Induced Fission
Physics: AQA A Level, topic 8.1.11
Induced Fission
Fission is caused by neutrons but not every neutron will induce fission.
Neutrons
- Not every neutron produced by fission induces fission.
- Some neutrons escape the fissionable material, while others interact with a nucleus without causing fission.
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.
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.
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.
Coolant
- The coolant transfers heat away from the reactor core to the heat exchanger.
- Coolants need to be fluid and have a relatively high specific heat capacity.
- Water, carbon dioxide, helium or liquid sodium can be used.
Shielding
- The shielding is a safety measure.
- Shielding reduces the penetration of radiation into the surroundings.
- Shields are usually made of thick concrete.
Test yourself on Induced Fission
Does every neutron cause fission?
Which of these statements is true about induced fission?
Which of these materials are used as coolants in nuclear reactors?