Nuclear fusion is, to put it simply, smashing very light atoms together to make heavier ones. In the process they release a large amount of energy which we can harness for electricity.
In principle this sounds straightforward, but in practice the technological hurdles we have to jump are quite high. Fusion occurs naturally all across the universe every star in the sky, including our own, is a giant fusion reactor. All life on Earth is a result of sunlight, and therefore a result of fusion. But what actually is fusion?
All matter in the universe is made up of atoms, incredibly small in size. These in turn are made up of protons and neutrons which form a nucleus, and electrons which orbit at a distance. The lightest atom of all is hydrogen-1. The “1” simply means that it has a mass of one atomic unit because it consists of just one proton and electrons have negligible mass. Uranium-238, on the other hand, is a heavy metal and has a mass of 238 atomic units (92 protons and 146 neutrons).
Atoms are unstable to some extent and will always try to become more stable. The most stable element of all is Iron-56, so heavy atoms like uranium-238 will split up to get closer to it, whilst light atoms like hydrogen-1 must join together to do so. Whenever an atom moves closer to Iron-56 it releases energy, as described by Einstein’s famous equation E=MC^2. Fission, the most common nuclear reaction on Earth, uses the splitting up method to release energy, whilst fusion uses the joining up approach.
We could try and use hydrogen-1 atoms, but they only consist of protons and all elements heavier than it require neutrons. Luckily, a form of hydrogen with one neutron exists called deuterium-2, and another with two neutrons called tritium-3. These can fuse into helium-4. Notice though that the masses don’t quite add up; two and three make five. This means that one of the neutrons is not needed and is released. Since we now have all the building blocks for heavier atoms deuterium and tritium seem good ingredients to use.
This all sounds good so far, all we have to do then is get some deuterium-2, and make it join up with tritium-3 atoms to make helium-4 and release energy! Well, yes, but it’s not easy. The nucleus of an atom will try to repel other nuclei if they get close due to their electric charge. Luckily, the strong nuclear force that holds nuclei together will take over and pull them together over extremely short distances. The trick is getting them close enough.
To increase the chances of them colliding we have to get them moving very fast indeed. We do this by heating them up – nice and warm at a toasty 25 million degrees. At this temperature the atoms lose their outer electrons and form a conductive fluid called plasma.
To heat up the plasma an electric current is run through it. The plasma is confined to a donut-shaped reactor called a tokamak by powerful magnetic fields, and because the plasma forms a closed circle it acts like a coil – it is effectively a giant transformer. Radio waves can be used to heat the plasma even further. If the plasma is kept hot enough, fusion will occur. Hopefully, the moment fusion occurs it produces more heat and if a chain reaction starts the heat from fusion is enough to keep the reactor running by itself. The reactor is now self-sustaining, and as long as sufficient fuel is available it will keep producing more and more heat.
This heat can be absorbed by a cooling system and used to heat water, driving steam turbines in much the same way as any conventional power plant. Once a reaction is self-sustaining it produces far more power than is put in to contain the plasma, to the point where a handful of fusion reactors could solve a country’s energy needs. Not only this, but the reactor’s only waste product is helium-4, completely harmless to life and great for inflating children’s balloons.
At present, fusion reactors that “break even” and make the same amount of power they need to run are relatively easy to build and exist all across the world, but a reactor that produces net power output is yet to be built. This is due to that fact the technology to cope with such intense temperatures has only recently been developed, although the first power generating plant, DEMO, is expected to be ready within the next 15 years.
The dream of cheap, clean energy is within our grasp, and the time is right to be pursuing it.