on this cheat sheet
Everything on the GCSE Physics Nuclear 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, which splits into two smaller nuclei, two neutrons and gamma rays.
Chain reaction
Neutrons released from one fission can cause further fissions in nearby nuclei, producing a self-sustaining chain (controlled in reactors, uncontrolled in nuclear weapons).
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.
- Fusion releases more energy per kilogram of fuel than fission.
A fusion event
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 | Deuterium (), tritium () |
| By-products | Two smaller nuclei, 2 neutrons, gamma rays | Helium nucleus, neutron, gamma rays |
| Conditions needed | Moderate temperatures (in reactors) | Extremely high temperature and pressure |
| Energy released | Large amount | Even larger amount per kg |
| Waste | Radioactive waste produced | Very little radioactive waste |
| Where it happens | Nuclear reactors in power stations | Stars (including the Sun); fusion reactors still in development |
| Current use | Generates electricity in nuclear power stations | Not yet used commercially – still being researched |