Nuclear Fusion and Nuclear Fission
Key idea: Compare nuclear fission and fusion, state precise definitions, and relate both processes to energy released from nuclear fuel using syllabus-safe O Level language.
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The core idea
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Learning objectives
- Describe atomic composition
- Use proton number, nucleon number and isotope
- Use and interpret nuclide notation
- Explain random and spontaneous nuclear decay
- Describe alpha, beta-minus and gamma radiation
- Compare ionising effect and penetrating power
- Use nuclide equations for radioactive decay
- Explain background radiation
- Use half-life in tables and decay curves
- State radioactivity applications and hazards
- Evaluate uses and hazards using half-life and radiation properties
- Relate fission and fusion to nuclear-energy release
1. Definition
A. Nuclear Fission
Nuclear fission is the splitting of a heavy nucleus into two (or more) smaller nuclei, releasing energy.
B. Nuclear Fusion
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy.
Learn the two meanings and relate both processes to energy released from nuclear fuel. You do not need energy–mass calculations or detailed nuclear power-station technology here.
2. Key Ideas
- Fission: heavy nucleus splits → two or more smaller nuclei + energy.
- Fusion: two light nuclei join → a heavier nucleus + energy.
- Both are changes to nuclei, not chemical reactions or changes of state.
- Both can release energy from nuclear fuel; fission is used in present nuclear power stations, while fusion powers the Sun and other stars.
| Feature | Fission | Fusion |
|---|---|---|
| Starting nuclei | one heavy nucleus | two light nuclei |
| Nuclear change | splits | joins |
| Product | smaller nuclei | a heavier nucleus |
| Energy | released | released |
3. Detailed Explanations
A. What Happens in Fission?
In fission, a heavy nucleus splits into two or more smaller nuclei and releases energy. In common fission examples, the heavy nucleus first absorbs a neutron and further neutrons may be released.
Those released neutrons can sometimes cause further fissions, producing a chain reaction. This is useful context, but detailed reactor control is beyond the 6091 requirement.
B. What Happens in Fusion?
In fusion, two light nuclei combine to form a heavier nucleus and release energy. Because both nuclei are positively charged and repel, extremely high temperature is needed to give them enough kinetic energy to get very close.
Fusion also releases energy and is the process that powers the Sun and many stars.
C. What your energy explanation needs
If the question asks you to relate the process to energy, state that the nuclear change releases energy from nuclear fuel. Do not add an energy–mass calculation unless the question provides extra information and asks you to use it.
4. Common Mistakes
- Mixing up the definitions: fission splits, fusion joins.
- Describing fusion as “melting” (fusion is a nuclear change, not a change of state).
- Forgetting that fission involves heavy nuclei while fusion involves light nuclei.
- Saying that fusion joins atoms or substances without specifying nuclei.
- Giving detailed reactor components when the question only asks for the meaning of fission or fusion.
5. Exam Tips
- Use the complete key phrases: “splitting of a heavy nucleus into smaller nuclei” and “joining of two light nuclei to form a heavier nucleus”.
- Include “releasing energy” when the question connects the process with nuclear fuel.
- Keep the answer at the command word: a “state” question needs the definition, not a reactor essay.
6. Worked Examples
Modelled example 1
Identify Fission vs Fusion
Problem
Study the worked solution
Identify the starting nucleus
Method
Recognise one heavy nucleus as the starting system.Reason
Fission acts on a heavy nucleus, whereas fusion begins with two light nuclei.Working
One heavy nucleus initially.Classify the change
Method
Name nuclear fission.Reason
The heavy nucleus splits into smaller nuclei and releases energy.Working
Heavy nucleus → smaller nuclei + energy = fission.
Guided practice 2
Conditions for Fusion
Problem
Connect charge, repulsion and kinetic energy
Hints
Hint 1: repulsion
Hint 2: temperature effect
View solution step by step
Identify the barrier
Method
State that positively charged nuclei repel each other.Reason
Like electric charges exert repulsive forces.Working
Positive nucleus ↔ positive nucleus: electrostatic repulsion.Use high temperature
Method
State that very high temperature gives nuclei very high kinetic energy.Reason
Some nuclei can then approach closely enough for fusion to occur.Working
Temperature ↑ → kinetic energy ↑ → closer approach.
Common misconception 3
Energy Release
Learner response
Separate initiation conditions from net process outcome
View solution step by step
State the shared outcome
Method
State that both fission and fusion can release energy from nuclear fuel.Reason
Both are nuclear changes with energy released by the process.Working
Fission → energy released; fusion → energy released.Interpret the fusion condition
Method
State that high temperature is required to initiate or sustain close collisions.Reason
That requirement does not mean a successful fusion event has no energy output.Working
High-temperature condition ≠ definition of net energy consumption.
Examiner practice 4
Fission in power stations
Examination question
Use complete definition language
View solution step by step
Name the process
1 markMethod
State nuclear fission.Reason
Uranium-235 is a heavy nucleus used as fission fuel.Working
Process: fission.Define the change
2 marksMethod
State that a heavy nucleus splits into two or more smaller nuclei and releases energy.Reason
This distinguishes fission from fusion and links it to power generation.Working
Heavy nucleus → smaller nuclei + energy.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark process, nuclear change and energy.
Challenge 5
Avoiding the “melting” trap
Cross-topic transfer
Compare what changes at particle and nuclear levels
Hints
Hint 1: fusion object
Hint 2: melting object
View solution step by step
Define nuclear fusion
Method
State that two light nuclei join to form a heavier nucleus.Reason
The change occurs within nuclei and produces a new nuclear product.Working
Light nucleus + light nucleus → heavier nucleus + energy.Define melting
Method
State that a solid becomes a liquid without changing its nuclei.Reason
Melting is a physical change in particle arrangement and motion, not a nuclear reaction.Working
Solid → liquid; nuclear identities unchanged.
7. Mind Stretchers
Mind stretcher 1: Why fusion is difficult on EarthExtension
Fusion powers the Sun, but controlled fusion is hard to achieve on Earth. Explain why.
Show Answer
The light nuclei are positively charged and repel. Extremely high temperature is needed so that they have enough kinetic energy to approach closely; producing and maintaining those conditions is difficult.
Mind stretcher 2: One-word checkExtension
A student writes, “Fusion joins small particles.” Why is this too vague for an exam definition?
Show Answer
It does not identify the particles as light nuclei, does not say they form a heavier nucleus, and does not mention the associated release of energy.
8. Practice and next step
Complete the Radioactivity Quiz and Radioactivity Structured Practice, then return to the O-Level Physics portal for mixed revision.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027