Everything on the GCSE Physics Fission & Fusion poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
Nuclear fission
Fission is the splitting of a large, unstable nucleus (usually U-235 or Pu-239) into two smaller nuclei.
- The nucleus normally has to absorb a neutron before it will split.
- Products: two smaller nuclei (about equal size), 2 neutrons, and gamma rays.
- Those neutrons can hit other nuclei, causing more fissions - this is a chain reaction.
A fission event
An incoming neutron is absorbed by a U-235 nucleus. The nucleus splits into two smaller nuclei and releases 2 neutrons and gamma rays. Those neutrons go on to trigger further fissions, sustaining the chain reaction.
Nuclear fusion
Fusion is the joining of two light nuclei (usually hydrogen isotopes: deuterium and tritium) to form a heavier nucleus.
- The product has slightly less mass than the two starting nuclei - the lost mass is released as energy (nuclear/kinetic energy).
- Fusion releases more energy per kilogram of fuel than fission.
A fusion event
A deuterium nucleus (²H) and a tritium nucleus (³H) fuse together to form a helium nucleus, releasing a neutron and energy in the form of gamma rays.
Fission vs fusion
| Fission | Fusion | |
|---|---|---|
| What happens? | Splitting a heavy nucleus into two smaller nuclei | Joining two light nuclei to form a heavier nucleus |
| Fuels | Uranium-235, Plutonium-239 (heavy elements) | Hydrogen isotopes: deuterium (²H), tritium (³H) |
| By-products | Two smaller nuclei, 2 neutrons, gamma rays (radiation) | Helium nucleus, a neutron, gamma rays (radiation) |
| Conditions needed | Moderate temperatures (in reactors) | Extremely high temperature and pressure |
| Energy released | Large amount | Even larger amount |
| Waste | Radioactive waste produced | Very little radioactive waste |
| Where it happens | Nuclear reactors in power stations around the world | Stars (e.g. the Sun and other stars), potential future fusion reactors (still in development) |
| Current use | Used in nuclear power stations to generate electricity | Not yet used - still being researched and developed |