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

Chain Reactions

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

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

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

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

Subcritical, critical and supercritical mass