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Physics · Atomic structure

Nuclear fission and fusion

Describe nuclear fission and chain reactions, how they are controlled in a reactor, and nuclear fusion.

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is a nucleus?

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    The centre of an atom, made of protons and neutrons

  2. 2

    What is an isotope?

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    An atom with the same protons but different neutrons

  3. 3

    What is energy conserved?

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    Energy cannot be created or destroyed

  4. 4

    What happens to mass in E = mc²?

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    It can be converted to energy

  5. 5

    What is a chain reaction?

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    A reaction that keeps itself going

Learning Objectives

  1. 1Describe nuclear fission and how a chain reaction occurs.
  2. 2Explain how the chain reaction is controlled in a nuclear reactor and uncontrolled in a nuclear weapon.
  3. 3Draw and interpret diagrams of fission and chain reactions.
  4. 4Describe nuclear fusion.

FISSION AND FUSION

Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium. Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

In both, energy is released. In fusion some of the mass is converted into the energy of radiation.

Nuclear Fission

Follow the steps.

  1. 1 A neutron is absorbed

    An unstable nucleus such as uranium-235 absorbs a neutron.
  2. 2 The nucleus splits

    Into two smaller nuclei, roughly equal in size.
  3. 3 Two or three neutrons are released

    Also gamma rays.
  4. 4 Energy is released

    All the fission products have kinetic energy.
  5. 5 Chain reaction

    The neutrons go on to cause further fission.

Controlled or Uncontrolled?

In a nuclear reactor

  • The chain reaction is controlled.
  • Control rods absorb excess neutrons so only one neutron from each fission causes another.
  • Energy is released steadily to heat water.

In a nuclear weapon

  • The chain reaction is uncontrolled.
  • Neutrons cause many more fissions each time.
  • A huge amount of energy is released in an instant: an explosion.

Describing a Chain Reaction

Explain how a chain reaction occurs in a fission reactor.

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  1. 1 Start A neutron is absorbed by a uranium-235 nucleus which splits
  2. 2 Neutrons released 2 or 3 neutrons are released
  3. 3 Continues These neutrons are absorbed by more nuclei, which also split, so the reaction keeps going

AnswerThe neutrons released in each fission cause further fissions, so the reaction sustains itself.

Fusion

Describe what happens in nuclear fusion.

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  1. 1 Two light nuclei Join together to form a heavier nucleus
  2. 2 Mass Some of the mass is converted to energy released as radiation
  3. 3 Conditions Very high temperature and pressure are needed to overcome the repulsion between the positive nuclei

AnswerLight nuclei join to form a heavier nucleus, releasing energy.

Comparing Them

Points to remember.

  • Fission

    A large nucleus splits into two smaller ones; releases neutrons; chain reaction.

  • Fusion

    Two small nuclei join; needs very high temperature; powers the Sun.

  • Both

    Release energy; fusion releases more per kilogram of fuel than fission.

  • Waste

    Fission produces radioactive waste; fusion products are largely not radioactive for long.

Fission or Fusion?

State whether each is fission or fusion: (a) uranium-235 absorbs a neutron and splits (b) hydrogen nuclei join to make helium (c) a chain reaction in a reactor (d) the process in the Sun.

1. Decide if a nucleus splits or joins.

A good answer shows: (a) Fission (b) Fusion (c) Fission (d) Fusion.

Can I...?

  1. 1Define fission.
  2. 2Define fusion.
  3. 3Describe a chain reaction.
  4. 4Describe control in a reactor.
  5. 5Describe uncontrolled fission.
  6. 6Draw and interpret a fission diagram.
  7. 7Describe fusion conditions.
  8. 8Compare fission and fusion.

Summary & Exam Focus

  • Fission: large nucleus splits into two smaller nuclei plus 2 to 3 neutrons and gamma.
  • Chain reaction: neutrons cause further fission.
  • Controlled in a reactor, uncontrolled in a weapon.
  • Fusion: two light nuclei join, releasing energy.

