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7.3.2Radioactivity and Particles

Nuclear Fission — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 14 of 17

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7What happens in fission

Fission is the splitting of a heavy nucleus into two lighter nuclei, called fission fragments. The most important fuels are uranium-235 and plutonium-239. A nucleus of uranium-235 is struck by a slow-moving neutron, which it absorbs. The nucleus becomes uranium-236, which is very unstable, becomes distorted and splits in two. A typical reaction is:

23592U + 10n → 14156Ba + 9236Kr + 3 10n + energy

Check that it balances

  • top numbers 235 + 1 = 236 and 141 + 92 + 3 = 236;
    • bottom numbers 92 + 0 = 92 and 56 + 36 + 0 = 92.
  • Many other pairs of fragments are possible, and either two or three neutrons are released each time.

Part 2 of 7Where the energy goes

The two fragments fly apart at high speed, carrying most of the released energy as kinetic energy.

Thermal energy
They quickly collide with surrounding atoms, so this kinetic energy becomes thermal energy.
Some energy is also carried off by the neutrons and by gamma radiation.
The fission fragments are themselves radioactive, usually with an excess of neutrons, so they decay by beta and gamma emission.
This is the main origin of the radioactive waste from reactors.

The energy from a single fission is about 3 × 10−11 J. That is small, but there are about 2.6 × 1021 nuclei in a gram of uranium-235, so a kilogram contains a vast number of potential reactions.

Part 3 of 7The chain reaction

Each fission releases two or three neutrons. If at least one of these goes on to cause another fission, the reaction sustains itself: this is a chain reaction.

The chain reaction
One neutron hits a U-235 nucleus, which splits and releases 3 neutrons.
Each of those can hit another U-235 nucleus, giving 9 neutrons.
The number of fissions grows rapidly, and the energy release soars.

If, on average, each fission leads to more than one further fission, the reaction grows exponentially, as in a nuclear weapon. If exactly one neutron on average causes another fission, the reaction proceeds at a steady rate, as in a nuclear reactor. If fewer than one does, the reaction dies out.

Part 4 of 7Conditions for a chain reaction

Conditions for a chain reaction
Enough fuel
Enough fuel in a small piece of uranium most neutrons escape through the surface without hitting another nucleus.
  • A critical mass is the minimum mass of fissile material needed to sustain a chain reaction.
Slow neutrons
U-235 absorbs slow (thermal) neutrons much more readily than fast ones.
  • The neutrons released in fission are fast, so reactors use a moderator to slow them down.
Neutrons must not be absorbed elsewhere
Neutrons must not be absorbed elsewhere impurities or the container can soak up neutrons, so the fuel is carefully designed.

Natural uranium is more than 99% uranium-238, which does not fission easily with slow neutrons. For use in many reactors the proportion of U-235 is raised in a process called enrichment, to a few percent.

Part 5 of 7Controlled and uncontrolled fission

In a nuclear power station, the reaction is controlled by absorbing surplus neutrons so that the average number of new fissions per fission is exactly one.

Nuclear weapon
In a nuclear weapon, two sub-critical pieces are rapidly brought together to form a supercritical mass,
  • so the reaction runs away within a tiny fraction of a second.
The next lesson describes how the reactor achieves control.

Fission releases no carbon dioxide, but the fragments and unused fuel are radioactive, and wastes must be stored for a very long time.

Part 6 of 7Slowing and absorbing neutrons

Neutrons are central to fission, so their speed and number matter.

Slowing and absorbing neutrons
A fast neutron leaving a fission has high kinetic energy and is likely to pass by a U-235 nucleus without being absorbed,
  • while a slow neutron stays near the nucleus longer and is captured.
This is why the neutrons must be slowed by collisions with light nuclei before they can sustain the chain reaction.

Neutrons can also be lostsome escape from the surface, and some are captured by uranium-238, which absorbs them without fission.

Slowing and absorbing neutrons
A chain reaction continues only if the losses are small enough that at least one neutron per fission still causes another fission.
This balance is why the geometry and the purity of the fuel are so important in the design of any reactor.

Part 7 of 7Worked examples

  1. Complete the fission equation: 23592U + 10n → 14456Ba + xyKr + 3 10n. Find x and y.

  2. In a chain reaction, each fission releases 3 neutrons and every neutron causes a new fission. Starting with 1 fission, how many fissions occur in the 5th generation?

  3. A fission releases 3.2 × 10−11 J. How many fissions per second are needed to produce 1.6 × 109 W of thermal power?

Key terms

TermMeaning
Fission fragmentsThe two smaller nuclei produced when a heavy nucleus splits.
Chain reactionA self-sustaining series of fissions caused by neutrons released in earlier fissions.
Critical massMinimum mass of fissile material needed to sustain a chain reaction.
EnrichmentIncreasing the proportion of uranium-235 in nuclear fuel.
Thermal neutronSlow-moving neutron, which is readily absorbed by uranium-235.

Avoid these mistakes

1
Saying the nucleus splits spontaneously in a reactor. It is induced when a neutron is absorbed.
2
Writing a fission equation that does not balance. Check both top and bottom numbers.
3
Forgetting that fission releases neutrons, which is what makes the chain reaction possible.
4
Thinking a chain reaction always means an explosion. In a reactor it is controlled to a steady rate.
5
Mixing up U-235 (fissile) with U-238, which makes up most natural uranium.

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