Nuclear fusion – the future power source?

Planet Earth is big. Sometimes it’s hard to grasp how big it is. A massive object orbiting the sun in space. So big that billions and billions of lifeforms find shelter side by side in an incredibly complex symbiosis. Among them are we, the humans.

We humans are a bit different from other lifeforms: we are obsessed with tools. Since the beginning of human evolution, we have been developing tools to deal with any obstacle life on earth brings about. Starting from more primitive tasks like obtaining food and shelter in order to survive to much more complex problems that lead to the modern technology that we know.

A brightly shining cone of plasma of blue color in a fusion reactor.
Plasma generated in a fusion reactor. The two main components, Deuterium and Tritium, are heated up to more than 100 million degrees Celsius in order to initialize the fusion reaction to Helium. Eye Steel Film from Canada, CC BY 2.0, via Wikimedia Commons

We humans are problem solvers. And we rely on tools and technology to do that.

However, this comes at a cost. Since the industrial revolution in the 19th century the machines and contraptions we build require a rather abstract resource we call energy. Energy is everywhere: any physical, chemical, biological etc. process requires energy to take place. Anything we see, hear, feel through our senses is a transfer of energy. Anything we do requires energy, be it you reading this text by looking at the screen emitting light in the form of photons, which then reach the sensory apparatus in your eyes, converting them into electrical signals inside your brain, processing them into information that you can understand. The energy for such a process is not created out of thin air: your body uses energy from the food you ate, burning it and converting it into electrical signals.

Any process run by a machine is no different: it requires a source of energy in order to take place. The first law of thermodynamics prevents us to generate energy out of nothing. It needs to be converted from one form or another.

With an ever-growing population of humans on planet earth, the increasing wealth, the natural needs of self-sustainment or the more artificially created needs such as a new smartphone every year, the demands for energy sources are steadily growing.

In most recent years humankind has been slowly coming to an important realization: it is impossible to harvest energy from a finite amount of resource with an ever-growing population and assume that this does not come to an end after some time. Planet earth is big. Very big. But its resources are not as infinite as we thought they might be.

Arrays of solar power panels on a field on a sunny day. In the background mountains are visible.
A solar power plant in California. Credit: USA.Gov – BLM – BUREAU OF LAND MANAGEMENT via Wikimedia Commons, Public domain.

Still today, the vast majority of energy consumption comes from finite resources: fossil fuels such as oil, coal and gas make up around 64% of the total electricity production, the most important source of energy. Solar and wind power make up a total of 7%. Nuclear power, a form of energy which on one hand is finite, but at the same time is abundantly available, makes up 10% [source].

Solar panels and wind parks – the solution for the energy crisis?

There are many attempts to solve the looming crisis of sustainable energy sources. The first step is to replace energy sources that rely on burning fossil fuels. Often times alternatives like wind or solar power are proposed as the new green sources of energy that might solve the issue at hand. Up to some extent this might even be a replacement, but there is an inherent flaw to these types of sources: the low energy or power density, which is the energy or power per unit volume. Without going into the details, the energy (or power) density determines how big or what number of power sources are needed to create a certain amount of energy. As an example we can look at solar panels and how many of those one would need to supply the whole world with energy. The total worldwide consumption of energy in the year 2020 amounted to roughly 23,000 billion kilowatt hours (23 x 1012 kWh), and with a total of 8760 hours per year making it an average of 2.6 Terawatt (2.6 x 1012 W) of constantly consumed power. The power per square meter the sun delivers is roughly 1400 W/m2 out of which a mere 100 W/m2 are usable [source]. Assuming one can harvest the full 100 W/m2 with perfect efficiency, an area of solar panels of 26,255,707,762 m2 or 26,255.7 km2 is needed to provide the required amount of power. That area is roughly the size of Belgium. In reality, this number might be even higher due to a lower quantum efficiency of the solar panels and losses in general.

Similarly using wind as a main source would require a lot of wind turbines spread all across the country and the oceans. In addition to the problem of low energy density of these sources, this would require a massive amount of material production, which needs to be recycled or wasted. For instance, wind turbines are made of a composite material of glass fibers and epoxy resin. Not very environmentally friendly indeed.