Exam focus

Describe what happens in nuclear fission. (3 marks) (3 marks)

Neutron absorbed, nucleus splits, neutrons and energy released.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Nuclear fission
The splitting of a large, unstable nucleus.
Nuclear fusion
The joining of two light nuclei to form a heavier nucleus.
Chain reaction
A reaction in which the products cause more of the same reaction.
Control rods
Rods that absorb neutrons to control a reactor.
Fission products
The smaller nuclei produced by fission.
Nuclear reactor
A device in which fission is controlled.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question State 2 marks Easier

What is meant by nuclear fission?

Mark scheme — 2 marks available

  • Splitting of a large unstable nucleus — 1 mark
  • Into two smaller nuclei — 1 mark

Model answer

The splitting of a large and unstable nucleus (such as uranium or plutonium) into two smaller nuclei.

2. Exam question Complete the diagram 3 marks Easier

The diagram shows part of a fission chain reaction. (a) Name the particle labelled X. (b) Explain how the diagram shows that a chain reaction can happen.

A neutron X hitting a uranium nucleus that splits and releases more neutrons, which hit other nuclei.

Mark scheme — 3 marks available

  • X is a neutron — 1 mark
  • Each fission releases more than one neutron — 1 mark
  • These cause more nuclei to split — 1 mark

Model answer

(a) A neutron. (b) Each fission releases two or three neutrons, which cause further nuclei to split, so the reaction continues.

3. Exam question Explain 3 marks Easier

Explain how the chain reaction is controlled in a nuclear reactor.

Mark scheme — 3 marks available

  • Control rods absorb neutrons — 1 mark
  • Only one neutron from each fission goes on to cause fission — 1 mark
  • So the rate of energy release is steady — 1 mark

Model answer

Control rods absorb some of the neutrons produced. This means on average only one neutron from each fission causes another fission, so the reaction continues steadily without getting out of control.

4. Exam question Describe 3 marks Easier

Describe what happens in nuclear fusion, and state one condition needed for it to happen.

Mark scheme — 3 marks available

  • Two light nuclei join — 1 mark
  • Energy released — 1 mark
  • Very high temperature or pressure needed — 1 mark

Model answer

Two light nuclei join to form a heavier nucleus, and some of the mass is converted to energy. Very high temperature (and pressure) is needed to overcome the repulsion between the nuclei.

5. Exam question Compare 4 marks Easier

Compare nuclear fission and nuclear fusion.

Mark scheme — 4 marks available

  • Fission: splitting — 1 mark
  • Fusion: joining — 1 mark
  • Both release energy — 1 mark
  • A difference in conditions or products — 1 mark

Model answer

Fission is the splitting of a large unstable nucleus into two smaller nuclei; fusion is the joining of two light nuclei to make a heavier one. Both release energy. Fission releases neutrons that cause a chain reaction; fusion needs very high temperature and pressure.

6. Multiple choice 1 mark Easier

In nuclear fission a large nucleus...

  1. A joins with another
  2. B becomes an atom
  3. C splits into two smaller nuclei Correct
  4. D loses all its protons

Why: That is what fission means.

7. Multiple choice 1 mark Core

Nuclear fusion is...

  1. A the splitting of a nucleus
  2. B the loss of electrons
  3. C a chemical reaction
  4. D the joining of two light nuclei Correct

Why: Light nuclei fuse to make a heavier one.

8. Multiple choice 1 mark Core

Control rods in a reactor...

  1. A absorb neutrons Correct
  2. B release neutrons
  3. C cool the room
  4. D make gamma rays

Why: They control the chain reaction.

9. Multiple choice 1 mark Core

How many neutrons does each fission of uranium-235 release?

  1. A 0
  2. B 2 or 3 Correct
  3. C 1 only
  4. D 100

Why: That is what allows a chain reaction.

10. Multiple choice 1 mark Stretch

Fusion happens in...

  1. A a fission reactor
  2. B a smoke detector
  3. C the Sun Correct
  4. D a battery

Why: Hydrogen nuclei fuse into helium.