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
- 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Recommended simulation work
Investigating the chain reaction
Track the neutrons produced in each fission and distinguish absorption by control rods from slowing by the moderator. The simplified model is useful for predicting whether a chain reaction dies away, stays steady or grows.
What to investigate
- Classify the behaviour you observe and explain it using the terms below.
- Use the terms induced fission, chain reaction, critical, moderator, control rods.
- Compare controlled chain reactions in a reactor with the requirements for fusion and with an uncontrolled chain reaction.
Model limits
Use this model to compare chain-reaction behaviour. Real reactor control also depends on neutron energies, geometry and delayed neutrons.
Explain your result
Link each change in reaction rate to neutron absorption, moderation or the number of neutrons available to induce further fissions.
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.
The moderator does not absorb neutrons to stop the reaction. That is the job of the control rods. The moderator slows neutrons.
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.
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
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.
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
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.
Knowledge Check
- Neutrons released in one fission event cause further fission events
- This process repeats so the reaction becomes self-sustaining
- They absorb neutrons
- This controls the reaction rate / keeps the reactor near critical
- 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-Style Questions
- 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
- Use 1 u c² ≈ 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
- 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
- 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
- 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.