The big idea: A nuclear reactor is a machine that lets uranium nuclei split (fission) in a slow, steady, controlled way — and turns the energy released into useful heat.
It has four key parts, and each one has one job:
- fuel — the stuff that splits - moderator — slows the neutrons down - control rods — absorb spare neutrons - heat exchanger — carries the heat away
Two words to know first: Fission = a heavy nucleus (like uranium-235) splitting into two smaller nuclei, releasing energy and 2–3 fast neutrons.
Chain reaction = those neutrons go on to split more nuclei, which release more neutrons, and so on. The reactor's job is to keep this chain steady — not dying out, not running away.
| Component | What it does (its function) | Typical material / form |
|---|---|---|
| Fuel | The material that undergoes fission and releases the energy | Enriched uranium-235 (or plutonium-239) rods |
| Moderator | Slows the fast neutrons down so they are far more likely to cause the next fission | Water or graphite around the fuel rods |
| Control rods | Absorb spare neutrons to keep the chain reaction steady (lower = slower, raise = faster) | Boron or cadmium rods |
| Heat exchanger | Carries the heat away from the core to boil water into steam, which turns a turbine | A coolant loop (water / CO₂) through a boiler |
The exam shortcut: Reactor questions follow a predictable pattern: match a component to its function. Lock in the three verbs:
- moderator → slows - control rods → absorb - heat exchanger → removes heat
Follow the energy through the reactor and each part's job makes sense. The fission happens in the fuel; the other three parts control it and collect the heat.
1 — The moderator slows the neutrons: Fission gives off fast neutrons, but a slow neutron is much more likely to be captured by the next uranium-235 nucleus and cause it to split.
So the moderator (water or graphite) is packed around the fuel to slow the neutrons down, keeping the chain reaction going. This is the single most-tested fact in 5.4.3.
2 — The control rods absorb the spare neutrons: Each fission makes 2–3 neutrons, but only one is needed to keep the chain steady. The control rods (boron or cadmium) soak up the extra neutrons.
- Push the rods further in → more neutrons absorbed → reaction slows. - Pull them out → fewer absorbed → reaction speeds up.
Drop them all the way in and the reaction stops (a shutdown).
3 — The heat exchanger removes the heat: The splitting fuel makes the core very hot. A coolant (water or CO₂) is pumped through the core, carries the heat to a heat exchanger, and there boils separate water into steam.
The steam spins a turbine connected to a generator — that's how the nuclear energy becomes electricity.
| Step | Component | Job |
|---|---|---|
| 1 | Fuel | Fissions and releases energy as fast neutrons + heat |
| 2 | Moderator | Slows those neutrons so they trigger more fissions |
| 3 | Control rods | Soak up just enough neutrons to keep the rate steady |
| 4 | Heat exchanger | Moves the heat out to make steam and drive a turbine |
Don't mix up the two 'neutron' jobs: The moderator and the control rods both act on neutrons, but in opposite ways:
- moderator → slows neutrons (to help fission) - control rods → absorb neutrons (to limit fission)
Swapping these two is the classic exam mistake.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
Try Smart Study Free7-day free trial • No card required
How this is tested: 5.4.3 is a recall-and-identify topic.
- Paper 1A (MCQ): identify the function of a named component (e.g. the moderator), identify a suitable material for the moderator, or pick which statements about the heat exchanger / control rods / moderator are correct. - Paper 2: outline or state what a component does, or explain what happens when the control rods are raised or lowered.
Classic trap: confusing the moderator (slows neutrons) with the control rods (absorb neutrons).
How to answer an 'identify the function' question: Name the component, then give its one job in a single verb phrase. Moderator → slows the neutrons. Control rods → absorb neutrons. Heat exchanger → transfers heat out to make steam. No calculation needed — just the right verb.
IB-style question — identify the function of the moderator
In a uranium-fuelled fission reactor, graphite is packed in blocks around the fuel rods. State the function of this graphite, and explain why it helps the chain reaction continue.
Solution
- Name the component the graphite is acting as:
- Graphite around the fuel is the moderator.
- State its function (the marking point):
- It slows down the fast neutrons released by fission.
- Explain why that keeps the chain reaction going:
- A slow neutron is far more likely to be absorbed by a uranium-235 nucleus and make it fission, so slowing the neutrons keeps fissions happening.
Final answer
The graphite is the moderator: it slows the fast neutrons down. Slow neutrons are much more likely to be captured by uranium-235 and cause further fission, so the chain reaction is sustained.