Uday Mehta

Engineer | Writer | Podcaster

What is Nuclear Fusion and How Will It Kill Me?

Editor's Note: Hey, it's Uday, I'm also the editor because I don't have the money to hire an editor.  I recently wrote a piece "Why is Nuclear Power Not in the Green New Deal?" which presents nuclear power as an economically and environmentally viable solution to the energy crisis and climate change. To help people understand the subtleties of nuclear power, I'm launching a set of companion pieces in a series I'm calling "How Will It Kill Me?" Previously, we talked about radiation, nuclear waste, and Chernobyl. Today, we talk fusion.

NuclearFusionHowKillMe.jpg

We've spent a lot of time talking about the various aspects of nuclear energy, but in this discussion we've ignored one crucial component. Thus far, 'nuclear power' has been used interchangeably with 'fission power', because every nuclear weapon and commercial nuclear reactor is built on the technology of fission. And while splitting the atom has gotten us this far, the time is coming for an even greater technology to take over. If fission is our current reality, then fusion is the dream.

Fission power is, as we've established in detail, holistically far greater than any other method of power generation in terms of efficiency (in terms of mass and volume of the fuel and the space required for a plant), environmental cleanliness (absolutely no Carbon emissions), consistency (compared to the variable wind and solar), cost (after you get past initial capital), and safety (in terms of both worker deaths and number of accidents). Fusion not only keeps all of fission's advantages, but also minimizes every fission-related drawback there is. So let's familiarize ourselves with the technology that could be the salvation of the human race.

A quick caveat – I've tried to make this as science-lite as possible, but yes, there will be science here. Don't gloss over these points, because if I included it, it's something you need to know.

 

Give me the science. What is fusion?

Quick recap before we get started: Protons (p) are positively charged particles, and Neutrons (n) are neutral particles. Together, these two particles make up the Nucleus of an atom. An Element (like Hydrogen, Helium, etc.) is defined by the number of protons it has. Isotopes of an element have different numbers of neutrons. The Atomic Weight is the total number of protons and neutrons. For example, Uranium is defined by its 92 protons. The isotope of Uranium with 143 neutrons has an atomic weight of 92p + 143n = 235, and is appropriately called Uranium-235.

In fission, you start with one big heavy thing and break it apart. Fusion is the complete opposite, the idea of combination. To fuse, you start with two small light things and force them together. Just how light? Almost as light as you can get – the two lightest elements – Hydrogen (1p) and Helium (2p) – work the best. There are five isotopes between these two elements: Hydrogen has Protium (1p+0n), Deuterium (1p+1n), and Tritium (1p+2n), while Helium has Helium-3 (2p+1n) and Alpha (2p+2n).

There are eight reactions that combine these isotopes in different ways. Let's take a look at one. Deuterium (1p+1n) and Tritium (1p+2n) combine to form Alpha (2p+2n), a free neutron (1n), and in the process release energy.


Okay, but where is this actually coming from?

You may have heard the saying 'greater than the sum of its parts', which comes into play here. Let's consider the reaction shown above. If we were to physically weigh each thing on a scale, the reactants (Deuterium and Tritium) would weigh more than the products (the Alpha and the free neutron). There's an imbalance.

But, the number of protons and neutrons on each side are equal. So how could there possibly be an imbalance? This comes from a phenomenon called the Binding Energy. The nucleus of an atom – which contains the protons and the neutrons – is held together by a fundamental force, the Strong Nuclear Force. The binding energy is the amount of energy you'd have to put in to overcome the strong nuclear force and break apart the nucleus. Some isotopes are more tightly bound than others, which means their binding energy is higher. This difference in binding energy shows up as a difference in mass (the imbalance) – remember, mass is just energy in a different form (E=mc^2).

 

Why only Hydrogen and Helium? Nothing else works?

While Hydrogen and Helium work best, Lithium-6 (3p+3n) is another element that can be fused on Earth in six possible combinations. But technically, quite a few isotopes *could* be fused.

The following is the binding energy curve, where the binding energy is shown for each isotope. Every isotope, due to its unique combination of protons and neutrons, will have a unique binding energy. And as long the binding energies of your fused product is greater than that of your reactants, you get that imbalance on the scale and energy out.

