Power, Unlimited Power! — A Primer on Fusion Energy

Introduction

We’re in an energy crisis.

That’s a pretty broad statement, so I’ll specify. By “we,” I mean literally everyone. By “energy crisis,” I don’t mean a localized shortage of energy; I mean there’s a fundamental issue with how the vast majority of our energy is obtained. I’m sure you can ascertain what I’m getting at, so I’ll leave my critique of fossil fuels for another time. In this post, I’d like to discuss how we might go about solving this problem. Renewable energy is the simple answer, but, once again, that’s a broad statement. At this moment in time, I believe fusion energy shows the most promise of any renewable energy source.

Fusion Energy and Donuts

In short, fusion draws upon the equivalence of mass and energy to convert matter into usable energy. Einstein famously proposed the equation E=mc2 to describe this relationship. Fusion is the same process that powers stars, so you may now realize how desirable fusion is. But what does the term “fusion” mean? Fusion is the process by which light elements are forced together and fuse, producing a heavier element. The resulting particle is less massive than the original particles, so energy is released.

Perhaps your scientifically curious mind may be questioning how this can be, given the laws of conservation of mass and energy. In fusion, we observe a net loss of mass and a net gain of energy. Thus, it is more accurate to say that the sum of mass and energy is conserved, rather than saying that mass and energy are individually conserved.

To achieve fusion, the fuels used (usually deuterium and tritium, which I’ll discuss later) must be stripped of their electrons and collide with other nuclei. Sounds simple enough, right? Well, reality is often disappointing. The nuclei of said fuels will be positively charged, so they absolutely despise being next to one another. If they are to fuse, the environment they are in must be very hot, under intense pressure, or both. For something like a star, those conditions are easily met. However, when you consider recreating these conditions on a mild planet such as Earth, the big puzzle of fusion is revealed.

A deuterium-tritium (D-T) reaction

The most promising design for a fusion reactor is called a tokamak (whose name originates from something Russian). Essentially, a tokamak is a big donut (also known as a torus to those who speak sciencey jargon) with some big magnets around it. The magnets keep the fuel spinning around the center of the donut, and the fuel is then heated in excess of 60 million degrees Celcius. This forces the nuclei together, violating all personal space. Fusion then occurs and releases energy. The energy is captured in the lithium-coated blanket surrounding the hot donut, which in turn heats some liquid to drive a turbine. There are other designs for fusion reactors as well. For example, one design concentrates several high-powered lasers at one single point to heat the fuel. However, to my knowledge, this design is less efficient than the tokamak.

A tokamak, AKA energy donut

The biggest goal of fusion energy is to achieve a gain factor greater than 1. This factor (notated Q) is energy out divided by energy in, which would mean that the reactor outputs more energy than is input. So far, no reactor has been able to sustain a Q of 1 or greater, resulting in a net energy loss. One significant roadblock to efficiency is the strength of the magnets used in tokamaks. Having more powerful magnets means more fuel will collide, and thus, the reaction will be more efficient. New powerful magnets (over 10 T) are being developed, and it is theorized that a reactor with these magnets would achieve a Q of greater than 10 (though that may be a long time away).

Equation for the fusion energy gain factor, Q

Fuels and Other Issues

As I mentioned earlier, the most common fuels for fusion are deuterium and tritium (a D-T reaction), which are isotopes of hydrogen. Deuterium is relatively abundant and can be harvested from seawater. Tritium is the difficult one, as it is quite rare and has a short half-life. It’s estimated that there may be only around 20 kg of tritium on the earth. Fortunately, fusion uses a minuscule amount of fuel, but finding tritium remains an issue all the same. An exciting idea has been tossed around for obtaining tritium, in which the fuel is created within a fusion reactor itself. After a D-T reaction takes place, a lone neutron is released in addition to the fused helium nucleus. The high-energy neutron can collide with lithium in the blanket, forming tritium. Many tokamak designs are modular, meaning components of the machine can be reconfigured for different purposes. One such module could be a collection system for tritium. If Q>1 is achieved, a fusion reactor will be almost entirely self-sustaining (save the deuterium and lithium, which are relatively easy to acquire).

So, if fusion is so promising, why don’t we have it yet? In a word, it’s expensive. The facilities and fuels used by fusion are incredibly costly, not to mention the billions we’ve already sunk into research. Naturally, this prompts another question. If fusion is so expensive, and there hasn’t been any truly successful reactor yet, why are we still spending so much money on it? That’s a perfectly reasonable question to ask, especially given other forms of renewable energy that we know to work like solar, wind, and hydroelectric. The main reason is the incredible implications fusion energy has. Even with the 20 kg of tritium on earth, if reactors reached even the slightest bit above the breakeven point (Q=1), fusion reactors could power the earth for thousands of years with clean, low-waste energy (unlimited power, in the words of the Emperor).

Despite the benefits, several concerns have been raised regarding the safety of fusion. I believe the majority of those concerns are misguided. The term “nuclear,” associated with fission energy (think “nuclear power plant”), is often applied to fusion energy as well. While the reaction does take place on the nuclear level, the similarities stop there. Fission energy requires a constant supply of coolant to prevent the fuel from overheating. If that cooling malfunctions, there will be a disaster (Fukushima, for example). In contrast, fusion energy does not require a constant energy supply to keep the reaction under control, just the opposite. If the reactor loses power, the reaction simply stops. The reactor itself may be damaged, but the fuel will quickly expand and cool off, preventing any large-scale disaster.

Discussion and Conclusion

It certainly seems that fusion has a long way to go. Especially in recent times, the viability of fusion has come into question. As much as I believe in fusion, I understand the sentiment of focusing on other renewable energy sources. Nonetheless, I think it’s essential to define what viability really means. Viability refers to “the ability to work successfully.” Indeed, energy sources that we know to work successfully are considered viable, given that we use them today. However, fusion’s inability to work successfully (if we define success as Q>1) does not detract from its viability because its theoretical ability to function still stands.

This post took quite a while to make; that’s because I feel it’s an issue worth discussing. In fact, fusion energy is one area I plan to do research in. Fusion energy is a deep and complex topic, and my hope in writing this post was to compile my knowledge into one document to explain fusion in simple terms. I do believe I’ll see fusion work successfully in my lifetime, and I’ll do whatever I can to ensure that. As for right now, I’m currently working on a post related to fluid dynamics (woo, turbulent flow mechanics!), so you should see that one soon.

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