Module 6 · Particles and Medical Physics

Fission and Fusion

Specification: OCR A H556 | Section: 6.4.4 Nuclear fission and fusion | Teaching frame: OCR A classroom resource with calculations, reactor context and exam practice

By the end of this topic you should be able to…

define fission and fusion and balance simple nuclear equations
link energy release to mass defect, binding energy and Einstein’s equation ΔE = c²Δm
explain induced fission, chain reactions, and the roles of fuel rods, control rods and moderator
compare the conditions needed for a reactor and for a bomb
describe why fusion requires extreme temperature and pressure, and why it powers stars
evaluate benefits, risks and environmental issues around nuclear power

Big idea: both fission and fusion release energy because the products move towards higher binding energy per nucleon. The route is different, but the physics ledger is the same.

Part 1

Core definitions

Nuclear fission is the splitting of a heavy nucleus into two smaller nuclei, usually after absorbing a neutron. The process releases energy and typically emits additional neutrons.

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. If the products are more tightly bound, energy is released.

Typical fission reaction²³⁵U + ¹₀n → ¹⁴¹Ba + ⁹²Kr + 3¹₀n + energy

Typical fusion reaction²₁H + ³₁H → ⁴₂He + ¹₀n + energy

Exam anchor

In OCR A questions, never just say “energy is released because things split” or “because nuclei join”. The mark-scoring explanation is that the products have a higher binding energy per nucleon, so total binding energy increases and the difference is released.

Heavy nuclei

Uranium-235 and plutonium-239 are common fissile fuels because they can undergo induced fission after absorbing a slow neutron.

Light nuclei

Hydrogen isotopes such as deuterium and tritium are used in fusion because the Coulomb barrier is lower than for heavier nuclei.

Energy source

In both cases, a small mass defect corresponds to a large energy release because c² is enormous.

Part 2

Mass defect, binding energy and why energy is released

The total mass of reactants in a nuclear reaction is usually slightly greater than the total mass of products. That missing mass is the mass defect, and it appears as released energy.

Mass–energy linkΔE = c²Δm

Binding energy is the energy required to separate a nucleus completely into free nucleons. A larger binding energy per nucleon means a more stable nucleus.

Useful conversion1 u = 931.5 MeV

The binding energy per nucleon curve peaks around iron. That is why:

Heavy nuclei release energy by splitting towards medium-mass nuclei.
Light nuclei release energy by joining towards medium-mass nuclei.

Common misconception

Fission does not release energy because the products are “smaller”. Fusion does not release energy simply because the product is “bigger”. The winning explanation is always about binding energy per nucleon and mass defect.

Worked example 1

A reaction has a mass defect of 0.186 u. Calculate the energy released in MeV and in joules.

Using 1 u = 931.5 MeV: E = 0.186 × 931.5 = 173 MeV

Convert to joules: 173 MeV = 173 × 10⁶ × 1.60 × 10⁻¹⁹ J

E = 2.77 × 10⁻¹¹ J

Part 3

Induced fission and the chain reaction

In induced fission, a heavy nucleus absorbs a neutron and becomes unstable. It deforms, splits, and emits two or three more neutrons. Those neutrons can trigger further fission events.

Chain reaction idea1 neutron in → fission → 2–3 neutrons out → more fissions

Criticality

Subcritical: fewer than one further fission caused on average by each fission event; reaction dies away
Critical: exactly one further fission caused on average; steady power output
Supercritical: more than one further fission caused on average; reaction rate rises rapidly

Reactor conditions

Maintain an approximately critical reaction. Neutrons are moderated and excess neutrons are absorbed so energy release is controlled.

Bomb conditions

Create a rapidly supercritical assembly. There is no moderation for steady output or control system designed to keep the reaction stable.

What OCR usually wants

If asked about the difference between a reactor and a bomb, mention rate control, criticality, and the role of control rods/moderation. “A bomb is uncontrolled” alone is often too vague.

Interactive

Recommended simulation work

Why the embedded version is gone

It was not reliably useful. A half-working or decorative simulation is worse than none, so this page now points students to a proper simulation and tells them exactly what to do with it.

Task 1 · Critical vs subcritical vs supercritical Open the PhET nuclear fission simulation and change the conditions until the reaction dies away, stays roughly steady, and then runs away. For each case, write one sentence explaining why that behaviour happens in terms of neutrons causing further fissions.

Task 2 · Reactor thinking Use the sim as a model for a reactor and explain what control rods are trying to achieve. Your answer must include the word critical and must distinguish slowing neutrons from absorbing neutrons.

Task 3 · Link to fusion Explain why this kind of simulation makes sense for fission but not for fusion. Include the ideas of a chain reaction, electrostatic repulsion, and high temperature.

What makes this useful

Students are not just “watching atoms”. They are classifying behaviour and explaining it with OCR vocabulary.
It directly supports mark-scheme language: induced fission, chain reaction, critical, moderator, control rods.
It creates a clean bridge to the exam question: “compare a fission reactor with nuclear fusion / a bomb”.

Teacher note

If you want a single in-page interactive later, build a proper reactor-control model. Do not use a decorative particle animation.

Exam payoff

This section now trains explanation, not spectacle.

