Len Explains Fusion Energy

The photo is of Lisa Meitner. The BFF of one of our kids is (quite likely) a great grand niece of this amazing woman. 

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MB: Len occasionally explains very complicated science things to me or our kids in such a way that we actually understand it. One of our kids asked him what the big deal was about fusion. Len wrote this and I thought some of you might like to read it.

Long live curious people and long live nerds. 

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In the 1930’s, Austrian physicist Lise Meitner and German chemist Otto Hahn were close friends and worked together at the University of Berlin. While they were there, they started a project that sounded very boring, but which wound up changing everything we knew about atoms and the nature of matter.

Before then, chemists knew that there were thousands of different chemicals, and you could change one chemical to another through different reactions. For example, you could burn gasoline, refine metals and even change long molecules like rubber with heat and pressure. But they also knew that every chemical, no matter how complicated, was made out of a little more than 50 basic elements. You could combine different elements in different ways, but you could never change one element to another. Alchemists boasted about changing lead to gold, but that was impossible. Chemistry was all about combining and rearranging.

Which meant that the physical properties of the individual elements were really important. Scientists knew a fair amount about iron, carbon, oxygen and more, but someone needed to make reference books that told the EXACT melting temperature and weight of every different element. It was like asking absolutely precisely how much does ice weigh: important but kind of boring.

Nevertheless, Meitner and Hahn started at it. They were especially interested in several heavy metals, including lead, gold and uranium. People knew what uranium was, but it wasn’t considered very important. You could make a bright orange glaze for china out of it (look for old Fiestaware), but no one had looked that carefully at the pure metal.

What the two scientists did was make very pure samples of the different elements. Like, really, really 100% pure. Then they did their measurements, which took some time. In the case of uranium, they finished by taking one last look at the sample to make sure it was still pure uranium. And, it wasn’t.

They were amazed. They knew that they had done their best to make it really pure, but there were clearly several other elements mixed into the sample. They checked their equipment to make sure that nothing was contaminating the process. Then they tried to purify the samples again, but after a short time they found the same other elements. There were several different elements in the mixture, but most of them were about half the weight of uranium. What a puzzle!

In the end, the only thing that could have happened was that the uranium atoms were breaking in half. Each piece was an atom of a different element, and those other elements were what Meitner and Hahn were seeing in their sample. So, elements could actually change.

Now, people knew a little about atoms. They knew that there was a very small center part (the nucleus) surrounded by a cloud of electrons that flew around pretty far away from the center. They knew that the nucleus contained protons and neutrons, but they were a little vague about exactly how things fit together. But different scientists worked hard to find out more.

What they discovered was that there were different assortments of protons and neutrons at the center of atoms.

This isn’t exactly what was happening, but think of this anyway. Stack some blocks together in a triangle. If you have six blocks, you can make a nice pyramid that is pretty stable. You can also make a good pyramid out of ten blocks. But how do you do eleven blocks? You stack them up, but not as neatly as six or ten. Atoms were more complicated, but the same thing was happening. Some arrangements fit together well, and stayed together for a long time … but other arrangements didn’t work as well.

Six dots

Ten dots

Oops. Not stable. 

They also discovered that some arrangement of protons and neutrons stored energy, like a spring, and when they changed the arrangement, that energy was released. When it was just a few uranium atoms at a time, like in Meitner and Hahn’s laboratory, the energy was small, but if you could make a lot of atoms split at once, it would release a lot more energy.

In an atom bomb, a lot of the uranium atoms split at the same time. Most of the energy is in the form of heat and x-rays, which causes a lot of damage. In a nuclear reactor, the atoms split in a controlled way, and the heat that is released is turned into steam and used to generate electricity.

Nuclear reactors are not without problems. Uranium is kind of rare, so it’s expensive. On top of that, all uranium atoms are not the same and only one kind (less than 1% of all uranium atoms) can actually split, and the process to separate out the good ones (it’s called “enrichment”) is very difficult. Finally, the atoms that are left over after the reaction (that’s nuclear waste) are very dangerous and must be kept away from people for years. Still, it makes electricity at a price people can afford.

Think about the blocks again. When uranium atoms split into two smaller atoms, that’s like a big, unstable pile of blocks falling down. There is another thing that can happen with atoms: sometimes two smaller atoms can come together to make a bigger atom that is stable. The splitting process (called “fission”) usually happens with big, heavy atoms like uranium. The coming together process (called “fusion”) only happens with a few, smaller atoms, like hydrogen.