A landscape on a sunny day with mountains and some clouds. On this landscape a very large amount of wind turbines are seen.
Wind turbines. Credit: PublicDomainImages, via Pixabay.

Don’t get me wrong. I’m not saying that these technologies aren’t useful. They might even be a good short- to mid-term solution. However, at some point we need to think more long-term. And that’s where nuclear fusion power comes into play.

Nuclear fusion – a potential power source

Nuclear fusion – this term might sound a bit intimidating. It somehow reminds of nuclear power plants along with a lot of associations with that. Chernobyl, Fukushima and all these catastrophic events might come into mind reminding us of the fragility of nuclear power plants (although I have to stress that modern nuclear power plants have become much safer in recent years).

However, nuclear fusion is different. It is all around us. In fact, life on earth as such relies on this process and it has existed all over the universe since the beginning of time. Just by looking up into the sky we can observe nuclear fusion at work constantly. All the stars, near and far and even the closest one – the sun – are producing energy by nuclear fusion on a big scale. This process is called the proton-proton-chain reaction. By fusing two protons and subsequent neutrino emission a deuterium nucleus is formed, which is subsequently fused with another proton to form a Helium-3 nucleus. The helium-3 nucleus fuses with another Helium-3 nucleus into a Helium-4 nucleus plus again two protons. Via this proton-proton-chain reaction, energy is emitted and the sun heats up; the thermal radiation of the hot matter is then sent into space. Even the tiny fraction of this energy that reaches earth (yes, the earth is big, but the sun is way bigger!) is enough to power life on our planet as we know it.

In the example with solar power panels above I explained how we can make use of the sunlight to create energy. Basically, just making use of the product of nuclear fusion. But what if we don’t just make use of the sun’s main product, photons, but instead become the sun? Is it possible to imitate on earth what the sun is doing, by fusing two Hydrogen nuclei into a Helium nucleus and directly exploit the energy that is produced from it?

Turns out it is possible. And the technology for this is more advanced than one might expect.

The picture is divided into left and right. On the right side a metal chamber, which is a so-called tokamak is shown from the inside. A person is seen working on it. On the right side a design for a stellarator is shown with a lot of pipes, plasma, and a person for scale.
A picture of a tokamak (left) and a sketch of a stellerator (right). The stellarator design on the right on the right is the design for the Wendelstein 7-X experiment in Greifswald, Germany. Credit: left: Rswilcox via Wikimedia Commons, CC BY-SA 4.0, right: T Klinger et al., and The Wendelstein 7-X Team, reproduced under CC BY 3.0.

The idea that it is nuclear fusion underlying the immense power production of the sun has already been around since 1920. Based on Albert Einstein’s discovery of the equivalence of mass and energy with the famous formula E=mc2, it was proposed that it is the process of merging of two atomic nuclei that is responsible for the sun’s emission of energy. In the 1950s, particularly during the cold war the research on nuclear fusion started to take off remarkably. Possibly powered by the technological race between the United States and the Soviet Union, a lot of progress has been made concerning the nuclear fusion research. Most notably, the so-called “Tokamak” (rus. токамактороидальная камера с магнитными катушками) was invented in 1950 by Andrei Sakharov and Igor Tamm as well as the “Stellarator” in 1952 by Lyman Spitzer, both designs aiming to act as a heating and confinement cavity for the Hydrogen-based plasma, used to drive the nuclear fusion process. Both the Tokamak and the Stellarator are based on magnetic coils generating a magnetic field that keeps the plasma in place and both designs are still subject to a lot of research nowadays. In the meantime, thermonuclear power technology has led to the invention of the hydrogen bomb; the first of these detonated in 1952, surpassing the power of nuclear bombs by far.

For some time, research on nuclear fusion was mainly focused on testing the Stellarator and Tokamak technology. The goal was to captivate hydrogen plasma and reach temperatures that would mimic the conditions in the sun, with temperatures exceeding 10 million °C. Nowadays, plasma temperatures up to over 100 million °C have been reached! Under these conditions, hydrogen nuclei can undergo fusion into Helium even without the help of the strong gravitational force of the sun.

