Nuclear Fusion

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.

• 7.23 — The difference between nuclear fusion and nuclear fission
• 7.24 — Fusion as the creation of larger nuclei with a loss of mass and a release of energy
• 7.25 — Fusion as the energy source for stars
• 7.26 — Why fusion does not happen at low temperatures and pressures

1 · Understand it

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

FUSION is the joining of two small nuclei to make a larger one. It is the opposite operation to fission, which splits a large nucleus into two — and, as the last lesson established, both release energy, because both move matter towards the most stable nuclei in the middle of the range.

The mechanism for the energy is exactly the one from fission. Weigh the nucleus produced and it is slightly LESS massive than the two nuclei that went into it. That missing mass has become energy, at the rate set by the square of the speed of light. Fusion releases more energy per kilogram of fuel than fission does, which is why it is worth the extraordinary difficulty of achieving it.

In the Sun, the process joins hydrogen nuclei together, and through a series of steps four hydrogen nuclei become one helium nucleus. About 600 million tonnes of hydrogen fuse every second, and roughly 4 million tonnes of MASS disappears each second in the process, which is where the sunlight comes from. The Sun has been doing this for about 4.6 billion years and has enough hydrogen for roughly as long again.

Now statement 7.26, which is the real content of the page. Two nuclei are both POSITIVELY charged, and like charges repel. Bring two hydrogen nuclei towards each other and they push each other apart, harder and harder the closer they get. To fuse, they must get close enough for the strong nuclear force to take over — and that force only acts over a distance comparable with the size of a nucleus itself. So they must be driven right up against one another against a repulsion that is screaming at them not to.

Think of it like pushing two magnets together the wrong way round

Hold two bar magnets with their north poles facing and push. Close up, the force fighting you becomes enormous, and if you push gently they simply slide apart. To get them to touch you have to move fast and hard — slam them together so quickly that they are in contact before the repulsion can turn them away. Now imagine having to do that with a hundred million pairs at once, without hands. That is fusion, and it explains both requirements at a stroke. TEMPERATURE is how fast the nuclei are moving, so an enormous temperature is how you get the speed to overcome the repulsion. PRESSURE is how tightly packed they are, so an enormous pressure is how you get enough near-misses per second for any of them to succeed.

Why both a high temperature and a high pressure are needed

1. Both nuclei are positively charged, so there is a strong electrostatic REPULSION between them.
2. The repulsion grows rapidly as they approach, and they must get extremely close before the strong nuclear force can pull them together.
3. A VERY HIGH TEMPERATURE means the nuclei are moving extremely fast, so they have enough kinetic energy to overcome that repulsion and get close enough.
4. A VERY HIGH PRESSURE means they are packed extremely densely, so collisions happen often enough for fusion to occur at a useful rate.
5. At everyday temperatures the nuclei simply do not have enough energy — they approach, are repelled, and turn away without ever getting close enough. So fusion does not happen at all.

In a star, both conditions are supplied by GRAVITY. The enormous mass of the star pulls its material inwards, compressing the core to a colossal pressure and heating it to about 15 million degrees at the centre of the Sun. Nothing on Earth is heavy enough to do this, which is why fusion is easy for a star and extremely hard for us.

It also explains why the Sun is stable. Fusion in the core pushes outwards; gravity pulls inwards; and the two balance. If fusion were to speed up, the core would expand, which lowers the pressure and slows the fusion back down. If it slowed, the core would contract, which raises the pressure and speeds it up again. That self-correcting balance is why a star can burn steadily for billions of years, and it is the subject of the next topic.

Fusion reactors on Earth are being built and none yet produces net power. The difficulty is not the physics but the containment: no material can touch a gas at a hundred million degrees without vaporising, so the fuel has to be held away from the walls by magnetic fields, and holding it there long enough and densely enough has taken decades and is not finished. The attraction is considerable — the fuel is hydrogen from water, and the product is helium, so there is no long-lived radioactive waste of the kind fission produces.

Comparing the two processes on the numbers that matter

State three ways in which fusion differs from fission, and explain why fusion is harder to achieve despite releasing more energy per kilogram.

1. ONE: fusion JOINS small nuclei; fission SPLITS a large one.
2. TWO: fusion uses light fuel such as hydrogen; fission uses heavy fuel such as uranium-235.
3. THREE: fusion produces helium and no long-lived radioactive waste; fission produces radioactive daughter nuclei that remain hazardous for thousands of years.
4. The difficulty: fusion requires the nuclei to overcome their mutual electrostatic repulsion, which needs temperatures of millions of degrees and enormous pressures.
5. Fission needs no such conditions, because the neutron that triggers it is UNCHARGED and is not repelled at all — it walks in at room temperature.

Answer: Fusion joins rather than splits, uses light rather than heavy fuel, and leaves no long-lived waste — but the charged nuclei must be forced together, while fission's neutron is not repelled at all.

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 · Nuclear fusion

The joining together of two small nuclei to form a larger nucleus. The mass of the nucleus formed is slightly less than the total mass of the nuclei that fused, and the missing mass is released as energy.

The vocabulary of this page

Electrostatic repulsion

The pushing apart of two positive charges. It is what two nuclei must overcome before they can fuse.

Strong nuclear force

The attraction that binds nucleons together. It acts only over distances about the size of a nucleus, which is why the nuclei must be forced so close.

Plasma

The extremely hot gas of separated nuclei and electrons in which fusion takes place, held away from the walls by magnetic fields.

