For IGCSE Physics, it is important that you know the difference between nuclear fission and fusion.

Nuclear Fission

- the process of splitting a large nucleus into smaller nucleus

- used as energy source to generate electricity in nuclear power plants

• When a neutron collides with the nucleus of uranium-235, the nucleus becomes uranium-236, which is highly unstable.
• The unstable nucleus splits into 2 daughter nuclei.
• 2 ~ 3 neutrons are also released during the fission process.
• Energy is released as kinetic energy by fast-moving neutrons.
• If the released neutrons collides with other uranium-235 nucleus, they can cause further fission reactions. This is called chain reaction.

The design of nuclear power plants can be found here.

Nuclear Fusion

- the process of fusing two small, light nuclei into one larger nucleus

- form of energy release in the Sun and other stars

• A common example of nuclear fusion that occurs in stars is when two isotopes of hydrogen nuclei fuse to become helium nucleus.
• There is a small amount of mass loss in the fusion reaction.
• This missing mass is converted into energy and released.
• Nuclear fusion requires fusing two positively charged nuclei. Bringing the two nuclei together will result in electrostatic repulsion.
• Nuclear fusion can only happen when enough there is enough energy to overcome the electrostatic repulsion. The process happens only in extremely high temperature and pressure.

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