AQA A Level Physics复习笔记8.4.5 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:

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