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An artist's impression of how laser beams from the NIF are concentrated on a fuel pellet to produce nuclear fusion

Nuclear fusion

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by Chris Woodford. Last updated: December 11, 2016.

When scientists "split the atom" in the early 20th century, they thought they were revealing how the world is built from basic bits of matter. What they didn't realize at the time was that they'd invented a completely new way of producing energy that would soon be deployed in atomic bombs and nuclear power plants. Ernest Rutherford, the scientist who led the early atom-smashing experiments, famously said that "anyone who expects a source of power from the transformation of the atom is talking moonshine." In one sense at least, he was correct: though it's widely used in many countries, nuclear power has proved hugely expensive and politically controversial, caused some catastrophic accidents, polluted seas, and generated horrible amounts of highly dangerous radioactive waste. Today's nuclear power plants release energy by splitting up large atoms (in what's called nuclear fission); tomorrow's plants could work using an entirely different form of nuclear power where small atoms are forced together to make bigger ones. Known as nuclear fusion, this is much cleaner and safer and could potentially solve our energy needs forever. What is fusion and how does it work? Let's take a closer look!

Photo: One way to make nuclear fusion is to confine atoms in a small space and then blast them with laser beams, as in this experimental setup at the National Ignition Facility (NIF) in California. More about this further down the article. Photo credit: Lawrence Livermore National Laboratory.

What is nuclear fusion?

A closeup of the Sun taken by the Solar and Heliospheric Observatory (SOHO)

Photo: The Earth is powered by nuclear fusion happening inside the Sun. Some time in the future, we might be able to create small-scale fusion power ourselves, here on Earth. This photo was taken by the Solar and Heliospheric Observatory (SOHO), a joint project of the ESA (European Space Agency) and NASA. Picture courtesy of NASA Goddard Space Flight Center.

It's not immediately obvious, but almost everything you're doing right now (from breathing in and out to tapping away on your computer) and everything you can see around you (trees blowing, grass growing, and cars speeding by your window) is powered by nuclear fusion—because that's what drives the Sun, and the Sun drives the Earth. If you could get close enough to the Sun to peer inside its core and see what's going on inside, you'd see atoms of hydrogen joining together ("fusing") to make atoms of helium, releasing huge amounts of energy in the process. Apart from making incredible amounts of heat inside the Sun itself (the core is at a temperature of at least 10 million degrees), the Sun's nuclear fusion also produces the solar energy that streams out across 150 million kilometers (93 million miles) of space sustaining pretty much all the life we see on Earth.

Now suppose we could make a machine that copied what happens inside the Sun here on Earth, only in a much smaller and more controllable way. In theory, we'd just feed in hydrogen (a simple, fairly safe gas we can make from water) at one end, smash its atoms together inside, and get helium (a clean and safe gas) out of the other end. In the process, we'd produce huge amounts of heat energy, which we could use to drive steam turbines and generators to produce electricity, much as in any conventional power plant. There'd be no pollution, no carbon dioxide (one of the so-called greenhouse gases that causes global warming and climate change), and no deadly nuclear waste. We'd have simple, clean, safe, nuclear power!

Why do we need yet another kind of energy?

225kW wind turbine in Staffordshire, England

Photo: Wind turbines can make clean, green energy, but it takes at least 1000 of them to replace a single coal or nuclear power plant. This is a relatively small wind turbine that generates peak power of just 225kW, so you would need about 9000 turbines like this to make as much energy as a large 2GW (2000 megawatt) coal or nuclear plant (assuming the turbine worked at maximum power all the time, which of course it doesn't). Typical large wind turbines generate 10 times more power than this one.

We've got lots of different ways of making the energy we need—oil, gas, coal, nuclear, wind, solar, wave, hydro, waste incineration, and biomass, to name just 10 of them—so why do we need any more? The answer is that three big problems are going to hit us hard in the next few decades:

  1. The world's population is still increasing and it's expected to rise from about 7.5 billion today to about 9 billion by 2050. Meanwhile, lots of the people in developing countries who currently use very little energy may well want to use more in future as their standard of living improves. So we'll have lots more people on the planet and each one of them, on average, using more energy than each person does today. It's been estimated that the world will need perhaps 2–3 times as much energy in 2050 as it does now.
  2. About 80 percent of our energy currently comes from fossil fuels such as oil, gas, and coal. We have only limited supplies of these fuels and oil and gas (in particular) are rapidly running out.
  3. The third problem we have is that fossil fuels make carbon dioxide gas when we burn them (or use them in engines) to release energy. That's creating climate change that could ultimately make our planet impossible to live on.

What's the answer? Renewable energy made from the Sun, the wind, the oceans, and other sources is one solution to these problems, but we're not building it quickly enough at the moment to make enough difference (for example, it takes thousands of wind turbines to make as much electricity as a single coal-fired power plant). If we could build nuclear fusion plants that make as much electricity as today's coal or nuclear fission plants, with none of their drawbacks, we could potentially solve Earth's energy problems forever, making all the power we need without wrecking the environment.

How can we make nuclear fusion happen on Earth?

There's obviously a huge difference between the center of the Sun and the inside of a power plant on Earth—so how could we make nuclear fusion reactions happen in practice?

