Induced Fission

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Induced Fission

Fission is caused by neutrons but not every neutron will induce fission.

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  • Not every neutron produced by fission induces fission.
  • Some neutrons escape the fissionable material, while others interact with a nucleus without causing fission.
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Maximising fission

  • We can enhance the number of fissions produced by neutrons by having a large amount of fissionable material.
  • There is a minimum amount of material needed for the chain reaction to keep going until all the material has undergone fission.
    • This is called the critical mass.

Components of a Nuclear Reactor

Induced nuclear fission is the primary source of energy in a nuclear reactor.

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Moderators (e.g. graphite)

  • Moderators reduce the kinetic energy (KE) of neutrons so that they can be absorbed by uranium nuclei.
  • Moderators need to reduce the KE of neutrons, by elastic collisions between the neutrons and the atoms of the moderator.
    • Moderators are usually water or graphite.
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Control rods

  • Control rods absorb neutrons to control the rate of fission reactions.
  • They must be made from a material that is good at absorbing neutrons, such as boron and cadmium.
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  • The coolant transfers heat away from the reactor core to the heat exchanger.
  • Coolants need to be fluid and have a relatively low specific heat capacity.
  • Water, carbon dioxide, helium or liquid sodium can be used.
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  • The shielding is a safety measure.
  • Shielding reduces the penetration of radiation into the surroundings.
  • Shields are usually made of thick concrete.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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