AQA A Level Physics

Revision Notes

8.4.5 Induced Fission

Induced Fission

  • Induced nuclear fission occurs:

When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron

  • For example, when a uranium-235 nucleus absorbs a neutron, it becomes a uranium-236 nucleus
  • This uranium-236 nucleus is highly unstable and will decay almost immediately, which is why it is not usually shown in nuclear decay equations
    • This isotope can then decay into smaller nuclei
  • One of the many decay reactions uranium-235 can undergo is shown below:
  • Neutrons involved in induced fission are known as thermal neutrons
  • Thermal neutrons have low energy and speed meaning they can induce fission
    • This is important as neutrons with too much energy will rebound away from the uranium-235 nucleus and fission will not take place

Chain Reactions & Critical Mass

  • The products of fission are two daughter nuclei and at least one neutron
  • The neutrons released during fission go on to cause more fission reactions leading to a chain reaction, where each fission goes on to cause at least one more fission
  • Nuclear reactions are designed to be self-sustaining yet very controlled
  • This can be achieved by using a precise amount of uranium fuel, known as the critical mass
  • The critical mass is defined as:

The minimum mass of fuel required to maintain a steady chain reaction

  • Using exactly the critical mass of fuel will mean that a single fission reaction follows the last
    • Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop
    • Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion
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