• 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:

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

   

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

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