As you can see, Alpha (Helium-4) is a bit of an anomaly. While the curve is largely smooth, Alpha has an abnormally high binding energy (for… reasons we won't get into). Consequently, fusing two smaller things into Helium would provide the greatest imbalance on the scale, and hence the greatest release of energy. So you *could* fuse say, two Boron-10 isotopes (5p+5n each) and get Neon-20 (10p+10n), but the payoff wouldn't be nearly as good. Plus, the energy mountain you have to climb might as well be Everest, because Boron-10 already has a high binding energy that must be overcome to break it apart and make the 'soup'.

The most tightly bound nucleus – the maximum of the binding energy curve – is Iron-56. That means that it is energetically favorable to fuse elements up to Iron, but anything beyond that will actually *cost* you energy to fuse. You'd climb the energy mountain, but it would be flat after you got to the summit. The scale would be tipped the opposite way, where the products weigh more than the reactants.

If instead of trying to fuse two moderately heavy things into something very heavy, you can instead break apart one very heavy thing into two moderately heavy things. You're just flipping the products and the reactants, going in the other direction. Now, your products weigh less than the reactants, and you get energy out. That's fission!

So fusion produces energy for everything lighter than iron, and fission produces energy for everything heavier than iron. Once you get to Iron, the mountain is so high that it's unlikely that you'd spontaneously have enough energy to break it up into a soup for fusion, and it doesn't have an imbalance that you can exploit for fission.

 

This seems just as complicated. Why is it better?

It's better because we no longer have to deal with things like Uranium and Plutonium. Each of our previous high-level concerns – radiation, waste, accidents – are inherent to the science of nuclear fission.

First, there is no Radioactivity involved. Remember that radioactivity comes from a severe imbalance in protons and neutrons. Heavy isotopes tend to be unstable and are hence radioactive. If you want to fission (break apart) something and get energy, it has to be something pretty heavy. After something heavy is fissioned, it makes other things that are still somewhat heavy and still radioactive. The six fuel isotopes and the waste products (stuff on the right side of the equation, which can include Tritium and Lithium-7, but primarily Alpha) we're dealing with in fusion are all light. They have so few protons and neutrons that most of these isotopes have no such instability. And since they're not radioactive, they don't need any kind of special long-term underground storage.

Second, the waste products aren't otherwise harmful. the waste products aren't harmful the main waste product (Alpha) is inert and non-toxic, and can just be released into the atmosphere. Lithium has a number of uses and is not harmful in any way. Tritium is the only one that's unstable, but it does not emit ionizing radiation (x-rays or gamma rays), Tritium emits beta particles, which are unable to penetrate skin, and is only problematic if inhaled or ingested (e.g. if it gets into drinking water). Even then, its half-life is just a couple weeks, so there's no risk of long-term accumulation.

Third, there's no real possibility of any kind of accident. For a Hydrogen explosion to occur, you need Oxygen. These fusion reactions are conducted in a vacuum, without anything present except for the specific isotopes involved in the reaction. Even if some air were to somehow leak into the chamber (which would be virtually impossible), the amount of Hydrogen is so low that their interaction probability is negligible. (More on this in a bit)

Fourth, the use of elements like Hydrogen and Helium is that the threat of Proliferation would no longer exist. They're not like Plutonium and Uranium, which you have to safeguard and track.

Fifth, the absolute best part is that Hydrogen is virtually limitless. There's plenty of Hydrogen and Helium around, and Deuterium can be distilled from water. Some of the reactions produce Tritium as a 'waste' product, but that Tritium can be used as a fuel for a different type of fusion reaction. A gallon of seawater has enough Deuterium to generate as much power as 300 gallons of gasoline. The oceans have enough fuel to help light world for millions of years.

 

So… why are we not doing this already?

As you'd imagine, there's a bit of a catch. In order to initiate fusion, you need very high temperatures, a tremendous amount of energy, and other extreme conditions. The fusion process is twofold – first, you form a plasma by heating a gas in an electromagnetic field, which strips away all of the electrons from your fuel. This breaks up the fuel and creates a 'soup' of protons and neutrons. Next, the particles must be accelerated toward each other at very high speeds to fuse them. You need these high speeds because of the Electrostatic force, which causes two entities with the same charge (say, two protons) to repel each other. If you can force the particles in the particle soup close enough together, the aforementioned Strong Nuclear Force will take over and hold the nucleus together. Because of these restrictions, fusion has not yet been accomplished in a manner or at a scale suitable for power generation.

