This topic summary covers Knowledge Organiser: Nuclear Fission and Fusion within Nuclear Fission & Fusion for GCSE Physics. Revise Nuclear Fission & Fusion in Atomic Structure for GCSE Physics with 13 exam-style questions and 25 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 18 of 18 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
- E = mc² — mass converts to energy
- Moderator slows; control rods absorb
- Fission = split; fusion = join
- Fusion cleaner but far harder to achieve
Key Equations
- E = mc² (mass-energy equivalence — small mass loss releases huge 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
Practice questions for Nuclear Fission & Fusion
What is nuclear fission?
Explain what is meant by a chain reaction in nuclear fission.
Quick recall flashcards
What is nuclear fission?
The splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and neutrons
What is nuclear fusion?
The joining of two small, light nuclei to form a larger nucleus, releasing energy