Nuclear Fusion – A Crash Course

Limitless carbon-free energy, the largest temperature gradients in the universe, international scientific collaboration. Doesn’t sound too bad, huh? If the words ‘nuclear fusion’ sound confusing (or worse, frightening) to you or you have a desire for the human race to exist beyond 100 years into the future, then you’re in the right place. It doesn’t matter if you can’t remember the different between your neutrons and protons or your fission and fusion – and what the hell is a tokamak? You’re in the right place. Welcome to your crash course on fusion.

The inside of a tokamak showing during plasma burning (left) and without plasma burning (right).

We all need energy. For cooking, travelling, heating, cooling and lighting our world, and for making everything you see around you. Fusion is the process that the sun uses to produce energy. Fundamentally, fusion occurs when a gas is heated until its atoms are moving so fast that when they collide with one another they merge (fuse) together, releasing energy as they combine from two atoms to one larger atom. That’s it, basically.

Its important to note that nuclear fusion is completely different to nuclear fission. Fission is the type of nuclear power that is currently powering your homes, whereas fusion is still being developed and proven as commercially viable. In fission we are splitting heavy atoms apart (normally uranium isotopes) whereas in fusion we are fusing light elements together.

Atomic Structure 101

Starting from the smallest level, an atom is the most fundamental sized component of an element. It consists of a central nucleus made up of protons (a type of positively charged particle) and neutrons (a type of particle with no charge) and is orbited by electrons (a negatively particle with approximately 2000 times less mass than a proton or neutron). Just like the heads of the people who think that vaccines contain microchips, most of the atom is empty space. To put into perspective just how much of everything is made of, well, nothing, if we had the element hydrogen, which has 1 proton and 1 neutron in its nucleus and the nucleus were the size of a basketball then its electron would be about the size of a golf ball and be 5 miles away!

Schematic of a Lithium atom. Notice it has 3 protons and 3 electrons so a charge of zero.

So hydrogen, H, has one proton. What if it had two? Well the proton number determines what type of element an atom belongs to. For example, helium (He) has two protons, lithium (Li) has 3 and so on throughout the periodic table. How about the number of electrons? Atoms are normally electrically neutral, so the number of positive protons is equal to the number of negative electrons. If atoms lose an electron they become positively charged and vice versa.

So the number of protons determines what element we have. An isotope however is an atom of an element with a different number of neutrons. For example, deuterium and tritium are both isotopes of hydrogen. So they each have 1 proton, but deuterium has 2 neutrons and tritium has 3. The number of neutrons in an atom can vary but it stays within a certain typical range otherwise they become too unstable and decay into other atoms.

The Periodic Table of the Elements. Each of the individual numbers represents the proton number and therefore a unique element.

States of Matter

Now, when I said ‘gas’ in our basic description of fusion I wasn’t strictly being true with you. As we know, solid, liquid and gas are all ‘states of matter’, i.e. different arrangements of molecules with different properties. For example, ice, liquid water and water vapour are the three states of water. There is a fourth state of matter called a plasma (actually, there’s many more which you can read about here, but those only exist in special conditions). We know that if we heat a liquid, it evaporates, producing a gas. Now if we continue heating that gas, the orbiting electrons will eventually gain so much energy that they will separate themselves from their parent atoms – this is called ionisation – leaving us with negatively charged free electrons and positively charged ions. Much in the same manner as the friend (normally me, the electron) that has had one too many jagerbombs on a night out and wanders off on a lone mission from the group (the parent nucleus).

Despite sounding pretty uncommon, plasmas are actually the most ubiquitous and abundant state of matter in the universe. Neon signs, the earths ionosphere and stars are all great examples. So now we have a plasma, great. But how do we get energy from that?

Ring Donuts and Tokamaks

Firstly, we need a way to confine this plasma. Enter the tokamak! A tokamak is a Russian acronym meaning “toroidal chamber with magnetic coils”. It is a torus (ring donut) shape that was first designed by Soviet researchers in the 1950’s as a more stable way to confine the plasma compared to other attempts, such as a stellarator.

Homer surrounded by ring (or torus) shaped donuts. A torus doesn’t have icing, of course.

