Induced Fission (AQA A Level Physics)

• Induced nuclear fission occurs when:

A stable nucleus splits into small nuclei due to the absorption of a slow-moving neutron

• For example, when a nucleus of uranium-235 absorbs a neutron, it becomes uranium-236
• A uranium-236 nucleus is highly unstable and will decay into two smaller nuclei almost immediately 
  
  • This is why it is not usually shown in nuclear decay equations

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

When a uranium-235 nucleus absorbs a neutron it may decay into two smaller daughter nuclei

Thermal Neutrons

• Neutrons involved in induced fission are known as thermal neutrons, meaning:
  
  • They have low kinetic energy
  • They are slow-moving

• Only slow-moving neutrons can be absorbed by uranium-235 nuclei and induce fission
• If a fast-moving neutron is incident on a uranium-235 nucleus it will rebound from it

• A thermal neutron is defined as:

A neutron which is in thermal equilibrium with its surroundings

• In a nuclear reactor, neutrons are slowed until they are in thermal equilibrium with the moderator
  • This corresponds to a core reactor temperature of about 300 K

• Thermal neutrons have kinetic energies associated with
  • Where  is the Boltzmann constant and is the thermodynamic temperature

• Typically, the average kinetic energy of a thermal neutron is:

J

eV

• Typically, a thermal neutron has a speed of:

m s⁻¹

Chain Reactions

• The products of fission are two daughter nuclei and two or three neutrons
• 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

Critical Mass

• 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