To answer “how nuclear fusion works,” perhaps we should first ask, “how does the sun work?”
Since the dawn of time, humanity has stood in awe of our sun. Ancient Egyptians venerated it as the god Ra, who sailed across the sky in a celestial boat as one might sail down the Nile; ancient Greeks worshiped it as Helios, who drove a chariot from horizon to horizon pulled by flaming horses. Many religions, ancient and modern, see the radiant, blinding disk in the sky as an icon of divine beings such as Aten, Utu, Tonatiuh, Sol Invictus, Ameratsu, Surya, etc. The sun gives us heat and light, our changing seasons, and makes all life and civilization on Earth possible. You might say, in fact, that our world revolves around the sun.*
*And you would be correct, because it does.
The first person in recorded history to say that our world revolves around the sun, literally and not just metaphorically, was the Greek astronomer Aristarchus of Samos, who lived during the 3rd century BC. Around the same time, another Greek astronomer and philosopher, Anaxagoras, suggested that the sun was not, in fact, the chariot of Helios and was instead a giant ball of flaming metal that orbited the Earth (people did not like being told this). Around the same time, Erastothenes of Cyrene, the Greek mathematician renowned for calculating the circumference of the Earth with astonishing precision, also calculated the distance from the sun to the Earth as being about 150 million kilometers (about 94 million miles). The sun is, in fact, 147 million kilometers away from the Earth at the closest point in our orbit and 153 million kilometers at the farthest point.
Over the next two thousand years or so, scientists and philosophers the world over, in the Mediterranean, in the Middle East, in Asia, and in Europe, learned more and more about the sun, but it wasn’t until the beginning of the modern scientific era in the 19th century AD that we had the tools to start tackling one of the biggest questions in the world—where does all the sun’s energy come from?
It wasn’t until the 20th century, after the discovery of radioactivity, that we figured it out. In 1904, Ernest Rutherford suggested that radioactive decay may be responsible for our sun’s output. Soon after, Albert Einstein developed his theory of mass-energy equivalence, best expressed in his famous formula E=mc2, and in 1920, Sir Arthur Eddington proposed that the sun could be producing energy, as expressed by Einstein’s work, by merging hydrogen atoms to create helium and thus giving out heat and light. Subrahmanyan Chandrasekhar and Hans Bethe developed the theoretical concept of what Eddington had proposed, now known as nuclear fusion, and calculated how the fusion reactions that power our sun worked.
As soon as we understood the nuclear furnace resting in the heart of our sun, which was in fact a giant ball of incandescent (mostly hydrogen) gas and not, as Anaxagoras had surmised, a fiery metal orb (good guess, though!), we started wondering—“Hey, can we do that here on Earth, too?”
And thus the quest for fusion energy began.
The Physics of Nuclear Fusion
Nuclear fusion is one of the simplest, and yet most powerful, physical processes in the universe. Two very excited, very hot, very energetic atoms collide with each other and turn into one atom, releasing a few leftover subatomic particles and leftover energy in the process.
After the Big Bang, the entire universe was an extremely hot, extremely energetic soup of very tiny subatomic particles—except it wasn’t quite fair to call them subatomic particles yet, since atoms didn’t exist at this point. Eventually, these tiny particles began to attract each other and bond, turning quarks into electrons, neutrons, and protons—the fundamental building blocks of matter. The hot, dense soup of the universe began to cool and curdle as it expanded, forming little lumps of hydrogen gas.
For a while, the universe was nothing but hydrogen, the simplest element. But gravity slowly began to pull some of these gas clouds closer together, and as the hydrogen atoms zipping around gained more energy in their increasingly-dense, increasingly-hot environment, they began to fuse with each other to form helium, the second-lightest element.
Many of these gas clouds became stars just like our sun—massive balls of hydrogen and helium. And in the dense cores of these stars, hydrogen and helium continued to fuse until they formed heavier and heavier elements. When the universe’s early stars died and erupted into novas and supernovas, they cast out clouds of all these heavier elements into space, which eventually became the nebulae, planets, asteroids, comets, and other interstellar bodies we know of.
