Nuclear FissionOxford AQA IGCSE Physics: Revision notes

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing energy.

Fission does not happen spontaneously in a controlled way in a reactor — it has to be triggered by a neutron hitting the right kind of nucleus.

Only certain heavy elements are 'fissionable' (can undergo fission). The two used in practice are:

• Uranium-235 — the fuel used in most nuclear reactors
• Plutonium-239 — also fissionable, used in some reactors and in weapons

Both are large, unstable nuclei that can absorb a neutron and split.

The step-by-step process of induced fission:

1. A slow-moving neutron is absorbed by a uranium-235 (or plutonium-239) nucleus
2. The nucleus becomes unstable
3. The unstable nucleus splits into two smaller daughter nuclei
4. Two or three neutrons are released
5. A large amount of energy is released

The energy released per fission event is much greater than the energy released in any chemical reaction, because it comes from the nucleus itself rather than from rearranging electrons.

Each fission event releases 2-3 neutrons. If these neutrons go on to cause further fission in nearby nuclei, this is called a chain reaction.

• In an uncontrolled chain reaction, the number of fissions increases rapidly (as in a nuclear weapon)
• In a nuclear reactor, control rods absorb some of the fission neutrons so that, on average, only one neutron from each fission goes on to cause another fission
• This keeps the reaction steady and controllable rather than growing out of control

The products of fission (the smaller daughter nuclei) are radioactive. This nuclear waste:

• Remains radioactive for a long time — some waste stays dangerous for thousands of years
• Must be stored and disposed of very carefully to protect people and the environment
• Is a major factor that must be weighed up when deciding whether to use nuclear power as an energy source