Chain Reactions from Fission (DP IB Physics): Revision Note

Katie M

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Katie M

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Chain Reactions

  • One of the many decay reactions uranium-235 can undergo is shown below:

Induced Fission Uranium, downloadable AS & A Level Physics revision notes

Uranium-235 decay chain from nuclear fission

  • 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

  • Only one extra neutron is required to induce a Uranium-235 nucleus to split by fission

  • During the fission, it produces 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, downloadable 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

  • 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

Chain Reaction, downloadable AS & A Level Physics revision notes

Only one thermal neutron is used to create another fission reaction in a controlled chain reaction

  • 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

Critical Mass, downloadable AS & A Level Physics revision notes

Subcritical, critical and supercritical mass

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Katie M

Author: Katie M

Expertise: Physics Content Creator

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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