What is nuclear fusion, how does it differ from nuclear fission, and why is it all the rage for clean energy? Tom explains.
Featuring Tom Merritt.
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Episode transcript:
I saw back to the future and the car ran on banana peels in a thing called Mr. Fusion
So where are our fusion reactors?
I heard we figured it out or something. No?
Confused?
Don’t be. Let’s help you Know A little more about nuclear fusion.
To solve… everything… we need more cheap clean energy. Yes there’s wind and solar and geothermal and lots of other options. But one dream that physicists particularly love is harnessing the power of nuclear fusion.
Nuclear fusion is the process of combining two atoms. If the atoms are light enough the process releases energy, called an exothermic reaction. If the atoms are heavy, the process retains energy, called endothermic. Exothermic means the energy goes out and endothermic means the energy stays in. So if you to generate power you want exothermic. And for an exothermic reaction you can fuse two light elements together. The fusion of light elements is what most people mean when they refer to fusion.
Two nuclear forces are important to understand why nuclear fusion of light elements makes power.
The nucleus of an atom is made of protons and neutrons. The force that keeps them bound together is called, creatively enough, the nuclear force.
There’s another force called the Coulomb force, named not after Columbo, but rather after French Physicist Charles Augustin de Coulomb who published three reports on electric and magnetism in 1785. In those reports he stated Coulomb’s law that stated that “the repulsive force that the two balls — [that were] electrified with the same kind of electricity — exert on each other, follows the inverse proportion of the square of the distance.” Basically the force derives from the charges in the two things multiplied by each other and gets stronger the closer they are. Except he had math to help you determine exactly how much that is.
So the nuclear force keeps neutrons and protons together in the nucleus. And Coulomb’s force repels like charges. Protons are positively charged so they normally will repel each other. If you keep protons close together in a nucleus, the nuclear force keeps them together, but pull them apart and the Coulomb force will cause them to split apart.
In elements smaller than iron and nickel, there aren’t many protons. So when you push two nuclei together the protons will resist but since there aren’t many of them they can get close enough that the nuclear force overcomes Coulomb. In fact there’s more nuclear force than you need, so some of it gets released as energy.
This is how the sun generates energy. It fuses together hydrogen atoms, which have one proton, turning them into Helium which has two protons.
Now that nuclear force is short range. The Coulomb force is infinite range. It takes a lot of energy to get two hydrogen atoms to get close enough so that their nuclei are so near that the nuclear force overcomes the Coulomb force and fuses them together. But it’s worth it because more energy is released as a result of the fusion than it takes to get the two atoms close together. Given the right conditions that extra energy can be used to force other hydrogen atoms into each other and so on and so on in a sustained reaction.
That’s what’s going on in the sun. And there’s enough hydrogen around that it will probably go on for around 5 billion years.
Great. So we just need to recreate the conditions of the sun. Heck we don’t even need a fraction of 5 billion years worth of energy so it can be a very small sun. Which is probably smart since a big sun would cause all kinds of problems.
The only obstacle is how to get it started. The sun did this by collapsing a giant cloud of spinning dust under its own gravity. Hard to replicate, especially without destroying the Earth. While it’s a tough problem it’s worth exploring.
Why? Well it doesn’t emit many byproducts. No CO2 at all. And it has a lot fewer downsides than its opposite, nuclear fission. Fission power works by splitting atoms apart to release energy. It also releases lots of radioactivity and radioactive waste. Because when you pull apart those protons a lot of other stuff leaves with it. Fusion generates much less radioactivity and very little nuclear waste. Your pushing things together not pulling them apart. So you do end up with some helium but that’s kind of about it.
And fuel is pretty easy to get. The whole planet is covered in hydrogen, albeit bound with oxygen in the form of water. And fusion fuel is energy dense. A cup of hydrogen could potentially power a house for a hundred years.
OK so its worth doing. How do we do it.
The big obstacles are getting the temperature pressure and duration needed to get the reaction going, and then the next obstacle is sustaining it long enough to generate enough power to start other reactions. The challenge with sustaining the reaction is that the neutrons released in fusion sometimes degrade the equipment you’re using to make the fusion happen. And the ways we use to create the energy to ignite the fusion sometimes destroy the equipment. Little things like that.
But we’re making progress.
There are two main approaches showing promise: the tokamak design and the internal confinement fusion or ICF design. There is also magnetized target fusion and inertial electrostatic confinement, but the majority of the work at the moment is being done on tokamak or ICF.
