IGCSE Physics / Fission and fusion
Inside a nuclear reactor revision notes
Edexcel International GCSE Physics (4PH1), chapter 25: Fission and fusion, specification point 7.21, 7.22. Free lesson.
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A nuclear reactor uses controlled fission to release energy steadily. Its parts keep the chain reaction at a suitable rate, transfer thermal energy and absorb harmful radiation.
What the main reactor parts do
The core is where the chain reaction happens. Fuel rods contain uranium-235, whose nuclei split in fission and release energy. Each other part has a specific job, so explain what it does when asked for its role. Fuel rods contain uranium-235, where fission releases energy. Coolant flows past the hot fuel rods and carries thermal energy to a heat exchanger. There, it is used to boil water and make steam. Shielding is thick concrete and steel around the reactor. It absorbs radiation and neutrons so they do not escape.
How the moderator keeps the chain reaction going
The moderator, usually graphite or water, fills the space around the rods. It slows neutrons down. Slow neutrons are more likely to be absorbed by uranium-235 nuclei, so the moderator helps the chain reaction continue. The moderator slows neutrons down. It does not absorb them to control the reaction. The control rods absorb neutrons. This difference is important: the moderator helps fission continue, while the control rods control its rate.
How control rods change the rate of fission
Control rods are made of boron and can be moved between the fuel rods. Lowering them further into the core makes them absorb more neutrons. This leaves fewer neutrons to be absorbed by uranium-235, so fewer fissions happen and less energy is released each second. Lower the rods further: more neutrons are absorbed, so the reaction slows. Lower them all the way: the chain reaction stops. Raise them: the reaction speeds up again.
Why the reactor needs shielding
The core gives out radiation, including gamma rays, as well as neutrons. Thick concrete and steel absorb them, preventing them from escaping and harming workers or people nearby. For a question about shielding, name what it absorbs and explain how this protects people outside the reactor.