SPEC 8463 • TOPIC 4.4.4 • PAPER 1 • PHYSICS ONLY
Nuclear Fission & Fusion
Two words that sound alike — and do opposite things. Fission splits heavy nuclei to power reactors. Fusion joins light nuclei to power stars.
Two Opposite Processes
Both are nuclear processes — they involve the nucleus, not the electrons. Both release energy. But what happens to the nuclei is completely opposite. Mixing them up is the single most common error in this topic.
Fission
Heavy nucleus SPLITS
A large unstable nucleus (like uranium-235) breaks apart into two smaller nuclei. Used in nuclear reactors and weapons.
Fusion
Light nuclei JOIN
Two small light nuclei (like hydrogen isotopes) combine to form one heavier nucleus. Powers stars; future power source.
Memory Hook
F-I-SSION = sp-LI-TTING (the "I" sounds match). FUSION = FUSE (join together). Different process, different fuel, different conditions.
Three Key Definitions
Nuclear Fission
The splitting of a large and unstable nucleus (e.g. uranium-235 or plutonium) into two smaller nuclei plus two or three neutrons, releasing energy.
Chain Reaction
A self-sustaining series of fissions in which neutrons released by one fission go on to trigger further fissions.
Nuclear Fusion
The joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation.
Fission Step-by-Step
Spontaneous fission is rare. To trigger fission reliably, you need a slow neutron to be absorbed by the heavy nucleus first.
StepWhat happens
1. AbsorbA slow neutron is absorbed by a U-235 nucleus, briefly forming U-236.
2. UnstableThe U-236 nucleus is unstable — too much energy, too many nucleons in one place.
3. SplitsThe nucleus splits into two smaller nuclei, roughly equal in size , releasing two or three neutrons and gamma rays .
4. Releases energyAll of the fission products have kinetic energy (the daughter nuclei and the neutrons all fly apart at high speed); gamma photons also carry energy. This kinetic energy is what becomes heat in a reactor.
Spec Wording — Memorise This Phrasing
The spec uses the exact phrasing "splits into two smaller nuclei, roughly equal in size". Mark schemes credit the words "two smaller nuclei" and reject "two atoms". Atoms have electrons; what splits is the nucleus.
A Single Fission Event
The spec requires you to draw and interpret diagrams like this one.
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Balancing a Fission Equation
Mass numbers and atomic numbers must balance, just like in lesson 4.03's nuclear equations.
EXAMPLE: ONE POSSIBLE FISSION OF U-235 ¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n
CheckLeft sideRight side
Mass numbers1 + 235 = 236141 + 92 + (3 × 1) = 236 ✓
Atomic numbers0 + 92 = 9256 + 36 + (3 × 0) = 92 ✓
Why "One Possible" Fission
U-235 can split in many ways — Ba/Kr is one common pair, but Sr/Xe, I/Y, and others also occur. The number of neutrons emitted is also 2 or 3, depending on which split. The spec only requires you to balance the equation given, not memorise specific products.
Chain Reaction
Each fission releases 2 or 3 neutrons. Each of these neutrons can be absorbed by another U-235 nucleus, triggering another fission. That fission releases more neutrons. And so on.
The Cascade
Gen 1: 1 fission → 3 neutrons released
Gen 2: Each of those 3 neutrons triggers a fission → 3 × 3 = 9 neutrons
Gen 3: 9 fissions → 27 neutrons
Gen 4: 27 fissions → 81 neutrons
...continues growing exponentially.
This is why even a tiny amount of fuel can release vast amounts of energy in a short time. The number of fissions per second can grow by a factor of 3 in each generation.
Chain Reaction Diagram
Past papers commonly ask you to "complete to show how a chain reaction may occur" — they want this exact tree pattern.
Controlled vs Uncontrolled
The same physics; the difference is whether the cascade is allowed to grow.
Nuclear Reactor
Controlled Chain Reaction
Control rods (typically boron or cadmium) are inserted between fuel rods. They absorb excess neutrons, so on average just one neutron per fission goes on to trigger the next.
Result: a steady rate of fissions. Energy is released as heat at a manageable rate, used to boil water → drive a turbine → generate electricity.
Nuclear Weapon
Uncontrolled Chain Reaction
No control rods. Every released neutron is free to trigger more fissions. The reaction grows exponentially — millions of fissions within microseconds.
Result: an enormous amount of energy released in an extremely short time. This is the explosion of a fission bomb.
Mark-Scheme Note
For "describe how the rate of fission is controlled", say: "control rods absorb (excess) neutrons". The word "absorb" is the key verb — generic phrases like "control rods slow it down" don't score.
Fusion: The Opposite Process
Two light nuclei combine to form one heavier nucleus. The mass of the products is slightly less than the mass of the reactants — that missing mass has been converted to the energy of radiation (E = mc²).
EXAMPLE: HYDROGEN ISOTOPE FUSION ²₁H + ³₁H → ⁴₂He + ¹₀n
CheckLeft sideRight side
Mass numbers2 + 3 = 54 + 1 = 5 ✓
Atomic numbers1 + 1 = 22 + 0 = 2 ✓
Fusion Fuel
Deuterium (²H, "heavy hydrogen") and tritium (³H, "heavier hydrogen") are the most easily fused isotopes. Deuterium is abundant in seawater. Tritium has to be manufactured. In stars, plain hydrogen (¹H) is fused — through a longer chain of reactions.
Spec Subtlety: "MAY be converted"
The spec says "some of the mass may be converted into the energy of radiation". The word "may" matters — only some of the mass converts to energy. Most remains as the heavier product nucleus. The tiny mass difference (Δm) becomes a large amount of energy via E = mc².
