How Reactors Work
In Module 3, you learned the science — fission, chain reactions, and criticality. Now discover the engineering that puts it all into practice, transforming nuclear energy into electricity safely and reliably.
Learning Objectives
By the end of this module, you will be able to:
In this module, you'll see exactly how reactor design puts Control, Cooling, and Containment into practice.
Control rods and moderation manage the chain reaction
Coolant systems remove heat and transfer it to turbines
Multiple barriers prevent radioactive release
From Fission to Electricity
Nuclear power is fundamentally about boiling water
Here's something that might surprise you: a nuclear power station is fundamentally a very sophisticated way of boiling water.
Think about how a coal power station works. It burns coal to produce heat. That heat boils water to make steam. The steam spins a turbine, and the turbine drives a generator to produce electricity.
A nuclear power station works exactly the same way — except the heat comes from the fission process you explored in Module 3. Everything downstream of the heat source — the turbines, generators, condensers, cooling towers — is conventional power station technology. There's nothing nuclear about it.
Remember that single fission of U-235 releases about 200 MeV — fifty million times more energy than burning a carbon atom. That's why a reactor core the size of a large car can power a city. Now let's see how we harness that energy.
Click "Start Chain" to watch energy flow from fission to electricity. Use Step to advance one stage at a time.
The nuclear process is only the heat source — everything else is conventional power station technology
All power stations work the same way — they just use different fuels to create heat.
The only difference is how we boil the water — the rest is identical
Let's explore what's inside the reactor core and how each component contributes to this process.
Fuel & Cladding
The first barrier of containment
At the heart of any nuclear reactor is the fuel. You learned in Module 3 that U-235 is the fissile isotope that sustains the chain reaction — but how do we actually arrange it inside a reactor?
The uranium is formed into small ceramic pellets of uranium dioxide (UO₂), each about the size of a fingertip. This ceramic form can withstand extremely high temperatures — crucial given the intense heat from fission. The pellets are made from uranium ore that's mined, chemically processed, and pressed into shape — we'll cover this fuel cycle in detail in Module 6.
These ceramic pellets are stacked inside long metal tubes called fuel rods. The rods are then bundled together into fuel assemblies. A large reactor might contain 150 to 250 fuel assemblies, holding 80 to 100 tonnes of uranium in total.
Click the buttons to explore each level of the fuel assembly — from a single pellet up to the full reactor core.
A ceramic cylinder of uranium dioxide (UO₂), about the size of a fingertip. The ceramic form can withstand temperatures over 2800°C.
Cladding: The metal tube surrounding fuel pellets, typically made from a zirconium alloy (sometimes called "zircaloy"). It's chosen because it's strong, resists corrosion, and doesn't absorb many neutrons — allowing them to continue the chain reaction.
Remember the fission products you learned about in Module 3 — those radioactive fragments created when uranium splits? The cladding serves as the first barrier of containment, keeping those fission products locked inside the fuel and preventing them from escaping into the coolant water.
The cladding is containment at its most fundamental level — right at the source. If you remember the 3 C's, this is where Containment begins.
Moderator & Coolant
Slowing neutrons and removing heat
In Module 3, you learned that fast neutrons from fission need to be slowed down to become thermal neutrons — they're about 500 times more likely to cause fission in U-235. You also explored different moderator materials. Now let's see how this works in a real reactor.
A moderator slows neutrons through collisions — like a billiard ball bouncing off cushions. In pressurised water reactors (PWRs), ordinary water serves as the moderator. The neutrons collide with hydrogen atoms and gradually slow down.
But here's something new: in a PWR, water has two critical jobs. It's not just the moderator — it's also the coolant.
Coolant: The fluid that flows through the reactor core, absorbing the intense heat generated by fission. This is how we extract useful energy from the nuclear reaction. In a PWR, water performs this role alongside its job as moderator.
In a pressurised water reactor, the water is kept under extremely high pressure — about 155 times atmospheric pressure (155 bar). This prevents it from boiling, even though it reaches temperatures around 325°C. Water normally boils at 100°C, but under high pressure, it stays liquid at much higher temperatures.