Searching for the right ingredients

Above I already wrote how the sun is creating energy via the proton-proton-chain. On earth, this process might not be feasible in exactly that way. In principle one could do fusion with any kinds of nuclei. However, to be efficient one needs to find the right materials. One of the most abundant substances here on earth is water. Water contains hydrogen, bound two oxygen. Hydrogen atoms can be separated from the oxygen atoms, creating two hydrogen atoms H and one oxygen atom. That’s good already, and perhaps hydrogen is a basis to a technology by itself, like hydrogen powered automobiles. But what we need for nuclear fusion is something (slightly) different. Hydrogen basically consists of a single proton as a nucleus and an electron bound to it. In order to run a sustainable nuclear fusion reaction, we need Deuterium: a hydrogen atom with an extra neutron in the nucleus. Deuterium is the heavier brother of hydrogen; it is a so-called isotope of it. But we also need an even heavier ingredient: Tritium. Tritium is even heavier than deuterium and has two more neutrons in the nucleus.

These are the two main ingredients. Deuterium and Tritium. Fusing theses two nuclei results in Helium and an additional neutron, which gets emitted in the process. Writing this into a formula would look like this:

21H + 31H  à  42He + 10n + 17.8 MeV

Where 21H is Deuterium (2 times elementary mass, 1 proton), 31H is Tritium, 42He is Helium, 10n a neutron and 17.8 MeV is the energy released by this process in units of electron-Volts (eV). This is the fundamental equation that is supposed to govern the nuclear fusion process in a Stellarator and Tokamak. The ejected neutron will be playing an important role, which I will talk about soon. Keep it in mind!

The thing about these two isotopes of Hydrogen is that they are much rarer than Hydrogen. Although that having said, Deuterium is relatively common and appears bound to oxygen as “heavy water”, the same way as Hydrogen is bound to oxygen. Within all the water on earth it appears with a rate of 0.015%, meaning that there is more than enough for the operation of nuclear fusion power sources. Tritium on the other hand is very rare: due to its radioactive decay with a half-life of 12.32 years it is very rare. It is estimated that in the whole biosphere of the earth there is only about 3.5 kg of tritium. Definitely not sufficient for our purposes.

Where does the Tritium come from?

On the official ITER website [https://www.iter.org/mach/TritiumBreeding] the following statement can be found: “A second source of tritium fortunately exists: tritium can be produced within the tokamak when neutrons escaping the plasma interact with a specific element—lithium—contained in the blanket. This concept of ‘breeding’ tritium during the fusion reaction is an important one for the future needs of a large-scale fusion power plant.”

Tritium breeding; sounds like the controlled reproduction of cattle on a farm. But this breeding of Tritium will be the key element towards fusion power. And that’s where the emitted neutrons come into play! The natural questions that come up might now be: Can they really find a regime within a vast parameter space where the fusion produces excess energy, more than is being put into the system while maintaining a stable Tritium supply produced by the neutrons emitted into the Lithium “blanket”, splitting it up into Tritium?

I just casually dropped a term that is actually the most important one regarding the realization a nuclear fusion power plant: excess energy.

The Q factor

It doesn’t require a lot of fantasy to understand what a fusion power plant eventually is supposed to do: produce more energy than is being used. Very easy. Almost as easy it is to quantify this excess energy production. Think of a certain power, Pin that is the power that is used to contain and heat the plasma. And then the power Pout, well the power we get out, mostly in the shape of neutron kinetic energy. Divide these two and you get the fusion energy gain factor Q= Pout/Pin . The so-called breakeven point is defined as Q=1, when Pout = Pin.

Many advances have been made until today in several test-facilities around the world towards a Q=1 nuclear fusion process. For a long time, since 1997, the record of Q=0.67 was set by the JET tokamak in the UK. Just this year, in 2021, the National Ignition Facility reached Q=0.7 and broke this record. Quite amazingly, this already took more than 20 years to achieve. There are other values of e.g. Q=1.25, but this value was the projected yield of the JET tokamak in Japan, where projected means that under the achieved conditions within the tokamak, an astonishing and world record-holding temperature of 558 million °C, more than half a billion of degrees, that Q-factor of Q=1.25 would have been achieved. The only problem was the Tritium supply, for which the technology is not implemented into this tokamak.

