Reaching for the Stars - An Introduction to Nuclear Fusion

Carl Sagan was one of the first scientists that made the secrets of the universe accessible to the average joe through his seminal series, Cosmos. Years after Sagan’s series, another modern-day science storyteller, Neil De Grasse Tyson relaunched the show and captivated a new generation of viewers with stories of the stars. That was my introduction to the cosmos and since then, I’ve been hooked. My curiosity led me to explore many topics like black holes, space-time, aurora’s and planets. But all these events seemed beyond reach and happening far far away. During this journey, I stumbled upon a concept that is basically, in the words of Dennis Whyte, “the underlying process that powers the entire universe” and furthermore, it was being attempted right here, on Earth.

Nuclear fusion forms the basis for all things created in the universe. It is the reason why the universe didn’t remain a mass of swirling hydrogen gas and the reason why elements like oxygen, iron and gold exist. Without fusion, the sun would not shine and life on Earth would not exist.

So what exactly is nuclear fusion ?

1. Concept

On the face of it, the concept is quite simple. It is the process by which two atomic nuclei fuse together to form a heavier one giving out a large amount of energy in the process. The reality however, is far from simple. To understand this let’s start with the basics -

1. Each element is made of atoms called protons, neutrons and electrons. For the purpose of fusion, we will concentrate only on neutrons and protons.

2. The atomic nuclei is made up of protons and neutrons. Protons have a positive charge while neutrons are, like you guessed, neutral.

3. Atoms with a similar charge repel each other while those with the opposite charge attract.

Now that we have the basics covered, let’s dive a little deeper. For two nuclei to fuse, we need to bring them really close together. However, both nuclei are made of protons and protons don’t like coming together (both positively charged). How do we force them to come together ? By increasing the temperature. When we increase the temperature of a substance, what we’re doing is making the atoms vibrate and move faster ( increasing their kinetic energy).

As the temperature is amped up the intensity of the vibrations keep increasing, bringing the atoms closer and closer. At a certain sweet spot, the energy of vibration overcomes the energy of repulsion between protons and lets them interact. Turns out, as soon as this distance barrier is breached, the protons realise, “Hmm, maybe we aren’t so bad for each other!” Another fundamental force, much stronger than their force of repulsion - the strong nuclear force (props for creativity) takes over and pulls both the nuclei, fusing them together.

Logically, the mass of the fused nuclei should be equivalent to the sum of the masses of the individual ones. For e.g. let’s imagine atom A has a mass of 1 g and atom B has a mass of 2 gms. After fusion, atom AB should have a mass of 1 + 2 = 3 gms.

However, the reality is different. This process was theorised by one of the greatest scientists that lived in his now eponymous equation - E = mc^2. Through this equation, Albert Einstein proved that when two atoms fuse, some portion of their mass is converted into energy, a lot of energy. C here represents the speed of light which in itself is a large number (3x10^8 m/s). This meant that through fusion, a small amount of mass can generate large amounts of energy. This theory finally helped us understand how the Sun has continued to generate energy for 4.5 billion years!

2. Conditions

Now that we’ve kind of understood the theoretical science behind it, let’s move on to what we need to do to make fusion happen on Earth.

I. The Fuel

Hydrogen, the most abundant element in the universe, is the primary fuel used for fusion in space. Elements intrinsically want to reach a state of stability and it is this need that causes them to either fuse or disintegrate. Hydrogen is fairly stable and the energy needed to fuse 2 atoms of Hydrogen together is large. Instead, we use two isotopes of Hydrogen. Isotopes are basically different versions of the same element. Elements get their chemical identity by the number of protons in their nucleus. The periodic table is built around this concept. For e.g. Hydrogen (top left in the image below) has 1 proton, Helium has 2, Lithium has 3 and so on. Variation in the number of neutrons, causes elements to have the same chemical properties but different physical properties, such as their mass and stability.

When we add a neutron to the commonly found Hydrogen, we get an isotope called Deuterium. Add one more neutron and we get an another isotope, Tritium. These two are a lot less stable than Hydrogen and the energy need to fuse them is a lot lesser. Deuterium and Tritium are the main sources of fuel used for fusion on Earth.

II. Temperature and Density

When we studied science in school, we always learned that there are 3 states of matter - solid (ice), liquid (water) and gas (steam). Ironically, the most prevalent state of matter in our universe was left out. This state, called plasma, is reached when we heat gases beyond a certain temperature. 99% of the visible universe is in the plasma state. The most common occurrences of plasma we see on Earth are lightning and the northern lights. For fusion to occur, we need to ensure that this fuel (Deuterium and Tritium) is in the plasma state.

The Sun, our solar system’s most efficient fusion reactor, uses Hydrogen as its primary fusion fuel. Using a mixture of high temperatures and intense gravity, the Sun ensures that the fusion switch is never turned off. At its core, temperatures of around 15 million degrees Celsius are reached. The Sun’s massive size, about 3.3 lakh times that of the Earth, produces at intense gravitation pull on of the mass surrounding it. The kinetic energy imparted by high temperatures, coupled with the crushing gravitational pull, cause increased number of interactions between hydrogen atoms drastically increasing the chances of fusion.

On Earth however, the force of gravity is far weaker. To overcome this, we need to heat the fuel to 10 times the temperature as that on the Sun. To reach optimum fusion conditions, the fuel needs to be heated to 150 million degrees celsius. This obviously can’t be done just by lighting a fire. Electromagnetism and lasers are used to heat up the fuel to this level. The two major methods being inertial confinement fusion and magnetic confinement fusion.(We will get deeper into both these in the next article)

III. Confinement

Reaching high levels of temperature is not the problem, maintaining it is. The Sun’s gravitation pull ensures that fusion doesn’t stop and continues to be self-sustaining. On Earth, once the first reaction is achieved, we need to give it time to remain in that state so self-sustaining reactions can occur. This simply means to let fusion produce enough heat to start another reaction which then starts another and so on. On Earth, this is the biggest challenge. Since the levels of temperature are so high, no material yet can be used to confine fusion. One of the ways devised by engineers is to suspend the plasma using magnetic or electrostatic fields. However, both haven’t yet been efficient enough to give the reaction the time it needs to become self sustaining.

