Chain Reactions and Reactors

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 7.20 — How a chain reaction is set up
• 7.21 — The role of control rods and the moderator
• 7.22 — The role of shielding around a nuclear reactor

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

One fission needs one neutron and produces two or three. That is the whole idea. Each of those neutrons can be absorbed by another uranium-235 nucleus and cause another fission, which produces two or three more, and so on. This self-sustaining sequence is a CHAIN REACTION, and it is what turns a single nuclear event into a source of power.

Left alone, the numbers are alarming. If every fission caused two more, one would become two, then four, eight, sixteen — and after eighty steps, which would take a small fraction of a second, essentially all the fuel would have gone at once. That is an uncontrolled chain reaction, and it is a bomb. A reactor exists to prevent exactly this while keeping the reaction going.

Think of it like a conversation that has to be kept at exactly one reply each

Picture a room where everyone who is spoken to must reply to two or three other people. If that is allowed, the room goes from one quiet remark to a deafening roar in seconds, and nothing can stop it. If instead most people are asked to stay silent, so that on average each remark produces exactly ONE reply, the conversation continues indefinitely at a steady volume. If fewer than one reply gets through, it dies out. A reactor is that room, and the control rods are the instruction to stay quiet. Notice that the target is not zero and not as many as possible — it is exactly one, which is a much harder thing to hold.

So the CONTROL RODS do one job: they absorb neutrons, and how many they absorb is adjusted by how far they are lowered into the reactor. Lowering them further absorbs more neutrons and slows the reaction; raising them absorbs fewer and speeds it up. They are made of boron or cadmium, materials that absorb neutrons readily. The reactor is held at the setting where each fission causes, on average, exactly one further fission — so the reaction is steady rather than growing or dying. Dropping the rods fully in shuts the reactor down.

The MODERATOR does a completely different job, and this is the distinction that costs most marks in the topic. The neutrons released by fission are travelling extremely fast, and a fast neutron is very poor at being absorbed by uranium-235 — it tends to pass straight through. The moderator, usually water or graphite, is there for the fast neutrons to collide with. Each collision transfers some kinetic energy away, and after many collisions the neutrons are travelling slowly. SLOW NEUTRONS ARE FAR MORE LIKELY TO BE ABSORBED and cause fission, so without a moderator the chain reaction would not sustain itself at all.

The two jobs, kept apart

1. Ask what is being changed. The CONTROL RODS change HOW MANY neutrons there are, by absorbing some and removing them from the process.
2. The MODERATOR changes HOW FAST the neutrons are travelling, by slowing them through collisions, and absorbs almost none of them.
3. So the control rods regulate the RATE of the reaction and can stop it entirely.
4. The moderator makes the reaction POSSIBLE in the first place, and it is not adjusted during operation.
5. A test for any answer: if it says the moderator slows the reaction DOWN, it is wrong. The moderator slows the NEUTRONS, which speeds the reaction UP by making absorption more likely.

That last point is the trap in one sentence, and it comes from the word 'slows' doing two different jobs. The moderator slows NEUTRONS; the control rods slow the REACTION. An answer that says the moderator slows the reaction down has reversed its purpose completely — remove the moderator and the reaction stops, which is the opposite of what a brake does.

SHIELDING is statement 7.22, and it protects the people outside. The reactor core is surrounded by very thick concrete, often several metres of it, sometimes with steel and lead. Its job is to absorb the intense gamma radiation and the neutrons escaping from the core, both of which would otherwise be lethal to anyone nearby. Concrete is used because it is a good absorber, can be made as thick as required, and is cheap enough to use in the quantities needed. The shielding is not part of controlling the reaction at all — it does nothing to the chain reaction and only stops what escapes.

Putting the whole station together: fission in the core releases kinetic energy, which heats the fuel rods and the coolant passing through the core. The hot coolant is used to boil water into steam, the steam turns a turbine, and the turbine turns a generator, which is the machine from Topic 6. Everything after the core is an ordinary power station — the only unusual part is the heat source.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Chain reaction

A self-sustaining sequence of fissions, in which neutrons released by one fission go on to be absorbed by other nuclei and cause further fissions.

The parts of a reactor, and what each is for

Fuel rods

Contain the uranium-235 in which fission takes place.

Moderator

Water or graphite. Slows the fast neutrons by collision, because slow neutrons are far more readily absorbed.

Control rods

Boron or cadmium. Absorb neutrons; raised and lowered to set the rate of the reaction.

Coolant

Carries the heat away from the core to boil water into steam.

Shielding

Metres of concrete absorbing gamma radiation and neutrons that escape the core.

Control rods against the moderator — the distinction most often lost

How a chain reaction is set up and held steady

1. A neutron is absorbed by a U-235 nucleus, which undergoes fission.
2. Two or three neutrons are released, travelling very fast.
3. The moderator slows these neutrons through repeated collisions, because slow neutrons are far more readily absorbed.
4. The control rods absorb some of the slowed neutrons, and how many is set by how far they are lowered.
5. The rods are positioned so that on average exactly ONE neutron from each fission goes on to cause another, giving a steady reaction.
6. Lowering the rods fully absorbs enough neutrons to shut the reactor down.

