Nuclear Fission & FusionEdexcel GCSE Physics: Revision notes
Section 1
What is nuclear fission?
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235) into two smaller "daughter" nuclei, releasing two or more neutrons and a large amount of energy.
Nuclear reactions — including fission, fusion and radioactive decay — can all be sources of energy. The products of nuclear fission are themselves radioactive.
Section 2
How is a controlled chain reaction achieved in a nuclear reactor?
In a chain reaction, the neutrons released by one fission event go on to trigger fission in further nuclei, releasing more neutrons and more energy. Left uncontrolled, this can escalate rapidly.
In a nuclear reactor the chain reaction is controlled using:
- Moderators (e.g. water or graphite) — slow down fast neutrons to the speed needed to cause further fission efficiently
- Control rods (e.g. boron) — absorb some neutrons, and can be raised or lowered to control the rate of the reaction
The thermal energy released by the controlled chain reaction heats water into steam, which drives turbines connected to generators to produce electricity — the same basic process as a fossil fuel power station, but with fission as the heat source.
Distinguish clearly: moderator slows neutrons down; control rods absorb neutrons to limit the reaction rate — a common exam confusion.
Section 3
What is nuclear fusion?
Nuclear fusion is the joining of two smaller nuclei to form one larger nucleus, with a loss of mass from the smaller nuclei, releasing a large amount of energy. Fusion is the energy source that powers stars, including the Sun.
Fission vs fusion: fission splits a large nucleus into smaller ones; fusion joins small nuclei into a larger one. Both release energy, but by opposite processes.
Section 4
Why is fusion so difficult to achieve on Earth?
Nuclei are positively charged, so they repel each other electrostatically. For fusion to happen, nuclei must be forced close enough together to overcome this repulsion, which requires:
- Extremely high temperatures — so nuclei collide with enough kinetic energy to get close together
- Extremely high pressures — to increase the density and rate of collisions
At low temperatures and pressures, electrostatic repulsion prevents nuclei ever getting close enough for the strong nuclear force to take over and fuse them. These extreme conditions make building a practical, economically viable fusion power station very difficult with current technology.
Think of trying to push two like poles of strong magnets together — huge force is needed to get them close enough for a different, stronger effect to take over.
Section 5
What are the advantages and disadvantages of nuclear power?
Advantages:
- No carbon dioxide emissions during generation
- Large amounts of energy from small amounts of fuel
Disadvantages:
- Radioactive waste disposal is difficult and long-lasting
- Risk of accidents with serious consequences
- Negative public perception/safety concerns
- High costs and long timescales to build and decommission reactors
For 'evaluate' questions, always give a balanced answer covering both advantages and disadvantages, and reach a justified conclusion.
Must Know
- Fission: large unstable nucleus (e.g. U-235) splits into two daughter nuclei, releasing 2+ neutrons and energy; products are radioactive
- Chain reaction: neutrons from one fission trigger further fission events
- Moderators slow neutrons down; control rods absorb neutrons to control the reaction rate
- Fusion: small nuclei join into a larger one, losing mass and releasing energy; powers stars
- Fusion needs extremely high temperature and pressure to overcome electrostatic repulsion between nuclei
- Nuclear power: no CO2 emissions, but waste disposal, safety and public perception are major issues
That's the notes covered.
Carry on to the next subtopic.