control rods absorb surplus neutrons (1 mark), so that the chain reaction proceeds at a steady rate rather than accelerating out of control (1 mark).
Common mistakes
Saying fission joins nuclei together.
Fission splits a nucleus; fusion joins nuclei.
Forgetting the neutron absorption step.
Spontaneous fission is rare — usually a neutron must be absorbed first.
Saying control rods slow the neutrons.
They absorb surplus neutrons. Moderators slow them down.
Saying the two nuclei produced are identical.
They are of roughly equal size, not identical.
Exam tips
- Describe fission as a sequence: absorb, split, emit, release.
- Say two or three neutrons are emitted.
- Explain control rods as absorbing surplus neutrons.
- Remember this is a Triple-only topic.
Key terms
- Nuclear fission
- The splitting of a large unstable nucleus.
- Chain reaction
- Fission neutrons causing further fissions.
- Control rod
- A rod absorbing surplus neutrons in a reactor.
- Spontaneous fission
- Fission occurring without absorbing a neutron.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.