Why Fusion Is So Hard
Both nuclei are positively charged (protons). Like charges repel — this electrostatic repulsion stops them getting close enough to fuse. To overcome it, you need to give the nuclei huge kinetic energy: extremely high temperatures.
ConditionWhy it's needed
Very high temperature (millions of degrees)Nuclei need enormous kinetic energy to overcome the electrostatic repulsion and get close enough for the strong nuclear force to fuse them.
Very high pressure / densityThe nuclei must be packed tightly enough that collisions are frequent.
~15M °C
Sun's core temperature
~100M °C
Needed in tokamak fusion reactor
~4×
Energy per kg vs fission
Why It Powers Stars but Not Cities (Yet)
Stars have huge mass — gravity compresses the core to the conditions needed for fusion. On Earth, achieving and sustaining these conditions in a controlled way is the central engineering challenge. Experimental reactors continue to advance, but fusion is not yet practical for commercial power generation.
Fission vs Fusion: Side-by-Side
FissionFusion
What happensHeavy nucleus splits into two lighter onesTwo light nuclei join into one heavier one
Typical fuelUranium-235, plutonium-239Hydrogen isotopes (²H, ³H)
TriggerAbsorbs a slow neutronExtreme temperature and pressure
Energy releasedYes — large amount per fissionYes — about 4× more per kg of fuel
Used inPower stations; nuclear weaponsStars (incl. the Sun); H-bombs; experimental reactors
Radioactive wasteSignificant — long-livedMuch less long-lived waste
ConditionsWorks at near room temperatureNeeds millions of degrees
PEE Worked Example
"Compare the processes of nuclear fission and nuclear fusion."4 marks
P
Fission is the splitting of a heavy nucleus into two smaller nuclei; fusion is the joining of two light nuclei to form a heavier nucleus.
E
Both are nuclear processes (they involve the nucleus, not electrons), and both release energy.
E
Fission can occur at low temperatures when a slow neutron is absorbed, whereas fusion requires extremely high temperatures (millions of degrees) to overcome the electrostatic repulsion between the positive nuclei.
E
Fission produces significant long-lived radioactive waste; fusion produces much less radioactive waste, and releases more energy per kg of fuel.
Examiner Alert: Two Recurring Traps
Trap 1: "Atoms" instead of "Nuclei"
Nov 2021 Q02.1 mark scheme states: "do not accept atoms". Fission splits the nucleus, not the whole atom. Fusion joins nuclei, not atoms. Atoms have electrons — those aren't involved in either process. Use the word "nuclei" every time.
Trap 2: Mixing Up Fission and Fusion
The persistent error in this topic. Phrases like "fusion splits heavy nuclei" or "fission requires high temperature" score zero. Burn the memory hook into place: fission = sp(LI)tting; fusion = join.
Vocabulary Bingo
Fission words: splits, neutron absorbed, chain reaction, control rods, reactor. Fusion words: joins, high temperature, mass converted to energy, stars, Sun. If a question uses any of these signal words, you know which process it's testing.
Common Errors That Cost Marks
✗ Weak
"Fission splits two atoms together."
✓ Strong
Fission splits one large unstable nucleus into two smaller nuclei (Nov 2021 MS: atoms is rejected).
✗ Weak
"Fission requires high temperature."
✓ Strong
Fission needs a slow neutron to be absorbed. Fusion is the one that requires high temperature.
✗ Weak
"Chain reactions always explode."
✓ Strong
Chain reactions are controlled in a reactor (control rods absorb excess neutrons) and uncontrolled in a weapon.
✗ Weak
"Fusion produces a lot of nuclear waste."
✓ Strong
Fusion produces much less long-lived radioactive waste than fission. This is one of its main advantages.
Your Turn
In a nuclear power station, uranium-235 undergoes fission inside the reactor.
Explain how a chain reaction occurs in the reactor and how the rate of fission is controlled.4 marks
Tests both the chain reaction mechanism (slide 7-8) and the role of control rods (slide 9). The mark scheme will want the word "absorb" applied to both neutrons-triggering-fission and control-rods-absorbing-excess-neutrons.
1
Each fission of a U-235 nucleus releases 2 or 3 neutrons.
2
Each of these neutrons can be absorbed by another U-235 nucleus, triggering another fission — this is the chain reaction.
3
To control the rate, control rods (typically boron or cadmium) are inserted between the fuel rods.
4
The control rods absorb excess neutrons, so on average just one neutron per fission triggers the next — keeping the rate steady.
Checkpoint — Module 4 Complete
You should now be able to:
✓
Define fission, fusion, and chain reaction using mark-scheme wording — always "nuclei", never "atoms".
✓
Draw and interpret a fission diagram (spec requirement) — neutron + U-235 → 2 smaller nuclei + 2–3 neutrons + γ + energy.
✓
Balance a fission equation — mass numbers and atomic numbers must balance on both sides.
✓
Draw and explain a chain reaction (spec requirement) — exponential growth controlled by neutron-absorbing rods.
✓
Distinguish controlled (reactor) from uncontrolled (weapon) — same physics, control rods make the difference.
✓
Describe fusion and its conditions — light nuclei join, mass converted to radiation, requires millions of degrees.
✓
Compare fission and fusion — splitting vs joining, fuels, conditions, waste, energy per kg.
Module 4 Done — Next: Module 5 (Forces)
This was the final lesson of the Atomic Structure module. Paper 1 atomic content is now fully covered: atomic structure, nuclear models, radioactive decay, half-lives, contamination, uses of radiation, and now fission & fusion. Module 5 (Forces) starts Paper 2 content.
Next: Two Opposite Processes