Nuclear Fission and Fusion
Two processes that both release enormous energy from the nucleus — by doing opposite things. Confusing them is the defining error of the topic, and it was recorded repeatedly.
1. The two definitions
FISSION — a large nucleus SPLITS into two smaller nuclei, releasing energy and neutrons. FUSION — two small nuclei JOIN to form a larger nucleus, releasing energy.
A memory hook that works:
Fission = split (both have the “s…i” of split, and think of a fissure, a crack). Fusion = fuse together (as in fusing two things into one).
| Fission | Fusion | |
|---|---|---|
| Process | large nucleus splits | small nuclei join |
| Typical fuel | uranium-235, plutonium-239 | hydrogen isotopes |
| Where | nuclear power stations | stars, including the Sun |
| Conditions | slow neutron absorbed | extremely high temperature and pressure |
| Products | two smaller nuclei + neutrons | helium + energy |
| Waste | radioactive waste | little or none |
2. Nuclear fission
The process:
- A slow-moving neutron is absorbed by a large unstable nucleus (e.g. uranium-235)
- The nucleus becomes unstable and splits into two smaller nuclei (the fission fragments)
- Two or three more neutrons are released
- Energy is released, mostly as kinetic energy of the fragments
The fission products vary — uranium doesn’t always split the same way. It is typically two smaller nuclei, but which ones differs between events. A student’s uncertainty about this was recorded, and the honest answer is that the products are not fixed.
Where the energy comes from:
A small amount of mass is converted into energy. The total mass of the products is very slightly less than the original.
Mass is not conserved in the everyday sense in nuclear reactions — mass and energy together are. Misunderstanding mass conservation here was recorded. In chemical reactions mass is conserved; in nuclear reactions a tiny mass is converted to a very large amount of energy.
Chain reaction
The neutrons released go on to cause further fissions, releasing more neutrons, and so on — a chain reaction.
- Controlled chain reaction → steady energy release → nuclear power station
- Uncontrolled chain reaction → nuclear weapon
The nuclear reactor
| Component | Job |
|---|---|
| Fuel rods | contain the uranium-235 |
| Control rods (boron or cadmium) | absorb neutrons to control the rate of the reaction |
| Moderator (graphite or water) | slows the neutrons so they can be absorbed and cause fission |
| Coolant | carries thermal energy away to make steam |
| Concrete shielding | absorbs radiation to protect workers |
Control rods ABSORB neutrons; the moderator SLOWS them. Both affect the reaction rate but in different ways — lowering the control rods absorbs more neutrons and slows the reaction down.
Energy transfer chain: nuclear → thermal → kinetic (steam turbine) → electrical (generator).
3. Nuclear fusion
The process:
Two light nuclei — usually hydrogen isotopes — join to form a heavier nucleus (helium), releasing energy.
Why it needs extreme conditions:
Nuclei are positively charged and repel each other. Only at very high temperature (millions of degrees) and pressure do they move fast enough to overcome that repulsion and get close enough to fuse.
The repulsion between positive nuclei is the reason fusion is hard — and it is the standard explanation mark. Confusion about the charge of nuclei was recorded; nuclei are always positive.
Fusion in stars: this is what powers the Sun — hydrogen fusing into helium. The outward pressure from fusion balances the inward pull of gravity, keeping the star stable.
Fusion on Earth: experimental reactors exist, but sustaining the required temperature and containing the plasma is extremely difficult, so fusion is not yet a practical power source.
4. Comparing them
Advantages of fusion (if it can be achieved):
- Fuel (hydrogen) is abundant — obtainable from water
- No long-lived radioactive waste
- No risk of a runaway chain reaction
Fission today:
- Works now and produces large amounts of energy from little fuel
- Produces no CO₂ during generation
- But creates radioactive waste needing long-term storage, and carries accident risk
Both release energy from the nucleus by converting a small amount of mass — that is what they have in common.
5. Framing your answer
State which process it is, then what happens to the nuclei, then the energy. Tutors flagged “frame answers properly for full marks” — a fission answer needs the neutron absorbed, the splitting, the further neutrons, and the energy released.
If the question gives you a nuclear equation, check that the top numbers and the bottom numbers balance, exactly as in decay equations.
6. Mistakes that cost marks
Confusing fission with fusion — the dominant error.
Saying fission joins nuclei, or fusion splits them.
Saying mass is conserved in a nuclear reaction.
Forgetting the neutrons released in fission.
Forgetting the chain reaction.
Mixing up control rods and the moderator.
Omitting the high temperature and pressure needed for fusion.
Not explaining fusion’s difficulty in terms of repulsion between positive nuclei.
Saying the Sun runs on fission.
Frequently asked questions
What is nuclear fission? A large nucleus splits into two smaller ones after absorbing a neutron, releasing energy and more neutrons.
What is nuclear fusion? Two small nuclei join to form a larger one, releasing energy.
How do I remember which is which? Fission = split; fusion = fuse together.
What is a chain reaction? Neutrons from one fission cause further fissions, and so on.
What do control rods do? Absorb neutrons to control the reaction rate.
What does the moderator do? Slows down the neutrons so they can cause fission.
Why does fusion need such high temperatures? To overcome the electrostatic repulsion between positively charged nuclei.
Which process powers the Sun? Fusion — hydrogen into helium.
Where does the energy come from? A small amount of mass is converted into energy.
Why isn’t fusion used for power on Earth? The temperature and containment requirements are not yet practical to sustain.
Quick revision checklist
- I can define fission and fusion without confusing them
- I have a memory hook for which is which
- I can describe fission step by step, including the neutron
- I know the products vary and include 2–3 neutrons
- I know energy comes from mass converted to energy
- I can explain a chain reaction
- I know the job of the control rods, moderator, coolant and shielding
- I can describe fusion and name the fuel
- I can explain why fusion needs extreme conditions
- I know fusion powers stars
- I can compare the advantages of each
- I can balance a nuclear equation
These notes cover nuclear fission and fusion in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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