Nuclear Fission and Fusion

Watch first

CognitoNuclear fission and fusion

Key facts & equations

Nuclear fission

Nuclear fission: a large unstable nucleus splits into two smaller nuclei + neutrons + energy.
Triggered by: neutron absorption. Commonly used: U-235, Pu-239.
Energy released: enormous amounts of energy as kinetic energy of fragments (heat).
Products: two smaller nuclei (fission fragments), 2-3 neutrons, gamma radiation.
Chain reaction: neutrons from fission trigger further fissions. Self-sustaining.
Critical mass: minimum mass for self-sustaining chain reaction.

Nuclear reactor

Moderator: slows down neutrons (often water or graphite). Slow neutrons more likely to cause fission.
Control rods: absorb neutrons (boron or cadmium). Inserted to reduce rate; withdrawn to increase.
Coolant: removes heat from reactor core (usually water or CO2). Transfers to heat exchanger → steam → turbine → generator.
Containment: thick concrete and steel shield prevents radiation escaping.
Waste: highly radioactive products with long half-lives — major disposal problem.

Nuclear fusion

Nuclear fusion: two light nuclei join to form a heavier nucleus + energy.
Example: ²₁H + ³₁H → ⁴₂He + ¹₀n + energy (deuterium + tritium → helium + neutron).
Energy released per kg of fuel: much greater than fission.
Conditions required: extremely high temperature (~10⁷ K) and high pressure — to overcome electrostatic repulsion.
Currently: fusion requires more energy input than it produces in lab conditions.
Potential: fusion fuels are abundant (hydrogen isotopes from seawater). No long-lived waste.

Fission vs fusion comparison

Fission: current technology. Used in power stations. Produces long-lived radioactive waste. Risk of meltdown.
Fusion: not yet achieved as power source. Fuel abundant, waste product is helium (safe). No risk of runaway reaction.
Both: based on E = mc² (mass defect → energy). Nuclear reactions, not chemical.
Fusion releases more energy per kg of fuel than fission.

Exam questions — 4 questions · 11 marks · AQA 8463 style

1 markChain reactionAQA 8463 style

What is meant by a chain reaction in a nuclear reactor?

A A series of chemical reactions in the reactor fuel

B Neutrons from one fission event trigger further fission events, releasing more neutrons, sustaining the reaction

C The reactor core heats the coolant which drives a chain of turbines

D The control rods absorb neutrons in a chain sequence

3 marksRole of control rodsAQA 8463 style

Explain the role of control rods in a nuclear reactor and describe what happens when they are fully inserted. (3 marks)

Hint: What they are made of → what they do (absorb neutrons) → effect on chain reaction → what happens when fully inserted.

+30 XP

1 markFusion conditionsAQA 8463 style

Why does nuclear fusion require extremely high temperatures?

A High temperature provides neutrons to trigger the fusion

B High temperature gives nuclei enough kinetic energy to overcome their electrostatic repulsion and get close enough for the strong nuclear force to act

C High temperature separates electrons from nuclei, making fusion easier

D High temperature provides gamma radiation needed for fusion

4 marksCompare fission and fusionAQA 8463 style

Compare nuclear fission and nuclear fusion as methods of generating energy. Consider: fuel, waste, safety, energy output per kg. (4 marks)

Hint: Compare on four points. Be specific — name the fuels and waste products rather than just saying more or less.

+40 XP

Nuclear Fission and Fusion

Nuclear fission

Nuclear reactor

Nuclear fusion

Fission vs fusion comparison

Module complete! 🎉