Remember the negative temperature coefficient from Module 3? Here's how it works in practice: if the reactor overheats and water density decreases (it expands), there's less moderation, fewer fissions occur, and power automatically drops. This built-in physics makes the reactor inherently stable.
Watch how temperature affects neutron moderation. Orange neutrons are fast — they bounce off uranium without causing fission. Blue neutrons are thermal (slow) — they can split U-235. See what happens as you increase the temperature!
🔄 SELF-REGULATING: As temperature rises, water becomes less dense. Fewer collisions mean neutrons stay fast — and fast neutrons can't cause fission in U-235. The reactor automatically reduces power. This is the negative temperature coefficient you learned about in Module 3!
The dual role of water — as both moderator and coolant — is where Control and Cooling work hand-in-hand. The physics of moderation provides passive control, whilst the coolant removes heat for power generation.
Control Rods
Active control of the chain reaction
In Module 3, you saw how control rods absorb neutrons and affect the k-factor — fewer neutrons means fewer fissions. Now let's see how this principle is applied in a real power station to control reactor power.
Remember: control rods are made of neutron-absorbing materials like boron or hafnium. When a neutron hits the rod, it's captured and removed from the chain reaction. You saw this visually in Module 3's chain reaction simulator. Here, those same physics give operators precise control over reactor power.
Rods In = Less Power
Insert control rods and they absorb neutrons. Fewer neutrons available means k drops below 1.0 — the chain reaction slows.
Rods Out = More Power
Withdraw control rods and more neutrons survive to cause fission. k rises above 1.0 — power increases.
In normal operation, control rods are adjusted gradually to maintain k = 1.0 (critical) at the desired power level. But there's also a critical safety function...
SCRAM: An emergency shutdown where all control rods drop rapidly into the core within seconds. This drives k well below 1.0, stopping the chain reaction almost immediately. The term may have originated from "Safety Control Rod Axe Man" in early reactor experiments.
In a real reactor, control rods are held UP by electromagnets. If power fails, the magnets release and gravity drops the rods automatically into the core — no human intervention or electrical power needed. The dark grey rods you see below slot between the orange fuel assemblies you explored on the previous page. This is fail-safe by design: the reactor shuts itself down.
The blue fuel rods (with orange pellets inside) generate neutrons through fission. The dark grey control rods absorb neutrons when inserted. Use the slider to adjust position, or hit SCRAM to trigger an emergency shutdown!
FAIL-SAFE: In a real reactor, control rods are held UP by electromagnets. If power fails, the magnets release and gravity drops the rods — no power needed for shutdown!
Control rods are the most direct form of Control. In Module 3 you learned WHY they work (absorbing neutrons reduces k). Now you've seen HOW they're implemented — slotting between fuel assemblies to manage reactor power and provide emergency shutdown capability.
The Reactor Circuit
How heat becomes electricity
Now let's see how all these components — the fuel, moderator, coolant, and control systems — work together to generate electricity. In a pressurised water reactor, there are two separate water circuits that never mix.
📖 Quick Reference:
Primary Circuit: High-pressure water through the reactor • Secondary Circuit: Water/steam through the turbines • Steam Generator: Heat exchanger connecting the two
Primary Circuit: The closed loop of high-pressure water that flows through the reactor core. This water picks up heat from the fuel, reaching about 325°C. Powerful pumps circulate this hot water to the steam generator.
Steam Generator: A large heat exchanger containing thousands of small tubes. Hot primary water flows through the tubes; secondary water surrounds them. Heat transfers through the tube walls without the fluids ever mixing. This separation is crucial because the primary water may contain trace radioactivity.
Watch how heat flows from the reactor core through the steam generator to produce electricity. Use the control rods slider — they work like an accelerator: withdraw them (right) for more power, insert them (left) to slow down. See how flow rates, temperatures, and turbine speed all respond!
The primary water (orange) has been in contact with the reactor core and may contain trace radioactivity. By keeping it separate from the secondary circuit (blue/white), radioactivity stays contained within the reactor building, whilst clean steam powers the turbines.
Multiple Barriers
Defence in depth keeps radioactivity contained
You learned in Module 3 that fission creates radioactive fission products — and that these continue to decay, producing heat even after the chain reaction stops. Nuclear safety relies on the principle of defence in depth: multiple independent barriers, each capable of keeping these materials contained even if others fail.
