Fission & FusionEdexcel IGCSE Physics: Revision notes
Section 1
Nuclear reactions as an energy source
Three types of nuclear reaction release energy: fission, fusion, and radioactive decay. Fission splits large nuclei apart; fusion joins small nuclei together; both release large amounts of energy compared with chemical reactions, because the energy comes from changes within the nucleus itself.
Section 2
Nuclear fission
A nucleus of uranium-235 can be split when it is struck by a slow-moving neutron. This process:
- Produces two radioactive daughter nuclei (roughly half the mass of the original).
- Releases a small number of neutrons.
- Releases a large amount of energy, mostly as the kinetic energy of the fission products.
A chain reaction can be set up if the neutrons released by one fission event go on to strike further U-235 nuclei, causing further fissions, releasing yet more neutrons, and so on.
One U-235 nucleus absorbs a neutron, splits into barium and krypton nuclei, and releases 2–3 neutrons plus energy — those neutrons can go on to split further U-235 nuclei.
Section 3
Controlling a nuclear reactor
In a nuclear power station, the chain reaction must be controlled so it releases energy steadily rather than all at once:
- Control rods (e.g. boron) absorb some of the neutrons produced, limiting the rate of fission and keeping the reaction steady rather than accelerating out of control.
- The moderator (e.g. graphite or water) slows down the fast neutrons released by fission, because slow neutrons are far more likely to cause further fission in U-235.
- Shielding (thick concrete and steel) around the reactor absorbs harmful radiation, protecting workers and the surrounding environment.
When explaining reactor control, name all three components (control rods, moderator, shielding) and state the specific job of each — examiners credit each function separately.
Section 4
Nuclear fusion
Nuclear fusion is the joining of two small nuclei to create a larger nucleus, with a loss of mass from the smaller nuclei accompanied by a release of energy. Fusion is the energy source that powers stars, including the Sun.
Fusion does not happen at low temperatures and pressures because nuclei are positively charged and so repel each other electrostatically. Extremely high temperatures and pressures (as found in stellar cores) are needed to give nuclei enough kinetic energy to overcome this electrostatic repulsion and get close enough for the strong nuclear force to bind them together.
A common error is stating fusion "just needs high pressure" — the key reason is that both temperature and pressure must be high enough for nuclei to overcome electrostatic repulsion between their positive charges.
Must Know
- fission splits large nuclei (e.g. U-235); fusion joins small nuclei together — both release energy
- U-235 fission: struck by a neutron → splits into two radioactive daughter nuclei + a small number of neutrons + energy
- a chain reaction occurs when neutrons from one fission trigger further fissions
- control rods absorb neutrons to control the reaction rate; the moderator slows neutrons so they cause further fission; shielding absorbs harmful radiation
- fusion creates a larger nucleus, releasing energy from a loss of mass
- fusion is the energy source of stars, but needs extremely high temperature/pressure to overcome electrostatic repulsion between nuclei
That's the notes covered.
Carry on to the next subtopic.