It takes a great deal of energy to induce nuclear fusion. Atomic nuclei, which contain positively-charged protons and neutral neutrons, do not want to come near each other under normal circumstances. The Coulomb force, which describes how like charges repel each other and opposite charges attract (as with the north and south poles of a magnet, for example), keeps these two atomic nuclei from colliding with each other. If you set two atoms on a direct collision course with the intention of making their nuclei smash into each other and stick together, you will need to accelerate them to very high speeds so that when they collide, the nuclear force, which compels protons to stick to neutrons, overcomes the repulsive Coulomb force.
Nuclear binding energy is the minimum amount of energy it takes to break apart an atomic nucleus. The denser the element, the more energy it takes to break its nucleus apart. When we cause nuclear fission or fusion, the nuclear binding energy can be released. This is how fission and fusion can be used to produce electricity.
For heavier elements, fusion does not release energy. But for lighter elements, such as hydrogen and helium, when two atoms combine, the resultant third atom is filled with excess energy and an extra neutron or two in its nucleus that is making it unstable. No atom ever wants to be unstable, and so it seeks to return to the nearest point of stability by releasing all that excess. It relieves itself by tossing out the extra neutron(s), with its leftover energy released as well.
Nuclear Fusion in the Sun
In the sun, nuclear fusion occurs mainly between hydrogen and helium, since that is the bulk of its composition. As a star’s life cycle goes on, heavier elements form in its hydrogen-rich core, where the mind-boggling heat and pressure squeezes atoms together over and over again. Our sun is a medium-sized star around the midpoint of its life cycle, having formed from a cloud of gas about five billion years ago. Outside of its core, roiling layers of superheated plasma give off heat and light which travel through the abyss of space to warm all of the planets and not-quite-planets (sorry, Pluto) in our solar system.
Eventually, about five billion years from now, the sun will exhaust the once-ample supply of hydrogen and helium in its core by fusing it all together into heavier elements. When that happens, the sun will violently shed what remains of its outer layers and leave behind a small gaseous core of carbon and other heavy elements. No longer massive enough to force these heavy elements to fuse, this remaining white dwarf will rest, inert, in the center of an expanding cloud of gas until it cools to become a black dwarf.
The sun’s fusion processes are on a scale so massive that it’s difficult to take it all in. In its core, the sun fuses over 600 million tons of hydrogen every second. It takes such a great deal of energy to produce nuclear fusion that in our modern and mature universe, nuclear fusion will only occur naturally inside stars like our sun. Even hydrogen, the lightest element, require a high energy input to fuse that simply cannot naturally occur anywhere else.
And, of course, us being humans, we learned about that process and asked ourselves if we could do it here on Earth (on a much smaller scale, of course). After we figured out nuclear fission and created the most destructive weapons the human race has ever known, the race for nuclear fusion—as a source not of destructive power but of energy enough to power our civilization without need for polluting fossil fuels like coal or oil—began.
How Do Nuclear Reactors Work?
There are two broad categories of nuclear reactors: fission reactors, which split heavy atoms apart into less-heavy atoms to produce byproducts such as neutron radiation, radioactive waste, and most importantly, excess energy released that can be converted to electricity to power our homes and industries; and fusion reactors, which combine light atoms into less-light atoms to produce byproducts such as neutron radiation and (in theory) excess energy.
Fusion vs. Fission
A nuclear fission reactor uses uranium as fuel. When a uranium atom becomes excited and destabilized by exposure to neutron radiation, it breaks apart into smaller atoms such as barium and krypton and releases more neutron radiation, which in turn excites and breaks apart more uranium atoms, causing a chain reaction. The energy released causes water in the reactor to boil, turning into steam and turning a turbine, which then produces electricity. Some of the lighter elements produced in these chain reactions are quite radioactive and take tens of thousands of years or longer to decay, making disposal problematic. It’s also possible for nuclear fission reactors to melt down if the chain reaction gets out of control, as what happened in Chernobyl and Three Mile Island. Modern reactors are designed with incredibly redundant safety and shutoff systems to prevent these sorts of disaster scenarios.
Not every nuclear fission reactor is a power plant designed to produce electricity. Non-power-generating research reactors are used for their neutron output for applications such as radiation survivability testing, neutron radiography, and medical isotope production.
Fusion nuclear reactors are an altogether different beast from fission reactors. For starters, fusion works with much lighter elements. In the sun, we mainly see hydrogen, the lightest element, fused together to create helium, the second-lightest element. Here on Earth, fusion reactors combine deuterium and tritium, two heavy hydrogen isotopes. We choose to use deuterium and tritium for nuclear fusion fuel instead of emulating the hydrogen-hydrogen and helium-helium fusion reactions like our sun. This is because while the sun’s method works fine due to its gargantuan mass and size, at our much more modest scale, we can more easily induce a fusion reaction with a deuterium atom colliding with another deuterium atom (or tritium atoms) than with a hydrogen or helium fusion reaction.