After achieving these mind-boggling conditions, fusion reactions finally start to occur. To explain it simply, Deuterium and Tritium fuse to form Helium and one neutron with the expulsion of a large amount of heat energy. As highlighted above, the atomic mass of Helium is slightly lesser than the sum of the atomic masses of Deuterium and Tritium. This loss in mass is converted into energy.

All that is great but how will this energy power my home ? The part that follows fusion is surprisingly routine. This heat energy is captured and used to produce steam, which drives a turbine to generate electricity. Pretty much the same way we generate electricity from fossil fuels.

It’s certain that theoretically and fundamentally, nuclear fusion on Earth is possible. The reason the world is trying so hard to overcome what is now an engineering challenge is the energy source’s numerous benefits.

3. Advantages of Fusion

I. Exponential Energy

The multiplier for fusion energy is the square of the speed of light. This number is a mammoth 9x10^16 meaning just a small amount of mass can produce large amounts of energy. Fusion can produce 4 million times more energy than fossil fuels. To put this into perspective, an average 4 person household uses 32 kilowatt-hours per day. The amount of energy released by burning one kilogram of coal is approximately 7 kilowatt-hours, while the energy released by burning one kilogram of oil is approximately 12 kilowatt-hours. In contrast, the amount of energy released by a fusion reaction involving one kilogram of deuterium and tritium could potentially be over 600,000 kilowatt-hours!

II. Availability of Fuel

Another problem with fossil fuels is that they are finite. As the world has undergone rapid electrification, the consumption of fossil fuels has risen substantially. Our reserves are depleting rapidly and it is predicted that we will run out in this century. Fusion fuel on the other hand is abundant. Deuterium can be easily distilled from any form of water and tritium is produced as a by-product of fusion, ensuring constant production. Till fusion reaches commercial utilisation, tritium can be sourced from Lithium. Due to the small amount of fuel needed, the current reserves of lithium can provide us with enough tritium to last us for a million years.

III. Clean Source of Energy

With the Earth moving towards green-house gas suffocation, the need for a clean and efficient source is critical. Nuclear fusion does not generate any green-house gases or pollutants. It is essentially a clean source of energy.

Radioactive waste generated from fusion is also minimal. The main source is the walls of the reactor that get activated by high-energy neutrons produced during the reaction. The radioactivity of the material goes down dramatically with time and within 50 years, almost 40% of the waste can be released without restriction.

IV. No Meltdowns

The chances of a Fukushima-like disaster happening are zero. Unlike in a fission reactor, the nuclear fusion reaction is very difficult to maintain due to the high temperature needed. Any slight disturbance to the conditions will halt the reaction. In fission, once the chain reaction starts, it’s very difficult to stop. In fusion, its difficult to keep it going!

Like all things in the world, fusion is not perfect. Highlighting its faults is equally important as it helps one figure out ways to overcome them.

4. Areas of Concern

I. Cost

A lot of people say that fusion is essentially free because the cost of the fuel is negligible. However, this is far from the truth. The amount of capital required to create and maintain a fusion reactor is extremely high. While this will significantly go down once the energy is produced en masse (like solar), we are still a long way from getting there. Significant capital will need to be sunk to commercially produce energy through fusion.

II. Untested

Commercial production of fusion energy involves certain assumptions, namely the production of tritium, that haven’t been tested yet. While in theory, the fusion reaction itself will generate enough tritium to sustain future requirements, this has not yet been proven in a reaction. Tritium is prohibitively expensive and is not a naturally occurring isotope of Hydrogen. This for me, is one of the major stumbling blocks of fusion.

III. Plasma Physics

The 4th state of matter behaves very differently from the other three so much so that an entire branch of physics is dedicated to its study. Reaching the conditions required for fusion needs the fuel to be in the plasma state. This makes our understanding of how matter behaves in this stage extremely important. While major progress has been made, we are still yet to fully understand plasma. This in-turn limits us in coming up with effective ways to sustain fusion.

5. The Future of Fusion

With all its drawbacks, fusion energy still has a lot of benefits that make investing in this technology worth it. Strides are already being made with a number of government and private projects receiving large amounts of funding. Private companies entering the ring have immense benefits as they can skip past the red tape that burdens most government projects. Healthy competition between the two will spur progress and help us reach the holy grail of a commercial fusion reactor.

Consumption and dependence on energy is increasing exponentially and the need to develop a clean energy source is paramount. Fusion offers us that alternative albeit not immediately. We may not live to see fusion produced commercially but our willingness to invest a fraction of the global GDP in exploring its viability will benefit future generations.

In the next part of this fusion series, we will look at some of the companies at the forefront of this revolution. Stay tuned for that! I hope this introduction gave you a glimpse into what fusion is and its potential. This article only scratches the surface and I’ve posted some links below incase you want to dig further!

Links for extended reading -

1. Fusion Physics - IAEA, Vienna
2. Dennis Whyte: Nuclear Fusion and the Future of Energy - Lex Fridman Podcast
3. The Star Builders - Arthur Turrell
4. Why does the Sun Shine ? - Arvin Ash

If you enjoyed this article, subscribe to this newsletter and get insight on technologies like this that have the potential to change the world!