How fusion works, and are we there yet?

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Thought experiment: You know these two people who are perfect for each other and should marry and form a single household. They don’t know each other yet, but the truth is, the moment they see each other they will fall in love and instantly want to be married forever. Unfortunately, they are living in two very different places. One is living on a permanent human colony on Mars, the other is in a penitentiary in Russia serving a life sentence. Getting them together is going to take a LOT of work.

But, once they are together, they will, as stated, become one, in a sense. So, you organize a meet-up. It is in a house that is for sale, and the real estate agent and closing company are there, so all it will take for the happy couple to have their own abode is a simple signature. There is a wedding officiant in place, and witnesses, so their marriage will also be a single set of signatures. You have had two moving companies go to each of their respective earthly US-based households where all their stuff has been stored, and that stuff is now ready to be moved into the house. All you need to do is get them into the living room of this house.

Then, you get them together. Never mind the details as to how, but it works. They fall in love on first sight, instantly sign the marriage certificate and the closing documents, and movers move all the stuff instantly into their new home. They are now inseparable.

But there is one problem. They have two blenders.

And two Crock-pots, and two electric can openers, and two couches and … well, it goes on and on. If two households are going to merge, some stuff has to go.

Now, take yourself out of that quaint and seemingly improbably metaphor and imagine that when they pack up all the stuff that is duplicated to bring it to Goodwill, all that physical material turns into energy, with the formula predicting the total amount of energy being E = MC2, where M equals the combined mass of the extra blender, extra couch, extra everything else.

There are two sets of forces relevant to nuclear fusion. One is the set of electrostatic forces that keep atoms from getting too near each other. Without that energy there would be nuclear fusion going on all the time. This repulsing energy is the problem in our parable of lack of propinquity, with one of our ideal couple being on Mars, the other being in a Russian prison. These two people are not going to get near each other unless those overwhelmingly difficult problems are solved. The nuclei of two atoms are not going to get near each other unless the electrostatic forces are somehow overcome.

The other force is the strong attractive nuclear force that causes protons and neutrons to bind together in the nucleus of an atom. All you need to do to get this attractive force, represented in our parable by love at first sight, to combine the atomic bits is to get the nuclei near each other, nearer than the electrostatic forces would normally allow. There are a few ways to do this. In an ideal world, you just push them together using some sort of magic pushing wand, but such a wand does not exist. The way the Lawrence Livermore lab does it is to heat the atoms up using lasers, so they are bouncing around so much that kinetic energy pushes some of the nuclei nearer than electrostatic forces would usually allow. The above outlined parable could have used, instead of overcoming the impossible distance to bring the couple together in their future living room, the dance floor of a techno-pop rave. (But there would be other problems with that analogy.)

Forget about the needed energy for a moment, and just think about the atoms/people and their stuff. The atoms are made up of protons and neutrons, and when they are combined, there has to be just the right combination of protons and neutrons or else the fusion of two nuclei will not have an extra blender. There are some combinations of neutrons and protons that will take in energy rather than put out energy. Starting at the lower end of the Periodic Table, most combinations, if you could get them to happen at all, would put out energy (extra blenders and Crock-pots), until you get to a certain point, then the atoms if combined would take in energy. Certain combinations, given important measures of the electrostatic forces and the makeup of the atomic nuclei, would be easier to make happen, and others are more difficult. The details are very technical and very weedy. Suffice it to say that decades of research indicates that a certain combination of Hydrogen atoms, including different Hydrogen isotopes (different isotopes have slightly different bits in the atomic nuclei) can work, while others not so much.