Nuclear Byproducts and Radioactive Half-Life
One of the huge benefits of nuclear fusion over fission, and what makes it such an attractive energy source compared to not only fission but also basically every other energy source, is the waste material it leaves behind. Fission reactors leave behind very heavy elements from the splitting of uranium atoms which remain highly radioactive for up to tens or hundreds of thousands of years.
Every unstable and radioactive isotope has a “half-life,” or the amount of time it takes for half of any given sample of the material to decay into a stabler isotope that is no longer radioactive. For example, uranium-235, the particular isotope of uranium used as nuclear fuel, has a half-life of over seven hundred million years, while molybdenum-99, an isotope used to produce contrast agents for medical imaging, has a half-life of roughly two and a half days.
A smorgasbord of radioactive byproducts are produced from uranium and plutonium fission, some of which have half-lives of days or hours and some of which have half-lives in excess of two hundred thousand years. How to store and dispose of long-lived nuclear waste is a major concern regarding fission power, but practically a nonissue in fusion energy. Deuterium-deuterium and deuterium-tritium reactions produce helium-3 and helium-4, two stable isotopes of helium.
Magnetic Confinement Reactors
Fusion reactions begin with plasma, the fourth fundamental state of matter. Plasma is a hot, electrically conductive gas of ions and unbound charged particles that forms the perfect crucible for nuclear fusion, and all of our technology used to instigate fusion involves wrangling and controlling this state of matter in a high-energy, high-intensity environment. When ions collide with each other at high speeds, they can more easily break the Coulomb barrier and fuse, releasing the ions’ nuclear binding energy. This is what happens in the core of our sun. To replicate that energy-creating process here on Earth, we need technology that controls the flow of superheated plasma.
Magnetic confinement fusion relies on using powerful magnetic fields to contain and control the movement of superheated plasma. As particles within the plasma are guided by a strong magnetic field, they collide with each other and fuse into new elements. The concept of magnetic confinement was first developed in the 1940s, and initial research left scientists optimistic that magnetic confinement would be the most feasible way to produce fusion energy.
The most well-explored and well-known type of magnetic confinement system is the tokamak reactor, first developed by Soviet scientists Igor Tamm and Andrei Sakharov in the 1950s based on Z-pinch machines. A tokamak is a doughnut-shaped fusion reactor that generates a helix-shaped magnetic field using powerful electromagnets placed in the inner ring. A similar fusion reactor design, called a stellarator, uses external magnets to apply a containment field to the superheated plasma within the reaction chamber. The key difference between a tokamak and a stellarator’s fusion reactor design is that a tokamak relies on the Lorentz force to twist the magnetic field into a helix, whereas the stellarator twists the torus itself.
In the 1970s, and with a glut of funding pouring into research institutions from governments due to the energy crisis, experimental tokamak and stellarator (but mostly tokamak) reactors began to pop up all over the world. It didn’t take long to discover that magnetic confinement fusion, while certainly capable of generating clean fusion energy, was much more difficult to pull off than expected. In order to kick-start a fusion reaction powerful enough to output more fusion energy than it takes to sustain it and then keep it running (which is the important thing), you need very powerful magnets to keep the plasma flowing smoothly through the tokamak’s ring.
While the United States’ share of that funding dried up in the mid-80s after then-president Ronald Reagan declared the energy crisis over, work on tokamak development continued. Design work began on ITER, or the International Thermonuclear Experimental Reactor, in 1988. This was a joint effort between researchers from the United States, Soviet Union, European Union and Japan, as fusion energy researchers had quickly discovered that no one nation had the resources to develop a powerful enough tokamak on their own. Similar to ITER is the Joint European Torus, or JET, located at Culham Centre for Fusion Energy in the United Kingdom. JET is the world’s largest operational magnetically confined plasma physics experiment and one of its primary current uses is to test and refine features from ITER’s design. JET is one of the only facilities in the world that makes more neutrons than us!
Currently, while advances in plasma science and materials science are still needed to develop a fusion reactor that can output more fusion energy than it takes in, tokamak reactors are still regarded as the most promising path to clean fusion energy.