A far more ambitious and perhaps even promising project is the International Thermonuclear Experimental Reactor (ITER) project, already initiated in 1988. ITER is an international research project on a nuclear fusion reactor, a cooperation of the European Union and twelve more countries (China, India, Japan, Russia, South Korea, USA, Australia, Kazakhstan, Thailand, UK and Switzerland). The goal of ITER is to build a tokamak nuclear fusion reactor aiming at a Q of 10 or higher . So ten times more energy out than in. Also, as I already mentioned before, it should not suffer from Tritium shortages and this should be handled by breeding the Tritium out of Lithium.

Controversies, misleads and a lot of confusion

All in all very ambitious and perhaps also doable. But does Q=10 actually mean a breakthrough in nuclear fusion power development? As far as my rather limited knowledge goes, I would say possibly, but not necessarily. There is indeed a dispute about this topic. The Q factor I was talking about was actually only one of a multitude of Q factors used to describe the efficiency of a nuclear fusion facility. The parameter here is called the scientific Q factor. It describes only the power that is pumped into the plasma and compares it to the output power. It does not include all the expenses of energy and finances that is used to actually run a nuclear fusion power plant. That actually means: the actual power that is used Ptot, is actually much larger than only the power that can be effectively transferred into the plasma. The power Pin that I was talking about before does not take into account losses of the system as a whole. Also it does not include how efficient the energy can actually be transferred into plasma. So just to give an example, if we (completely arbitrarily) assume that the power to heat and contain the plasma can only be transferred with an efficiency of 50%, we end up with a Pin that is two times larger than the case we discussed. Remember, Q is defined by the ration Pout/Pin. As a consequency, this would render the Q two times smaller than the original one.

In a youtube video (also posted here), the prominent youtuber and physicist Dr. Sabine Hossenfelder makes exactly this point. The points out that most of the definitions of Q do not take into account these additional factors. She specifically quotes a statement by the head of the engineering department of ITER saying that ITER is estimate to be consuming a total power of 440 MW, while the aim is to produce 500 MW. As I mentioned before, the goal is a Q-factor of 10, so this means that this Q factor was calculated using an input value of merely 50 MW. These 50 MW only assume tha actual energy going into the plasma. With the 440 MW input, the actual Q factor would reduce to 1.136. She continues saying that not all the generated power can be converted into electric power and assumes an efficiency of 50%, meaning that actually usable output power might even be only 250 MW. This reduces the Q even further down to 0.57, below the breakeven point.

There are however other voices around on the internet that defend the popular definition of the Q factor. One among those I posted below.

In this video the author explains how considering the Qtot instead of Qplasma might be missing the point: the main focus should be on the research on nuclear fusion itself first and a pure research facility, like ITER is supposed to be, should not concern themselves with calculating all the cost and energy expenses in the first place. Another important point is also relating to the purely scientific purpose of ITER: the tokamak is designed in a way that parameters may be changed or scanned, like e.g. the radio frequencies applied to heat the plasma. However by desgining these components in a tunable way almost always leads to a decrease in efficiency, compared to components which are made to run on fixed parameters. That way, an actual power plant might even be designed in a much more efficient fashion, but to do to, actual physics research such as ITER provides, is needed.

A slightly optimistic conclusion

Now back to the most pressing question: is a nuclear fusion based power plant around the corner? How long might it take until we actually see a power plant being commissioned into public use? Having read about nuclear fusion and written this blog post I would now say: I still don’t know. As you might have seen above, there are still a lot of uncertainties and obstacles to be overcome. But I do think that most of us might be able to see a nuclear fusion power plant within our lifetime. And I honestly am looking forward to it.

Arthur Schoenberg

I am a PhD candidate at DESY and work on topics like ultrafast lasers, nonlinear optics and UV generation in gases.

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