The tokamak uses a magnetic field to confine hydrogen plasma in a torus shape. A torus is a donut shape. The kind of donut with a hole in it. It’s donut power. Homer Simpson by rights should have worked at a fusion reactor. But I digress.
Essentially in a tokamak reactor, a set of magnetic coils and a central magnet (a solenoid to be precise) create opposing fields that result in a twisted magnetic field that confines the hydrogen plasma. Twist the plasma enough and it fuses.
Internal confinement fusion or ICF, puts hydrogen in a pellet about the size of a pinhead. The outside of the pellet is bombarded with high energy beams. It could be photons, electrons, or ions. Most ICF reactors these days use lasers. The beam heats the outside of the pellet enough that it explodes both outward but also accelerating inward compressing the hydrogen and triggering fusion.
OK so we have some ideas. Let’s give a progress report.
We’ve been working on nuclear fusion for awhile. The first patent for a fusion reactor was registered in 1946 by the United Kingdom Atomic Energy Authority. And fusion ignition was first achieved just a few years later! In 1952! That’s a good pace right?
Just a couple of snags. One was that it used nuclear fission to start the reaction, so that kind of negated the benefits of fusion over fission. And two it was a bomb. So it kind of just blew things up rather than actually “powering” things. The bomb was called Ivy Mike though. So that was cute.
Much more useful for power generation was the first *controlled* fusion reaction using the Scylla 1 machine at Los Alamos National Laboratory in 1958. It technically created fusion but not for long, not efficiently and not at scale.
And that was the story of fusion power for decades. It’s always just a few years off. But there’s always a little progress and by the mid 2000s, momentum built Tokamak and ICF designs.
That sort of catches us up to now.
The biggest Tokamak project is Iter, which is an intergovernmental organization created in 2007 by China, Europe, India, Japan, Korea, Russia and the US. It began construction of a reactor site in 2013, the tokamak itself in 2020 and is expected to conduct its first reaction in 2025.
Another tokamak reactor is already up and running. On December 21, 2021, scientists at the Joint European Torus – aka JET– in the UK generated a record 59 megajoules over the course of 5 seconds by fusion. The most ever generated. It still wasn’t putting out more energy than they put in though.
Energy gain is what scientists began to look for. When can you say that you got more energy out than you put in, that’s energy gain.
Well, on December 14, 2022, scientists from the National Ignition Facility at Lawrence Livermore Labs in Livermore, California announced that for the first time they had developed a fusion reaction that produced more energy than it took in. They achieved energy gain.
The National Ignition facility used the “world’s biggest laser” focused on a cylindrical device with a capsule about the size of a pea, designed based on work with computer models to optimize its reaction. That capsule was filled with isotopes of hydrogen– deuterium and tritium. Laserbeams heated the capsule contents into a plasma that fused the atoms into helium and released energy and neutrons. At 1:03 AM on December 5th, the process was able to produce 3.15 megajoules of energy, 50 percent more than the 2.05 megajoules used by the 192 laser beams– about the same energy used by a hair dryer running for 15 minutes, but compressed into a millionth of a second. Still, that’s only 1.1 megajoules. Enough to boil water in a couple of of kettles.
So we’re there right? Not exactly.
The experiment produced more energy than went into the experiment, but that is not the same as producing more energy than was used in the experiment overall. The lasers are very inefficient. It took more than 300 megajoules of energy to create the 2.05 megajoules that the lasers put out. This is still scientifically important, because we’ve shown that if we had a perfectly efficient input, fusion could generate net positive energy. But it’s not a practical demonstration of a reactor we could use today. To do that we’ll have to see if we can repeat the process using more energy-efficient lasers.
There are other practicalities to overcome as well. This was one short reaction that destroyed the capsule and the sensors around it. Remember those stray neutrons? Yeah those and exploding capsules tend to play havoc with the equipment.
To be commercially viable, a fusion reactor needs to create multiple ignitions per minute without destroying everything nearby. Something of a trick when each ignition is designed to crush and obliterate your fuel capsule into helium.
Also the whole thing is still pricy. Like if you have to ask you can’t afford it pricy. The experiment cost a few billion dollars.
At a press conference, Kim Budil, Lawrence Livermore National Laboratory director, said “We need the private sector to get in the game. It’s really important that there has been this incredible amount of US public dollars going into this breakthrough, but all of the steps that we’ll take that will be necessary to get this to commercial level will still require public research and private research.” Among the private companies working on fusion are Tokamak Energy in the UK and Commonwealth Fusion Systems which spun out of MIT and more.
So nuclear fusion. It’s just a few years down the road.
In other words I hope you know a little more about nuclear fusion.