Statement 7.22 — shielding

• Several metres of thick CONCRETE surround the core, sometimes with steel and lead
• It absorbs the intense GAMMA radiation and the NEUTRONS escaping from the core
• Both would otherwise be lethal to anyone working nearby
• Concrete is used because it absorbs well, can be made any thickness, and is cheap in bulk
• Shielding plays NO part in controlling the reaction — it only stops what escapes

From fission to electricity

• Fission in the core releases energy, mostly as kinetic energy of the fragments
• This heats the fuel rods and the coolant passing through the core
• The hot coolant boils water into steam
• The steam turns a turbine, which turns a generator
• Everything after the core is an ordinary power station — only the heat source is unusual

Model answer [4 marks]

Explain the difference between the role of the control rods and the role of the moderator in a nuclear reactor. [4]

The control rods absorb neutrons, removing them so that they cannot go on to cause further fissions, and they are raised and lowered to adjust how many are absorbed and therefore how fast the reaction proceeds. The moderator does not absorb neutrons but slows them down, because the neutrons released by fission travel too fast to be readily absorbed by uranium-235. Slow neutrons are much more likely to be absorbed and cause fission, so the moderator makes the chain reaction possible in the first place. The control rods therefore regulate the rate of the reaction, while the moderator is what allows it to be sustained at all.

Model answer [5 marks]

Explain how a chain reaction is set up and how it is kept steady rather than growing. [5]

When a uranium-235 nucleus absorbs a neutron it undergoes fission and releases two or three further neutrons. Each of these can be absorbed by another uranium-235 nucleus and cause a further fission, so the reaction sustains itself. If every fission caused two or three more, the number of fissions would multiply extremely rapidly and the reaction would run out of control. Control rods made of boron or cadmium are therefore lowered into the core to absorb some of the neutrons. They are positioned so that on average exactly one neutron from each fission goes on to cause another fission, which keeps the rate of the reaction constant.

Not this: The moderator slows the reaction down, and the control rods speed it up.

This: Exactly backwards. The moderator slows the NEUTRONS, which makes them far more likely to be absorbed, so it makes the reaction possible — remove it and the chain reaction stops. The CONTROL RODS are what slow the reaction, by absorbing neutrons.

Mark-losing trap. Control rods change HOW MANY neutrons. The moderator changes HOW FAST they go.

Mark-losing trap. The moderator slows NEUTRONS, not the reaction — slow neutrons cause MORE fission, not less.

Mark-losing trap. A steady reactor means exactly ONE neutron per fission goes on to cause another.

Mark-losing trap. Shielding protects people outside and does nothing to control the reaction.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is the purpose of the control rods in a nuclear reactor?
2. Grade 7 · Explain [2 marks] — Explain why a chain reaction is possible at all when a uranium-235 nucleus undergoes fission.
3. Grade 8 · Explain [3 marks] — A student writes that the moderator 'slows the reaction down so the reactor does not overheat'. Explain what is wrong with this.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of how a nuclear reactor sustains a chain reaction and keeps it steady.
5. 9+ · Analyse [6 marks] — An engineer proposes a reactor with no control rods, arguing that the reaction can be regulated entirely by adding or removing moderator, since 'the moderator decides how many neutrons get slowed down and therefore how many fissions happen'. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who directed the first uncontrolled chain reaction ever built, and the one who worked out which uranium actually splits. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

J. Robert Oppenheimer — the one who directed the first uncontrolled chain reaction ever built

This page is about holding a chain reaction at exactly one neutron per fission, and Oppenheimer directed the project that deliberately did not. He is worth asking because he understood both the physics and what it would mean: he ran Los Alamos with complete commitment, watched the Trinity test, and then spent the rest of his career arguing against the weapons he had helped create, losing his security clearance for it in 1954. He is the right figure for the question a reactor page raises and cannot answer — what the difference between a power station and a bomb actually consists of, and who decides which gets built.

• “What is the physical difference between a reactor and a bomb?”
• “What happens if nothing absorbs the surplus neutrons?”
• “What did you think watching the Trinity test?”
• “Should the scientists have refused?”
• “Why did you argue against the weapons afterwards?”

Niels Bohr — the one who worked out which uranium actually splits

Natural uranium is over 99% uranium-238, and a reactor runs on the fraction of a per cent that is uranium-235 — a fact that shapes everything from enrichment to non-proliferation. Bohr worked out why: applying the liquid-drop model, he showed in 1939 that it is the rare U-235 that fissions with slow neutrons, while the abundant U-238 mostly absorbs them without splitting. He is the right second figure here because that result explains why the moderator matters, why the fuel has to be enriched, and why fission was so much harder to use than to discover.

• “Why does uranium-235 split when uranium-238 does not?”
• “Why do slow neutrons cause fission better than fast ones?”
• “What does the liquid-drop picture actually predict?”
• “Why does the fuel have to be enriched at all?”
• “How quickly did you see what fission would lead to?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against J. Robert Oppenheimer in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Nuclear Fission
• Next lesson: Nuclear Fusion
• Atomic Structure and Isotopes — A nucleus ten thousand times smaller than the atom around it, and why two atoms of the same element can weigh different amounts.
• Alpha, Beta and Gamma — Three emissions from unstable nuclei, arriving at random — and why the one that is easiest to stop is the one to be most careful with.
• Nuclear Equations — What each kind of decay does to the numbers — and why losing a negative particle makes the charge go up.
• Detecting Radiation and Background — How you measure something invisible, and why the counter clicks even with no source in the room.
• Half-Life — Half of what is left, every time — which is why a radioactive sample never quite runs out.
• All of Physics · Incandio Science