Part 4

The basic fission reactor

OCR A expects the basic structure of a fission reactor: fuel rods, control rods and moderator.

Fuel rods

Contain fissile material such as uranium-235. Fission in the rods releases energy and neutrons.

Control rods

Made from neutron-absorbing materials such as boron. Inserting them further absorbs more neutrons and reduces the reaction rate.

Moderator

Slows fast neutrons to thermal energies, making induced fission of U-235 more likely.

Why slow neutrons matter

U-235 is much more likely to absorb a slow neutron than a fast one. The moderator increases the probability that released neutrons cause further fission rather than escaping.

Common misconception

The moderator does not absorb neutrons to stop the reaction. That is the job of the control rods. The moderator slows neutrons.

Part 5

Fusion conditions and why stars can do it

Fusion requires positively charged nuclei to get close enough for the strong nuclear force to overcome electrostatic repulsion. This means particles must have enormous kinetic energy.

Fusion conditionVery high temperature + sufficient density/pressure + confinement time

In stars, gravity compresses the gas so strongly that the temperature and pressure in the core become high enough for fusion to occur. In the Sun, hydrogen nuclei ultimately fuse into helium.

Why fusion is hard on Earth

Temperatures of order 10⁷–10⁸ K are needed
No solid container can touch the plasma
The plasma must be confined magnetically or inertially long enough for fusion to happen

OCR phrasing to remember

Fusion requires high temperature so nuclei have enough kinetic energy to overcome electrostatic repulsion and get close enough for the strong nuclear force to act.

Worked example 2

A fusion reaction releases 17.6 MeV. Estimate the energy in joules from one reaction.

1 MeV = 1.60 × 10⁻¹³ J

E = 17.6 × 1.60 × 10⁻¹³

E = 2.82 × 10⁻¹² J

Part 6

Benefits, risks and environmental issues

Potential benefits

Very high energy density compared with chemical fuels
Low direct carbon dioxide emissions during generation
Reliable baseload electricity from fission reactors
Fusion fuel sources could be abundant if technology matures

Risks and drawbacks

Long-term management of radioactive waste
Accident risk and public concern
Very high build/decommissioning cost
Fusion is technologically difficult and not yet a routine power source

Balanced classroom view

OCR A often rewards balanced evaluation. A good answer does not simply say nuclear power is “good” or “bad”; it weighs energy security, carbon emissions, cost, accident risk, and waste management.

Retrieval

Knowledge Check

What is meant by a chain reaction in nuclear fission?

2 marks

Neutrons released in one fission event cause further fission events
This process repeats so the reaction becomes self-sustaining

Why are control rods used in a reactor?

2 marks

They absorb neutrons
This controls the reaction rate / keeps the reactor near critical

Why does fusion require a very high temperature?

2 marks

Nuclei are positively charged and repel
High temperature gives them enough kinetic energy to overcome electrostatic repulsion and get close enough for the strong force to act

Exam Practice

Exam-Style Questions

Explain, using the binding energy per nucleon curve, why both the fission of uranium and the fusion of hydrogen can release energy.

4 marks

Products move towards nuclei with higher binding energy per nucleon
In fission, heavy nuclei split into medium-mass nuclei with greater binding energy per nucleon
In fusion, very light nuclei combine into a nucleus with greater binding energy per nucleon
The increase in total binding energy is released as energy / corresponds to a mass defect

A fission reaction has a mass defect of 0.215 u. Calculate the energy released in joules.

3 marks

Use 1 u = 931.5 MeV, so E = 0.215 × 931.5 = 200 MeV
Convert: 200 MeV = 200 × 10⁶ × 1.60 × 10⁻¹⁹ J
3.2 × 10⁻¹¹ J

Describe the roles of the moderator and control rods in a thermal nuclear reactor.

4 marks

Moderator slows fast neutrons
Slow neutrons are more likely to cause fission in U-235
Control rods absorb neutrons
Therefore control rods regulate the reaction rate / prevent runaway increase

Explain why fusion is possible in the core of stars but difficult to achieve sustainably on Earth.

5 marks

Fusion requires very high temperature so nuclei can overcome electrostatic repulsion
Stars provide enormous pressure and confinement due to gravity
This keeps the plasma dense and hot enough for long enough
On Earth, producing and confining plasma at these temperatures is technologically difficult
Containment is hard because no material vessel can directly hold such hot plasma

A student claims: “Fusion is always better than fission because it is more powerful and has no risks.” Evaluate this statement.

6 marks

Fusion has advantages: abundant fuel sources / potentially less long-lived waste / no CO₂ during generation
Fusion releases large energy per unit mass
However it still involves neutron radiation and engineering challenges
Commercial fusion remains difficult and expensive
Fission is established and provides reliable large-scale electricity now
A balanced judgement is required, not an absolute claim

Topic Summary

Fission

Heavy nucleus splits; releases energy and neutrons; can sustain a chain reaction.

Fusion

Light nuclei join; needs extreme temperature and pressure; powers stars.

Reactor physics

Moderator slows neutrons. Control rods absorb neutrons. Reactors aim for criticality.

Mass defect

Released energy comes from missing mass: ΔE = c²Δm.

ΔE = c²Δm

1 u = 931.5 MeV

higher binding energy per nucleon → more stable

critical / subcritical / supercritical