Knowledge Organiser: Nuclear Fission and Fusion
Key Terms
- Fission: Splitting heavy nucleus
- Fusion: Joining light nuclei
- Chain reaction: Each fission triggers more
- Moderator: Slows neutrons (graphite/water)
- Control rods: Absorb neutrons (boron)
Fission Facts
- Fuel: uranium-235 or plutonium-239
- Releases 2–3 neutrons per fission
- Controlled = power station
- Uncontrolled = nuclear bomb
- Produces radioactive waste
Fusion Facts
- Fuel: deuterium + tritium
- Requires 100+ million °C
- Powers the Sun and stars
- Products: helium (non-radioactive)
- Not yet commercially available
Exam Tips
- Some mass is converted to energy in fission and fusion
- Moderator slows; control rods absorb
- Fission = split; fusion = join
- Fusion cleaner but far harder to achieve
Key Equations
- No equation needed: a small loss of mass releases a huge amount of energy
- Fission: large nucleus + neutron → two smaller nuclei + 2-3 neutrons + energy
- Fusion: two small nuclei → larger nucleus + energy (requires extremely high temperature)
- Chain reaction: each fission releases neutrons that trigger further fissions
Common Mistakes
- Confusing fission and fusion: Fission = splitting a large nucleus (e.g. uranium-235); fusion = joining small nuclei (e.g. hydrogen isotopes) — they are opposite processes
- Confusing moderator and control rods: The moderator (graphite or water) slows down neutrons so they can trigger further fissions; control rods (boron) absorb neutrons to control the reaction rate
- Saying fusion produces no radioactive waste: Fusion produces much less long-lived radioactive waste than fission, but the reactor materials do become mildly radioactive — it is not completely clean
- Thinking chain reactions are uncontrolled in power stations: In a nuclear reactor, the chain reaction is controlled (each fission triggers exactly one more); in a bomb it is uncontrolled (each triggers multiple)
- Forgetting why fusion is so difficult: Fusion requires extremely high temperatures (~10⁷ K) to give nuclei enough energy to overcome electrostatic repulsion and fuse — this is why commercial fusion power remains a challenge
Memory Aid
- Fission vs Fusion: "FiSSion = Splitting (two S's like a crack). FuSion = fuSed together." Or think: "Fission is like dividing cells (divides); Fusion is like welding (joins)."
- Nuclear reactor components: Use the mnemonic FMCC: Fuel rods, Moderator, Control rods, Coolant. Each has a specific job: fuel provides the fissions, moderator slows neutrons, control rods absorb them, coolant removes heat.
- Why fusion needs extreme temperature: Positive charges repel. To push two positive nuclei together, you need enormous kinetic energy (temperature). The Sun's gravity helps; on Earth we need magnetic confinement. Think: "Fusion needs to force FURY — 100 million degrees FURY."