Defence in Depth: A safety philosophy using multiple layers of protection. If one barrier fails, others remain to prevent radioactive release. Each barrier is independent — a failure in one doesn't automatically compromise the others.
Reactor Pressure Vessel (RPV): The massive steel vessel containing the reactor core and primary coolant. Typically about 20cm thick, it's designed to withstand extremely high pressures and temperatures, forming the second barrier to radioactive release.
Containment Building: A reinforced concrete structure (often with a steel liner) surrounding the reactor. Designed to withstand internal pressure from accidents and external events like aircraft impact. This is the final barrier preventing release to the environment.
Each barrier is independent. Even if fuel cladding fails on some rods, the radioactive material is still contained within the pressure vessel. If there's a leak in the pressure vessel, the containment building provides the final barrier. This layered approach means multiple things must go wrong simultaneously for any radioactive release.
This is the third C — Containment — implemented through multiple layers of protection. It's designed to ensure that even in a severe accident, radioactive materials stay where they belong.
Safety Measures
Three types of protection working together
Beyond the physical barriers, nuclear plants employ three types of safety measures working together. Understanding these helps you appreciate why nuclear is so heavily regulated — and why it's so safe.
Passive Safety
Relies on natural forces — physics and gravity — rather than human action or powered equipment.
- Negative thermal feedback (the physics you explored in Module 3)
- Gravity-fed water systems (no pumps needed)
- Natural convection cooling
- Control rods fall by gravity on power loss
Engineered Safety
Purpose-built equipment designed to respond to abnormal conditions automatically or on demand.
- Emergency core cooling systems
- Sensors and monitoring equipment
- Back-up diesel generators
- Battery power for essential systems
Administrative Safety
The human element — ensuring people know what to do and do it correctly, every time.
- Rigorous training programmes
- Detailed operating procedures
- Clear signage and communication
- Regular emergency drills
All three types work together. Passive safety — like the thermal feedback you simulated in Module 3 — provides the foundation that works even when everything else fails. Engineered systems add multiple layers of active protection. And administrative controls ensure people understand and maintain the safety systems.
Managing the chain reaction through control rods, moderation, and thermal feedback
Removing heat via the coolant, steam generator, and ultimate heat sink
Multiple physical barriers keeping radioactive materials isolated
🎮 Component Matching Game
Match each reactor component to its primary function. Click a component, then click its matching function.
Components
Functions
1. In a pressurised water reactor (PWR), what prevents the primary coolant water from boiling even at 325°C?
2. What is the primary purpose of fuel cladding in a nuclear reactor?
3. What happens during a SCRAM?
4. Why do PWRs have separate primary and secondary circuits?
5. Which of these is an example of passive safety?
🎯 Key Takeaways
What to remember from this module
1. From Science to Engineering
Module 3 taught you the science — fission, chain reactions, criticality. This module showed how that science becomes engineering: fuel assemblies, coolant circuits, and containment structures that turn nuclear energy into electricity.
2. Core Components
The reactor core contains fuel (uranium pellets in cladding), moderator (slows neutrons), coolant (removes heat), and control rods (manages the k-factor). Each component you learned about in Module 3 has a physical form in the reactor.
3. Multiple Barriers
Three independent barriers — fuel cladding, pressure vessel, and containment building — keep fission products contained. This defence in depth means multiple things must fail simultaneously for any release.
4. Three Safety Types
Passive safety (physics-based, like thermal feedback), engineered systems (equipment), and administrative controls (human procedures) work together. The physics you explored in Module 3 is the foundation of passive safety.
In Module 3, you learned about Control, Cooling, and Containment as principles. Now you've seen how they're implemented:
Control rods + moderation + thermal feedback manage the chain reaction and k-factor
Primary coolant + steam generator + emergency systems remove heat continuously
Cladding + pressure vessel + containment building keep fission products isolated
In the next module, we'll explore different reactor types — Magnox, AGR, PWR, and SMRs — and see how each design implements Control, Cooling, and Containment differently based on their historical development and engineering choices.