Now because the ions and electrons within the plasma are charged we can control them with magnetic fields. This is because there exists a tight relationship between the electric and magnetic forces and this interaction is called electromagnetism. It is understood that a moving electric charge induces a magnetic field in its vicinity and also that a magnetic field can cause motion in a charged particle!

Now if you’re asking what causes these electromagnetic forces this happens, I could talk about electron spin and go on to describe more detail but I’m afraid that I can’t give you a satisfactory answer. These phenomena have been observed to exist. We can form equations governing them and make predictions about what might happen, but on a fundamental level we can’t say why. I can’t explain this as eloquently as Richard Feynman so I seriously recommend watching this short video. It’s honestly fascinating if you’re interested in science or just in thinking clearly.

Gang Signs and Magnets

Thumbs up? No really, make a thumbs up with your right hand. You’re about to learn Amperes right hand rule. This rule allows us to easily understand the relationship between the direction of a electric current and the magnetic field that it induces. If the electric current is passing through your thumb (from positive at the bottom of your first to negative at the tip of your thumb) then a magnetic field is produced in the direction your fingers are curled. Do not actually run a current through your thumb. This is a bad idea and you will get no fusion.

The central solenoid is a length of coiled wires with a current through them (a tightly coiled spring shape basically). Use the right hand rule again but this time the current is curling through your fingers (from hand to fingertips) and the magnetic field is in the direction of your thumb.

Now, stay with me on this. Two directions of magnetic field are required to keep the plasma in its place. One toroidal (the direction of the Equator around a globe) and one poloidal (the direction if you travelled directly from the North to the South pole). As you may be able to figure out, we require two directions of electric current for this: a current though poloidal wires will produce a toroidal magnetic field, and a current through toroidal wires will produce a poloidal magnetic field.

A schematic of the coils and field lines in a tokamak. Notice here that the magnetic coils have been named relative to the field that they induce. For example the toroidal field coil is a coil in the poloidal direction that produces a field in the toroidal direction. Taken from ITER.

And now, energy?

Right. We’ve confined our hot (like super hot, 150 million degrees celsius hot in the future ITER tokamak) plasma with our magnetic fields and can now reap the rewards. Once we’ve heated the plasma to a sufficient temperature, the nuclei of deuterium and tritium (two isotopes of hydrogen) can fuse together into a helium nucleus and release a neutron and energy that we can turn into electricity by using the standard steam cycle. In other words, the heat generated via the fusion process is used to evaporate water into steam and drive a turbine. I’ve always found it a bit comical that regardless of the sophistication of the method to produce the heat, we still just boil water to get electricity. Underwhelming but simple and effective.

Now we could go into more detail about how when the deuterium and tritium fuse we actually get energy out, as well as more detail on how that heat is captured by the tokamak but I’ll save that for a future post. I feel like it will be more valuable to satisfy a couple of concerns you may have:

A typical day in the office… Remember this is a fission plant, not a fusion plant! It’s also a cartoon.

Addressing the ‘Dangers’

What if it explodes? If something goes wrong, say the current through the magnets stops and the confinement of the plasma is lost, then the plasma will just expand slightly, cool and the fusion will stop (while producing some significant damage to the tokamak walls). In other words, it is inherently impossible for there to be a runaway reaction that you might imagine with a fission plant leading to a explosion.

What about all the radioactive waste? There is no radioactive waste. The components that are close to the plasma will become radioactive after exposure (which makes maintenance difficult without the use of cool robots) but for only several thousands of years compared to millions of years for radioactive waste from fission reactors.

Do we have sufficient fuel? Yes and no. Deuterium is abundant in water as heavy hydrogen. Tritium is more scarce, the ITER experiment will use up 90% of the worlds stockpile during its operation. Thankfully the neutrons emitted from the fusion process can produce tritium through the induced fission of lithium-6 in components called tritium breeding blankets. These blankets are within the fusion reactor itself so in this way the fusion process basically creates its own fuel. Pretty cool right?

So there we have it. I hope this has shone some light on what all the hype is about. Thanks for reading and let me know what you think or if you have any questions! 🙂

Some other cool info for you to check out:

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