In both fusion and fission, you need to jump-start the reaction, give a little push in the right direction. For fission, that 'push' is not too difficult because the heavy isotope is already unstable. Think of it like going up a small speedbump followed by a hilly downward road. The car can't get over the speedbump by itself. You have to lightly hit the gas pedal to get past the bump, but after that, the car will roll down the rest of the road by itself. When it comes to fusion, that little speedbump is more like a mountain. You do get a lot of energy out, but you have to put a lot of energy in (under some highly specific conditions) to begin with.

The silver lining in this complex setup is that it makes meltdown impossible in a fusion reactor. Since such high temperatures are needed, if power is lost (as it was in Fukushima), the temperature will drop and the plasma will turn back into a gas, thus stopping the reaction. Even a slight drop will make it impossible for fusion to occur, so you don't need to rely on a coolant. Also, fusion doesn't involve a chain reaction like fission does, so the rest of the fuel in the reactor won't fuse by itself. Similarly, it's impossible to create a fusion-powered weapon, because achieving fusion conditions is hard enough in a stable, full-sized power plant environment, let alone in a small portable bomb hurtling through the air.

 

In that case, how do we even know that this works?

We have an excellent test case: the Sun. All of the energy on Earth is directly or indirectly from the light and heat given off by the Sun, which is generated via fusion. You might remember from elementary school science class that stars are big balls of gas, but importantly, these gases are Hydrogen and Helium, the specific gases required for nuclear fusion. The Sun is about halfway through its lifespan, meaning that today it still has half of the Hydrogen it had at the beginning of the universe. Every second, it converts 600 million tons of Hydrogen into Helium. After it burns through its remaining supply, it will expand and become a red giant, swallowing Mercury and Venus and making Earth uninhabitable, but it's a long time until we have to worry about things like that. Some stars, depending on their mass, will actually start to fuse all the Helium produced from the original Hydrogen fusion.

 

Why do we have to talk about this now?

Like any scientific development, it helps if there is public support. Such support can eventually lead to legislation, more available money, and increased interest among young scientists. And public support is built from public education. On that note, let's do a top-10 to make sure you're all educated up.

  1. Fusion is combining elements, whereas fission is breaking them up.

  2. The sun and other stars use fusion as their method of energy generation.

  3. To conduct fusion on Earth, you have to achieve extremely high sun-like temperatures.

  4. Fusion requires a lot of input energy, but gives you enough output energy to make it worth it.

  5. Theoretically, anything lighter than Iron can be fused and result in a net energy output, whereas anything heavier than Iron would have to be fissioned.

  6. The primary fission fuels are isotopes of Hydrogen and Helium, which are virtually unlimited and do not need to be safeguarded.

  7. Fusion waste is largely harmless because it does not emit ionizing radiation, and hence does not need to be kept in long-term underground storage.

  8. It is impossible to create a fusion-powered bomb.

  9. It is impossible to have a Fukushima or Chernobyl-like meltdown in a fusion reactor.

  10. Fusion is just as environmentally clean and cost-effective as fission.

The nuclear industry may have taken far longer to evolve into full form if it hadn't been for The Manhattan Project, which was a direct result of the arms race surrounding World War 2. Moonshots have worked since the original one – you know, the one that actually got us to the moon. Whether it's government-based like Obama's cancer moonshot or industry-based like Google's Waymo, they spur both engagement and investment. Yet, we are entering an election cycle where no candidates (with the exceptions of Cory Booker and the non-debate-qualifying Seth Moulton) openly support nuclear power, despite the fact that climate change is widely recognized as the country's biggest geopolitical threat.

It is imperative that the country learn from the mistakes of regulating nuclear thus far and recognize the need for change as the industry prepares to take its next big step. Fusion is nature's way of powering the universe. It's about time it becomes humanity’s too.

World-class journalism in your inbox.

* required