Hydrogen, the lightest element and simplest atom, normally has one proton in its nucleus, and one electron. Since electrons often interact with the rest of the world by taking short or medium length trips to visit other atoms, a hydrogen atom is, essentially, a proton that at any given moment may have a sort of open relationship with an electron somewhere. Deuterium is a form (isotope) of hydrogen that has the usual one proton plus one neutron. It is heavier than regular hydrogen, and is a stable (not radioactive) nucleus. It is also very rare. Something like one in ten thousand hydrogen atoms is Deuterium. Tritium is a special form of hydrogen that has one proton and two neutrons. This isotope of hydrogen is radioactive. As a radioactive element, it decays into an isotope of Helium (releasing beta energy) with a half-life of about 4,500 days. Tritium is produced in a nuclear reactor (there are several methods) so it can be used for scientific purposes.

The fusion reaction that works best is combining a nucleus of deuterium, with a nucleus of tritium. The result is the nucleus of a helium atom (two protons and two neutrons). So, one neutron and one proton plus one neutron and two protons equals two neutrons and three protons, but the helium atom does not use that extra proton.

That extra proton is the extra blender, except it is not a blender, but rather, energy. (I’m oversimplifying here a little. Some of the energy is alpha radiation, some of it is in highly energetic extra neutrons which are captured to heat up an appropriate substance). The amount of energy released from one such reaction of just the two hydrogen atoms is about 1.9516042893337081e-19 horsepower. Obviously, in an actual fusion reactor, gazillions of atoms would be combined every second. The extra energy produced in in the latest experiment at Lawrence Livermore was about enough to bring five gallons of tap water to boil. In an actual fusion power plant, the energy produced by fusion would be used to heat a metal or liquid, to run a turbine to produce electricity, with some of the waste heat inherent I this process (close to half) possibly being put to some use as well.

The radiation produced from a controlled fusion reaction is short lived, and/or will affect only a small amount of material which is easily handled. The reaction does not produce any radioactive material with a long half-life, or that is toxic (both of those problems result from fission nuclear reactors of which we use many to produce electricity). In theory, deuterium can be “mined” from water, and this is fairly routine. Tritium is produced from nuclear reactors of the common fission type, so when people tell that using fusion reactors to produce electricity at a commercial scale does not produce long lived nuclear waste, check your wallet. The tritium production required to feed a large scale fusion industry will require fission reactions, which do produce this waste.

Prior to the latest Lawrence Livermore success most experts, when asked how long the first fusion reactors might be available, have typically said “I don’t know, maybe 40 years.” With this result, the best realistic estimate is probably the same. The big problems with using fusion have yet to be addressed. The reactions that happen now are ephemeral. The reaction itself can ruin the equipment used to make the reaction, and produces a byproduct in the form of extra elemental dust, as it were, that has to be removed instantly or it ruins the reaction. These are surmountable problems, but not easily fixed, and the short term prognosis is uncertain at best.

To understand what has to happen next, let’s try another analogy. Let’s say it is 1800, and someone has the idea that blowing up gasoline or kerosene can move something. So they invent a “car” that has cans of flammable liquid in the back. When a can is ignited, it causes a great explosion that moves the car forward a little. That kind of works, but is not ideal. The better way would be to somehow control the explosions, capture the kinetic energy, and convert that into turning wheels. In theory, that is possible, but in 1800, the metals, electronics, and other materials needed to accomplish this are about a century away in the future.

Today’s fusion experiment is to a future fusion reactor what a 19th century steam-punk submarine imagined in fiction is to a modern attack submarine. Quaint, at best. But hopeful and very cool. Cool in a hot-fusion kinda way, but cool.

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3 thoughts on “How fusion works, and are we there yet?

  1. Greg:

    Nice explanation! I agree we are still probably 40 or 50 years away. But progress!

    When the lab reports they got out more energy than they put in – they are only talking about the energy in the laser beam versus what they got out. In reality it takes a lot more energy to power the equipment which generates the laser beam, so we are not really getting more out than we put in – if you factor in all the energy required to produce the laser beam (which is a lot more than the actual energy in the laser beam).

    Still – fusion would be the best solution if we can get it to work, at scale and consistently. I also read that we have a very small amount of tritium, which is the preferred fuel for fusion. So even if we get fusion to work – once we scale it up will we run out of fuel? Not sure.

    Still – nice article.

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