Edexcel IGCSE Physics · Spec 7.18-7.24
Nuclear Reactors and Fusion Energy
Covers trace energy transfer in fission, Distinguish moderator, control rods and shielding and Explain mass loss and energy release in fusion.
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Nuclear Reactors and Fusion Energy
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Explained
Fission, fusion, and what a reactor actually controls
A nuclear power station is a steam engine with an unusual boiler. The nuclear part produces heat, and everything after that is familiar: heat turns water to steam, steam turns a turbine, the turbine drives a generator. Knowing where the nuclear physics stops and the ordinary engineering starts makes this topic much easier to answer.
How fission releases energy
A neutron is absorbed by a large unstable nucleus, which becomes so unstable that it splits into smaller nuclei. The split releases energy and, importantly, releases more neutrons. Those neutrons can be absorbed by further nuclei, and the process repeats as a chain reaction.
Most of the energy first appears as kinetic energy of the fast moving fission products. They collide with surrounding material, which is what turns that kinetic energy into heat.
The three components that get confused
- The moderator slows the neutrons down. Slow neutrons are absorbed more readily, so the chain reaction can continue.
- The control rods absorb neutrons and remove them from the reaction. Lowering them further slows the reaction; raising them speeds it up.
- The shielding, usually thick concrete, stops radiation escaping. It plays no part in controlling the reaction at all.
The distinction to hold on to is that the moderator changes neutron speed while the control rods change neutron number. Both affect the rate of the reaction, but by different means.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Physics are unusually direct here. On a question about the chain reaction, markers were instructed to look for reference to neutron absorption, rather than the significantly vaguer phrases collision with the nucleus or hitting the nucleus.
That is worth writing down exactly. The neutron is absorbed. It does not simply strike or bump into the nucleus, and answers using those words were not credited.
Reports on the wider topic also note that many candidates struggled to balance nuclear equations for alpha decay, which suggests the equations are worth practising alongside the descriptive work.
Fusion, and why mass matters
Fusion is the opposite process. Two light nuclei combine to form a heavier one, and energy is released. This is what powers the Sun, where hydrogen nuclei fuse to form helium.
The mass of the products is slightly less than the mass of the original nuclei. That missing mass is the source of the energy. Fusion needs extremely high temperatures and pressures, because two positively charged nuclei repel each other and must be forced close enough for the strong nuclear force to take over.
That requirement is the reason fusion is not yet used for power generation, while fission has been for decades.
Spec 7.18-7.24
What you need to know
- Trace energy transfer in fission
- Distinguish moderator, control rods and shielding
- Explain mass loss and energy release in fusion
Active recall
Quick check
Answer each question before opening the answer.
What is the moderator's job?
To slow neutrons
What do control rods absorb?
Neutrons
Why does fusion release energy?
Some reactant mass is converted to energy
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