Chain Reactions (Oxford AQA IGCSE Physics)

Revision Note

Chain Reactions

  • For fission to occur, a nucleus must first absorb one neutron

  • The nucleus splits into two smaller nuclei (called daughter nuclei) of roughly equal size as well as two or three neutrons which move away at high speed

  • Each of these new neutrons can start another fission reaction, which again creates further excess neutrons

  • This process is called a chain reaction

Chain reaction analogy

Chain reaction analogy, for IGCSE & GCSE Physics revision notes
The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more

Controlled chain reactions

  • In a nuclear reactor, a chain reaction is required to keep the reactor running

  • The kinetic energy of the fission products is transferred to water, causing it to boil

  • The steam from this is used to generate electricity

  • For the reactor to work safely in a stable state, the number of free neutrons in the reactor needs to be kept constant

    • This means some neutrons must be removed from the reactor

  • To do this, nuclear reactors contain control rods

Structure of a nuclear reactor

The components of a nuclear reactor,  showing the control rods
The overall purpose of the reactor is to transfer the energy released from nuclear reactions
  • Control rods are made of a material, such as boron, which can absorb neutrons without becoming dangerously unstable

  • The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods

    • Lowering the rods further decreases the rate of fission, as more neutrons are absorbed

    • Raising the rods increases the rate of fission, as fewer neutrons are absorbed

  • This is adjusted automatically so that, on average, only one neutron produced by each fission event goes on to cause another fission event

  • In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way to absorb all of the neutrons so no further reactions can take place

Uncontrolled chain reactions

  • Because each new fission reaction transfers energy, uncontrolled chain reactions can be dangerous

  • The number of neutrons available increases quickly, so the number of reactions does too

  • A nuclear weapon uses an uncontrolled chain reaction to release a huge amount of energy in a short period of time as an explosion

Worked Example

The diagram shows the nuclear fission process for an atom of uranium-235.

A neutron is shown to be travelling toward a uranium-235 nucleus. After the fission reaction 2 neutrons and 2 daughter nuclei are produced

Complete the diagram to show how the fission process starts a chain reaction.

Answer:

Each neutron moves towards another uranium-235 nucleus. Each nucleus produces 2 more neutrons and so on.

Step 1: Draw the neutrons to show that they hit other U-235 nuclei

  • It is the neutrons hitting the uranium-235 nuclei which causes the fission reactions

  • The daughter nuclei do not need to be shown, only the neutrons and uranium-235 nuclei

Step 2: Draw the splitting of the U-235 nuclei to show they produce two or more neutrons

  • The number of neutrons increases with each fission reaction

  • Each reaction requires one neutron but releases two

  • More reactions happen as the number of neutrons increases

Examiner Tip

You need to be able to draw and interpret diagrams of nuclear fission and chain reactions. Generally, things move to the right as time goes on in these diagrams, but it is important to read all the information carefully on questions like this. If you have to draw a diagram in an exam remember that the clarity of the information is important, not how pretty it looks!

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