(c) Fission and fusion

Chain Reaction:

• The three neutrons produced by the fission may hit other nuclei of uranium 235, causing the process to repeat.
• For a chain reaction to occur, there is a minimum mass of uranium 235 required. This is known as the critical mass.

Moderator:

• Graphite is used as a moderator.
• The purpose of the moderator is to absorb some of the kinetic energy of the neutrons to slow them down.
• This is because slow neutrons are more easily absorbed by uranium 235 nuclei.

Control rods:

• Made of boron or cadmium.
• The purpose of the control rods is to absorb neutrons and completely remove them from the fission process.
• Helps adjust the rate of nuclear fission in the reactor.

Fission – Larger nuclei are split into smaller nuclei.

Fusion – Two smaller nuclei collide and combine to form a larger nucleus.

Fusion:

• Isotopes of hydrogen collide at high speed.
• Mass before > mass after

• For nuclear fusion to occur, very high temperatures are required to overcome the repulsive force between the positively charged nuclei of each isotope.
• High pressures are also needed to increase the chance of fusion between the nuclei.