There are two broad categories of fusion reactors: magnetic confinement reactors and inertial confinement reactors.
Inertial Confinement Reactors
Inertial confinement fusion relies on shooting a high-energy laser beams at a fuel pellet target containing deuterium and tritium fuel for the reaction. The impact of the high-energy beam causes shockwaves to travel through the fuel pellet target, heating and compressing it to induce fusion reactions.
This method of inducing nuclear fusion reactions was first suggested in the 1950s, and in the 1970s, high-energy ICF research suggested that it could be a more promising path to fusion energy than tokamak and stellarator reactors. However, over the next two decades, researchers gradually discovered more and more hurdles that needed to be overcome in order to reach ignition, and estimations regarding how much energy the laser beams needed to induce fusion doubled on a yearly basis.
The National Ignition Facility at the Lawrence Livermore National Laboratory in Livermore, California is the largest and most energetic ICF system in the world. Completed in 2009, as of 2015 this system has only been able to reach one-third of the conditions needed for ignition. The NIF is currently used mainly for materials science and weapon research.
Other Places Where Nuclear Fusion Happens
Nuclear fusion reactions only naturally occur in stars, but here on Earth, nuclear fusion isn’t just happening at ITER and other fusion energy research centers. There are also fusion research facilities exploring fusion projects such as colliding beam fusion, which involves accelerating a beam of ions into a stationary target or another beam to induce a nuclear fusion reaction, similar to inertial confinement fusion. While this artificial fusion experiment doesn’t have much potential for fusion power generation, it has other uses in research and industry that are no less important.*
*Nuclear fusion also occurs inside thermonuclear or fusion bombs, also known as hydrogen bombs, which every sane person on Earth hopes we never, ever, ever have to use.
As it turns out, one of the most immediately useful outputs of fusion reactions—particularly deuterium-deuterium and deuterium-tritium reactions—isn’t energy, but rather neutron radiation. Neutron radiation is a byproduct of all nuclear processes, including fission and fusion, and since the 1950s, industrial and research applications such as neutron radiography and medical isotope production have depended on fission reactors for their high neutron yield. But recent developments in colliding beam fusion, or accelerator fusion, is making fusion a more convenient way to produce neutrons than fission.
On the largest scale of colliding beam fusion are enormous particle accelerators such as the Spallation Neutron Source at Oak Ridge National Laboratory, which produce massive neutron yields and are primarily used for neutron scattering research. Scientists use neutron scattering to better understand the molecular composition of materials such as metals, polymers, biological samples, and superconductors.
On the smallest scale of colliding beam fusion are sealed-tube neutron sources, which are very small accelerators—small enough to fit on a table or workbench, and often small enough to be used for fieldwork—that induce fusion by shooting a beam of deuterium or tritium ions at a deuterium or tritium target. The smaller the neutron source, the lower its yield, and these tiny sealed-tube sources tend to be used mostly for work which only needs a low neutron yield from a portable source, such as oil well logging, coal analysis, and most applications of neutron activation analysis. These sealed-tube sources are widely used in the petroleum industry.
In between massive spallation sources and tiny sealed-tube neutron sources are Phoenix’s high-flux neutron generators. These high-flux neutron generators work under the same basic principles as sealed-tube sources, except massively scaled up from tabletop-sized neutron emitters so that they can be used in the same high-yield industrial and research niches as fission reactors. Phoenix’s systems rely on inertial electrostatic fusion, not magnetic confinement fusion—meaning that the plasma is contained by a strong electric field, not a magnetic field.
The Phoenix Neutron Imaging Center in Fitchburg, Wisconsin uses a high-yield accelerator-based source to perform neutron radiography, which is crucial for aerospace manufacturers; SHINE Medical Technologies in Janesville, Wisconsin aims to produce a third of the world’s supply of medical radioisotopes in the coming years using accelerator-based neutron generators.
Coming back full circle to humanity’s quest to tame the power of the sun, high-yield fusion neutron sources, though ill-suited to generating the scientific holy grail of a fusion power plant, can be used to help us attain that goal. To make fusion power a reality, we need stronger materials to use in a fusion system and reactor, such as superconducting magnets and shielding material that can withstand the intense operating conditions, and through techniques such as neutron scattering and radiation hardening, we can design and develop the fusion reactor for the power plant of tomorrow.