# Nuclear Fission and Fusion — GCSE Physics
Nuclear reactions release millions of times more energy than chemical reactions. Fission powers our nuclear reactors and nuclear weapons, while fusion powers the Sun and stars. Understanding both processes is an important part of GCSE Physics.
1. Nuclear Fission
Fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and additional neutrons.
How It Works
- A slow-moving (thermal) neutron is absorbed by a large nucleus (e.g., uranium-235 or plutonium-239)
- The nucleus becomes unstable
- It splits into two smaller (daughter) nuclei of roughly equal size
- Two or three neutrons are released
- Energy is released (as kinetic energy of the products and gamma radiation)
Nuclear Equation Example
(Note: The exact daughter products vary.)
2. Chain Reactions
The neutrons released by fission can go on to cause more fission events, which release more neutrons, which cause more fission... This is a chain reaction.
- Uncontrolled chain reaction: Each fission causes more than one subsequent fission → exponential growth → nuclear explosion (atomic bomb)
- Controlled chain reaction: Exactly one neutron from each fission causes the next fission → steady energy release → nuclear reactor
3. Nuclear Reactors
Components
| Component | Function |
|---|---|
| Fuel rods | Contain fissile material (uranium-235 or plutonium-239) |
| Moderator | Slows down neutrons (e.g., graphite or water) — slow neutrons are more likely to cause fission |
| Control rods | Absorb excess neutrons (e.g., boron or cadmium) — inserted further to slow the reaction, withdrawn to speed it up |
| Coolant | Carries thermal energy away from the reactor (e.g., water, CO₂) → used to make steam → turns turbines → generator → electricity |
| Shielding | Thick concrete and steel absorb radiation to protect workers |
How Power Output Is Controlled
- Lower control rods → absorb more neutrons → fewer fissions → less power
- Raise control rods → absorb fewer neutrons → more fissions → more power
- For emergency shutdown (SCRAM), all control rods are fully inserted
4. Nuclear Fusion
Fusion is the joining of two small, light nuclei to form a larger nucleus, releasing energy.
This is the process that powers the Sun and other stars.
Example: Hydrogen Fusion
(Deuterium + Tritium → Helium + neutron + energy)
Conditions for Fusion
Fusion requires extremely high temperatures (millions of °C) and high pressures because:
- Nuclei are both positively charged — they repel each other (electrostatic repulsion)
- Only at very high temperatures do the nuclei have enough kinetic energy to overcome this repulsion and get close enough for the strong nuclear force to bind them
These extreme conditions make fusion very difficult to achieve and sustain on Earth.
5. Fission vs Fusion
| Property | Fission | Fusion |
|---|---|---|
| Process | Splitting large nuclei | Joining small nuclei |
| Fuels | Uranium-235, Plutonium-239 | Hydrogen isotopes (deuterium, tritium) |
| Conditions | Slow neutron needed | Extremely high temperature and pressure |
| Energy released | Large | Even larger (per unit mass) |
| Waste | Radioactive waste products | Very little radioactive waste |
| Current technology | Used in nuclear power stations | Not yet viable for power (research ongoing — ITER, JET) |
| In nature | Not common | Powers the Sun and stars |
6. Where Does the Energy Come From?
In both fission and fusion, the total mass of the products is slightly less than the total mass of the reactants. This "missing" mass has been converted to energy according to Einstein's equation:
Where:
- = energy released (J)
- = mass difference (kg)
- = speed of light ( m/s)
Even a tiny mass difference produces enormous energy because is so large.
Worked Example: Example 1
Question: In a nuclear reactor, explain the role of the moderator.
Answer: The moderator slows down the fast-moving neutrons produced by fission. Slow (thermal) neutrons are more likely to be absorbed by uranium-235 nuclei and cause further fission. Common moderators include graphite and water.
Worked Example: Example 2
Question: Explain why fusion is difficult to achieve on Earth.
Answer: Fusion requires nuclei to get very close together, but they are both positively charged and repel each other (electrostatic repulsion). To overcome this, the nuclei need extremely high kinetic energy, requiring temperatures of millions of degrees and very high pressures. Containing such hot plasma is a major engineering challenge.
8. Practice Questions
- Define nuclear fission. (2 marks)
- Explain how control rods are used to control the rate of reaction in a nuclear reactor. (3 marks)
- Define nuclear fusion and state where it naturally occurs. (2 marks)
- Compare the waste products of fission and fusion reactions. (2 marks)
- Explain, in terms of mass and energy, why nuclear reactions release so much energy. (3 marks)
Answers
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Summary
- Fission: large nucleus splits → 2 smaller nuclei + neutrons + energy
- Chain reaction: neutrons cause further fissions; controlled in reactors
- Reactor components: fuel rods, moderator, control rods, coolant, shielding
- Fusion: small nuclei join → larger nucleus + energy
- Requires extreme temperature and pressure (millions of °C)
- Both release energy via (mass-energy equivalence)
- Fusion produces less waste but isn't yet achievable for power generation