In the sun, two hydrogen atoms combine to make one bigger atom of helium, and that coming-together process also releases a lot of energy. In fact, it release mores energy per atom than uranium.

But there is a big problem. To make fusion happen, you need to get the nuclei (that’s one nucleus and then another one: nuclei – hey, science has crazy word rules) very close together. But each nucleus has a proton, which has a strong positive charge. When you get two positive charges close to one another, they push each other way, which means that nothing happens. In the sun, the tremendous heat and pressure in the center squeeze them close enough to stick together, but nothing like that is found on earth.

In the early 1950’s, two scientists (Teller and Ulam) figured out a way to use an atom bomb (the fission kind) to smash the hydrogen atoms together with enough force that the nuclei would stick together. How they did so was a big secret (not a secret any longer, and actually a very interesting solution), but it’s not something that can be controlled, like a nuclear reactor, so it wasn’t anything that could be used (except for hydrogen bombs).

Since then, people have been trying to get that same energy without a bomb, but the positive charges are too strong. Even the hottest fire or strongest squeeze won’t get the nuclei close enough, and if it did, it would destroy the equipment used to make it work.

But they keep trying, because fusion would be such a good source of energy. For one thing, hydrogen atoms are in water, which is easier to get than uranium. Also, the end result is helium, which is not dangerous as spent nuclear fuel.

(To be fair, plain hydrogen doesn’t work, you need deuterium. But that’s still found in water and it’s easier to get than enriching uranium. Also, the energy that comes away from the fusion reaction would make some of the equipment radioactive, but it would not be quite as dangerous as nuclear waste. So, all in all, it’s a good deal.)

Lately, people have been trying two ways to make fusion work in a controlled way.

The first way is called a Tokomak, and it’s a big machine that uses plasma, which is a gas that is heated very hot. That’s not hot enough to allow fusion, but it is hot enough to make the gas conduct electricity – and not so hot that it melts the machine. In a very clever way, scientists put an electric current through the plasma. The current acts like an electromagnet which squeezes the hydrogen gas at the very center. By squeezing and heating the hydrogen atoms, they are trying to get the nuclei close enough to one another to fuse into helium, and release more energy. Since the very hot hydrogen never touches the metal walls of the machine, they should be able to keep the reaction going and making energy without destroying their equipment.

So far, Tokomak has not worked, although they are getting closer.

There is another approach. That is to take tiny pellets (about 2mm in diameter, which is a little over 1/16th of an inch) and blast them from several different sides with laser beams. The lasers heat the outside of the pellets and make them explode, which means that the very center of the pellet is heated and squeezed at the same time – squeezed and heated so hard that a few of the hydrogen atoms come together and make helium. Since the pellets are falling through the air (actually, a vacuum) when this happens, the heat doesn’t melt the equipment, and might be used make electricity.

Recently, scientists in California made that work, sort of. What they said was that the fusion reaction actually released more energy than the energy from the laser beams that were smashing the pellet.

That’s really cool, but it isn’t really practical yet. For one, the pellets are made through an outrageously complicated process, so they cost about $1 million each. Also, hydrogen is a gas, so the pellets have to be frozen to about 430 degrees below zero and held in a solid gold container that has all the air sucked out of it. Finally, while it’s true that the energy produced is more than what the pellet absorbs from the lasers, those lasers are incredibly expensive and use so much energy themselves that they can only fire once every eight hours. So, it’s going to be a while before this is really practical.

In fact, the total amount of energy released was about 500 calories, which is about the same as you would get eating a jelly doughnut.

But, hey, a jelly doughnut today, maybe cheap, clean electricity tomorrow.

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Here are two good articles with information about the recent laser experiments:

https://news.harvard.edu/gazette/story/2022/12/why-nuclear-fusion-is-so-exciting/#:~:text=Fusion%20is%20very%20hard%20to,can%20actually%20undergo%20this%20reaction.

https://www.scientificamerican.com/article/high-powered-lasers-deliver-fusion-energy-breakthrough/

Here is a little bit about Meitner and Hahn. Otto Hahn received the Nobel Prize, although Meitner worked alongside him. Hahn was a man and Meitner was a woman. In addition, the two worked at the University of Berlin as the Nazis were coming to power, and Meitner was Jewish. Although the two remained friends, it was clearly unfair that she did not receive the acclaim that he did. http://large.stanford.edu/courses/2018/ph241/goronzy1/#:~:text=Austrian%20physicist%20Lise%20Meitner%20and,and%20Meitner%20as%20the%20theorist.