Fusion against fission — statement 7.23

Statement 7.26 — why fusion needs extreme conditions

1. Both nuclei are positively charged, so they repel each other electrostatically.
2. The repulsion increases rapidly as they approach.
3. They must come extremely close before the strong nuclear force can act and hold them together.
4. A very high TEMPERATURE gives the nuclei enough kinetic energy to overcome the repulsion.
5. A very high PRESSURE packs them densely enough for collisions to be frequent.
6. At ordinary temperatures the nuclei are repelled long before they are close enough, so fusion does not occur.

Fusion in stars — statement 7.25

• Hydrogen nuclei fuse, and through several steps four hydrogen nuclei become one helium nucleus
• GRAVITY supplies both conditions: the star's mass compresses the core and heats it to about 15 million degrees
• About 600 million tonnes of hydrogen fuse each second in the Sun, and about 4 million tonnes of MASS disappears
• Fusion pushes outwards and gravity pulls inwards, and the balance between them keeps the star stable
• The Sun has been fusing for about 4.6 billion years and has roughly as long again

Why we cannot yet do it — and why we keep trying

• No material can contain a gas at a hundred million degrees, so the fuel must be held by MAGNETIC FIELDS away from the walls
• Holding it densely enough and long enough for net energy gain has taken decades and is not finished
• The fuel is hydrogen obtainable from water — effectively unlimited
• The product is helium, so there is no long-lived radioactive waste
• There can be no runaway chain reaction, because the reaction stops the moment the conditions fail

Model answer [4 marks]

Explain why nuclear fusion does not occur at low temperatures and pressures. [4]

Both nuclei taking part are positively charged, so there is a strong electrostatic repulsion between them, which increases rapidly as they approach one another. They must be brought extremely close together before the strong nuclear force can act and hold them together. A very high temperature is needed so that the nuclei are moving fast enough to have sufficient kinetic energy to overcome this repulsion, and a very high pressure is needed so that they are packed closely enough for collisions to happen frequently. At low temperatures the nuclei do not have enough energy to get close enough, so they are simply repelled and fusion does not occur.

Model answer [5 marks]

Explain how the Sun releases energy, and why it has remained stable for billions of years. [5]

In the core of the Sun, hydrogen nuclei fuse together to form helium nuclei. The mass of the helium formed is slightly less than the total mass of the hydrogen nuclei that fused, and this missing mass is released as energy. The conditions required are supplied by gravity: the enormous mass of the Sun compresses its core to a very high pressure and heats it to around 15 million degrees, which is enough for the nuclei to overcome their mutual repulsion. The Sun is stable because the outward push from fusion in the core is balanced by the inward pull of gravity. If fusion sped up, the core would expand and cool slightly, which would slow the fusion again, so the balance is self-correcting.

Not this: Fusion is dangerous in the same way as fission, because a fusion reactor could run out of control.

This: A runaway fusion reaction is impossible on Earth. Fusion needs conditions so extreme that they must be actively maintained, so any fault causes the conditions to fail and the reaction stops instantly. There is also no long-lived radioactive waste, because the product is helium.

Mark-losing trap. Fusion JOINS small nuclei; fission SPLITS a large one. Both release energy — that is not a contradiction.

Mark-losing trap. The reason fusion needs extreme conditions is ELECTROSTATIC REPULSION between two positive nuclei.

Mark-losing trap. Fission needs no special conditions because its trigger, the neutron, is UNCHARGED and is not repelled.

Mark-losing trap. In a star it is GRAVITY that supplies the temperature and pressure — which is why stars must be enormous.

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 happens to two small nuclei during nuclear fusion?
2. Grade 7 · Explain [3 marks] — Explain why nuclear fusion requires an extremely high temperature.
3. Grade 8 · Explain [3 marks] — Explain why fission can be made to happen at ordinary temperatures but fusion cannot.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of how the Sun produces energy and why it remains stable.
5. 9+ · Evaluate [6 marks] — A commentator writes: 'Fusion releases more energy per kilogram than fission and produces no long-lived waste, so fusion power stations should have replaced fission ones decades ago. The only reason they have not is a lack of funding.' Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who supplied the accounting that explained why the Sun is old, and the one who did the same mass-into-energy calculation for the other process. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Albert Einstein — the one who supplied the accounting that explained why the Sun is old

Before mass could be converted to energy, the Sun was a genuine crisis in physics. Kelvin calculated that if it were powered by gravitational contraction — the best available idea — it could be at most a few tens of millions of years old, and he used that to argue against Darwin, who needed far longer for evolution. Geology said hundreds of millions of years; physics said no. Einstein's relation of 1905 dissolved the problem by making an entirely different energy source possible, one that could run for billions of years on a fuel supply the Sun plainly had. He is the right figure to ask how a single equation settled an argument between two sciences.

• “How can the Sun keep going for billions of years?”
• “Why did the age of the Sun cause such an argument?”
• “How can mass simply turn into energy?”
• “Did you expect the equation to explain the stars?”
• “What is actually lost when four hydrogen nuclei become helium?”

Lise Meitner — the one who did the same mass-into-energy calculation for the other process

Meitner is the person who first carried out, for fission, exactly the calculation this page describes for fusion: work out the mass of everything before, the mass of everything after, and convert the difference. She is the right figure for statement 7.23 because she can explain what at first looks like a contradiction — that opposite processes both release energy — from the inside of having done the arithmetic. The answer is that both move material towards the most stable nuclei in the middle of the range, and it is the position of the fuel rather than the direction of the process that decides.

• “Why do splitting and joining both release energy?”
• “How do you calculate the energy from a change in mass?”
• “What makes some nuclei more stable than others?”
• “Which releases more per kilogram, and why?”
• “Would you have worked on fusion, given the chance?”

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 Arthur Eddington 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: Chain Reactions and Reactors
• Next lesson: The Universe and Gravity
• 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