How does nuclear fusion actually work and why is it so hard when we already have nuclear fission?
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Okay, so.
Fission is ‘easy’ because the radioactive materials that are used for fuel are already unstable and will easily break down to more stable materials, releasing the energy needed.
Fusion is a whole other ballgame.
It’s forcing two atoms together into one atom.
Fusion is ‘easy’ in the heart of a star because the insane amounts of pressure within the star’s interior. With that kind of pressure, you don’t need insane amounts of energy to force the atoms to fuse.
We can’t produce those levels of pressure, so we have to compensate by increasing the energy. We ‘can’ do it, but it takes a lot of energy and the whole point is to create a situation where we’re getting substantially more energy out of the fusion than we’re putting in.
And we’re not there yet.
Fission works with big atoms that don’t want to hold together, so they’re easy to split.
Fusion works with small atoms that really don’t like each other, so it takes extreme environments to get them to combine, like stars or particle accelerators.
Controlled nuclear fusion requires enormous temperature and pressure. You have to re-create the conditions of the inside of the sun, inside a laboratory.
This isn’t like fission. In fission, your material is already unstable. It wants to break down. All you have to do is coax it along to break down at the right rate.
In fusion though, you have to crush together very stable atoms. That’s why large temperatures and pressures are required.
Fission is like a stone at the top of a hill. The stone will roll down naturally. If you gather enough stones at the top of a hill, then the main problem is controlling it so that not too many stones roll down at the same time. But while that control is not simple, the rolling down itself is “natural”. Once you gather enough stones at the top of a hill (isolating fissionable material), the fission itself is self sustaining (ie once it starts, it will continue)
Fusion is like having a lot of stones at the bottom of a steep hill. To even start making power, a lot of effort is needed to push the stones to the top of the hill. And if you don’t push enough stones to the top, there won’t be enough usable energy. The hill in this case is a lot of pressure and a very high temperature. Unless there are conditions like the core of a star – fusion is not self sustaining.
Fusion works by combining small atoms to make bigger ones. We know how to do it, not only can we see the Sun do it but we can use fission devices and lasers to cause it to happen.
The problem is controlling it. The Sun is super large, so we can’t do it that way, and it’s super dangerous, so we’re fortunately 93M miles away.
The fission pumped scheme works, that’s how hydrogen bombs work, but it’s not controllable. Using atomic bombs as a power source isn’t really something you want.
The laser scheme is safe, which is good, but it is being done on a very tiny scale – to avoid the possibility of accidents. It’s never going to be a good idea to make a hydrogen-bomb sized fusion reactor – what if it blew up.
So, we have super tiny and safe schemes that we’re trying to scale up safely. That’s going to take a very long time, because we’re already using pretty exotic stuff like superconducting magnets to control the super tiny version. The levels of magnetic field required to keep a power-plant sized reaction under control are difficult to achieve.
Nuclear fission and fusion are actually opposites. Nuclear fission is when the nucleus of an atom is split. This is what occurs in power plants and atomic bombs. Fusion is when two nuclei are combined. This is what occurs in stars and hydrogen bombs.
Fusion is a challenge because it requires such insanely high temperatures (around the temp of the core of the Sun) to drive the fusion reactions.
Fusion works by having the nuclei of two atoms combine together. For small light atoms (lighter than iron), this releases more energy than it takes to cause the fusion.
The problem is that it is very hard to get a large number of atoms to fuse continuously. It is fairly easy to get a tiny handful of atoms to fuse in a laboratory, or to get a fairly large number to fuse for a split second. Getting lots of atoms to fuse in a sustained reaction has proved much harder than anyone initially expected. The only way we have found to reliably cause fusion is to create really high temperatures and pressures. It is very hard to maintain temperatures and pressures this high, especially when there is fusion occuring inside the high temperature plasma, as it really wants to expand and cool down. It is so hot that you can’t contain it in something like a high pressure tank, because it would just melt the tank. So they have to use magnetic fields to trap the high temperature fusion reaction in a sort of force field. But creating a stable magnetic force field is really, really hard.
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