A nuclear reactor’s whole job is to hold a chain reaction at exactly the right pace — fast enough to make useful heat, but never running away. It does this with four key components working together: control rods to tune the reaction, a moderator to keep it going, a heat exchanger to harvest the energy, and shielding to keep everyone safe.
📘 What you need to know
A reactor keeps a chain reaction running steadily by controlling the number and energy of free neutrons
The four main components are control rods, moderator, heat exchanger, and shielding
Control rods absorb neutrons; raising or lowering them tunes the fission rate (fully lowered = shutdown)
Moderator slows fast neutrons to thermal speeds so they can cause fission — it’s a poor neutron absorber (e.g. water, graphite)
Heat exchanger transfers heat from the reactor coolant to a separate water system to make steam
Shielding (thick steel and concrete) absorbs the radiation, keeping the surroundings safe
The steam produced drives a turbine and generator to make electricity
The Overall Idea
Fission in the fuel rods releases heat. A coolant (usually water) flows through the core, picks up that heat, and carries it to a heat exchanger, where it boils a separate supply of water into steam. The steam spins a turbine, which drives a generator to make electricity — exactly like a fossil-fuel power station, but with fission as the heat source instead of burning fuel.
Heat from fission in the fuel rods is carried by the coolant to the heat exchanger, which boils separate water into steam to drive a turbine and generator. Control rods and the moderator tune the reaction; shielding keeps the surroundings safe.
Control Rods
Purpose: to absorb neutrons. Control rods are made of a material that soaks up neutrons without becoming dangerously unstable itself. By raising or lowering them among the fuel rods, operators control exactly how many neutrons are available to cause fission:
Lowering the rods further absorbs more neutrons, so the fission rate decreases.
Raising the rods absorbs fewer neutrons, so the fission rate increases.
This is adjusted automatically so that exactly one neutron from each fission goes on to cause the next — keeping the reactor critical and steady. In an emergency, the rods are lowered all the way to absorb all the free neutrons and shut the reactor down.
lower the rods
→ absorb more neutrons →
slower fission
Moderator
Purpose: to slow down neutrons. The neutrons released by fission are fast, but U-235 only readily absorbs slow (thermal) neutrons. The moderator — a material like water or graphite that surrounds the fuel rods — slows them down. Fast neutrons collide with the moderator’s molecules and lose momentum until they’re in thermal equilibrium with it (hence “thermal neutron”). Crucially, the moderator must be a poor absorber of neutrons, so it slows them without soaking them up.
fast neutrons
→ collide with moderator →
slow thermal neutrons
→ cause →
more fission
Heat Exchanger
Purpose: to transfer heat efficiently between water systems. A reactor has separate water systems that must stay apart — the coolant that runs through the radioactive core, and the water that becomes steam for the turbine. The coolant is pumped into the reactor cold, absorbs the heat from fission, and carries it to the heat exchanger. There, it passes its heat to the second water supply, boiling it into steam. Keeping the systems separate stops radioactive coolant from reaching the turbine.
Shielding
Purpose: to house the reactor and absorb hazardous radiation. The daughter nuclei and neutrons produced by fission are radioactive, so the entire reactor is wrapped in shielding — walls of steel and concrete that can be nearly two metres thick. This absorbs the emissions so that the environment around the reactor stays safe.
Quick recap:control rods absorb neutrons to tune the fission rate; the moderator slows neutrons so they can cause fission; the heat exchanger moves heat to a separate water loop to make steam; and shielding absorbs the radiation to keep everyone safe.
🧭 Matching each part to its job
Control rods → absorb neutrons → control the number of neutrons and the fission rate
Moderator → slow neutrons → control the energy of neutrons so they cause fission
Coolant → carry heat out of the core to the heat exchanger
Heat exchanger → boil separate water into steam to drive the turbine
Shielding → absorb radiation → keep the surroundings safe
WE 1
State the purpose of the control rods and the moderator, and explain how each helps keep a chain reaction going at a steady rate.
Control rods
purpose: to absorb neutrons
lowering them absorbs more neutrons and slows fission; raising them speeds it up
→ tuned so exactly one neutron per fission causes the next = steadyModerator
purpose: to slow down the fast neutrons from fission
slow (thermal) neutrons are absorbed by U-235, so they can cause fission
→ keeps enough fissions happening to sustain the chain
WE 2
(a) Explain the role of the heat exchanger. (b) Describe how a reactor can be shut down quickly in an emergency.
Part (a) — heat exchanger
the coolant carries heat from the core to the heat exchanger
there it transfers this heat to a separate water supply, boiling it to steam
→ the steam then drives the turbine and generatorPart (b) — emergency shutdown
the control rods are lowered fully into the core
they absorb all the free neutrons, so no more fissions can occur
→ the chain reaction stops (emergency shutdown)
💡 Top tips
Control rods = number of neutrons; moderator = energy of neutrons. Two different jobs — don’t mix them
The moderator must be a poor absorber — it slows neutrons but mustn’t soak them up
Keep the water systems separate: the heat exchanger stops radioactive coolant reaching the turbine
Emergency shutdown = lower the control rods fully to absorb all the neutrons
⚠ Common mistakes
Swapping control rods and moderator — rods absorb neutrons, the moderator slows them
Saying the moderator absorbs neutrons — it must be a poor absorber, or the chain would stop
Thinking the reactor coolant directly drives the turbine — it heats a separate water system via the heat exchanger
Forgetting that only one neutron per fission needs to continue the chain for a steady, critical reactor
Up next: Radioactive Waste Management. You’ve seen how a reactor runs — the last piece is what to do with the dangerous by-products, from low-level waste to spent fuel, plus the pros and cons of nuclear power.
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