It turns out that, instead of using ordinary hydrogen atoms, the best approach is to use other isotopes (different atomic forms) of hydrogen that are slightly heavier. An ordinary hydrogen atom has just one proton in the nucleus and one electron in the space around it, but these heavier hydrogen isotopes are built differently (and are much less stable as a result). One of them, known as deuterium, has one proton and one neutron in the nucleus and one electron in the outside space. The other, called tritium, has one proton and two neutrons and two electrons outside. Smash together an atom of deuterium and an atom of tritium and the bits we have to play with are two protons, three neutrons, and two electrons. From these, we can make one very stable atom of helium (which has two protons, two neutrons, and two electrons) and have one neutron left over. By converting two unstable atoms (one each of deuterium and tritium) into one stable helium atom, we release a great deal of energy.

Diagram showing the nuclear reaction between deuterium and tritium that produces helium and a spare neutron.

Artwork: The nuclear reaction that produces fusion energy. Protons are shown as blue and neutrons as red. (Electrons aren't shown.)

Now what we have here sounds a little bit like a chemical reaction, where we might react something like sodium metal with chlorine gas to get sodium chloride (table salt). Chemical reactions can release energy if the products (the things you end up with) are more stable than the reactants (the things you start off with). However, you get much more energy out of a nuclear reaction (where the atoms you start with are changed into completely different atoms) than a chemical reaction (where the atoms retain their identity but join or rearrange into different molecules). That's because the amount of energy needed to hold the nucleus of one atom together (known as its binding energy) is vastly greater than the amount of energy involved in binding two different atoms together. (In other words, there's more energy that can be released when a nucleus breaks apart than when a molecule breaks apart.)

How much energy is released in a fusion reaction?

If you compare the mass of a deuterium atom plus a tritium atom (what you start off with) with the mass of a helium atom and a neutron (what you finish with), you'll find some mass is "lost" during a fusion reaction. That's the mass that's converted into energy as the nuclei fuse together; the amount of energy is precisely related to the amount of mass by Einstein's famous equation E=mc2, which shows that a tiny amount of energy can produce a huge amount of mass (because c, the speed of light, is a very large number and c squared, which is c×c is even bigger). The amount of energy released in one single reaction is 17.59 MeV (17.59 million electron volts), which is one of those baffling measurements only physicists use and understand. Translating that into more meaningful terms, you'd need about 3,500,000,000,000 (3.5 million million) of those reactions happening every single second to light a 10-watt, low-energy lamp. That sounds a lot, but remember that 1g (0.04 oz) of ordinary, pure hydrogen contains 600,000,000,000,000,000,000,000 (600,000 million million million) atoms and you can see we could potentially make a huge amount of fusion power with very little fuel at all. (The figures are slightly different if we take account of deuterium and tritium, but the basic point—a small amount of matter contains an awful lot of atoms—still holds.)

How would nuclear fusion plants work in practice?

As we've just discovered, the basis of nuclear fusion on Earth is converting deuterium and tritium to helium, with a big release of power as two unstable atoms rearrange themselves to make one stable atom. It sounds simple enough, but no-one has yet managed to make fusion work on a big enough scale to generate commercial amounts of power. That's because there are huge practical difficulties involved. To make two atoms fuse, you have to push their nuclei closer and closer together. The trouble is, the nucleus of every atom has a relatively large positive, electrical charge so two nuclei repel one another in much the same way as the north poles of two magnets ("like poles repel"). The nearer the nuclei get, the more energy it takes to get them any closer—and the amount of energy keeps on increasing. In fact, the repulsive force between two nuclei quadruples every time you halve the distance (if you're interested, that's because of Coulomb's Law). In practice, this means you have to use a vast amount of energy to make atoms fuse together in the hope that you can release even more energy in the end. So far, scientists have not even got to the point where they can "break even" (release anything like as much energy as they use). That's why nuclear fusion is currently being studied only in scientific laboratories and why, for the moment at least, it's not a practical or viable source of power.

Confinement

What are scientists actually doing in those fusion labs? To make atoms fuse, we have to heat them up to make plasmas (ultra-hot soups of gases in which the atoms get so hot that they're blown apart into their constituent nuclei and electrons), then hold them tightly together in a confined space at temperatures at least as hot as you get in the center of the Sun. So the problem of generating energy by fusion becomes a slightly different one: how can we confine a super-hot plasma so it stays hot and energetic enough for the atoms inside it to overcome their natural repulsion and fuse together?

Artwork showing the ring-shaped plasma confined inside a tokamak.

Photo: The donut-shaped plasma confined inside a tokamak. Artwork by Kwan-Liu Ma, University of California, Davis, courtesy of Argonne National Laboratory, published on Flickr under a Creative Commons Licence.

In stars like the Sun, gravity is the force that achieves this. On Earth, we have to achieve the same end artificially and there are two basic ways to do this. In one method, called magnetic confinement, deuterium and tritium are heated to sun-core temperatures of about 100 million degrees. Then a super-strong magnetic field is used to trap them in a donut-shape known as a torus. A machine that does this is known as a tokamak, a word coined in Russia for just this piece of apparatus (the biggest in the world is currently operating at a lab known as JET, Joint European Torus, a few km/miles south of Oxford in England). The other method of fusion is called inertial confinement and involves firing a powerful laser at a pellet of fuel so the atoms inside it are instantly heated and fused together. The fuel burns before it can blow apart, so its own mass (and therefore inertia) is effectively what confines it. One example of this technique is being used at NIF (National Ignition Facility) at Lawrence Berkeley National Laboratory in California, where 192 laser beams (collectively making the world's biggest laser) are fired simultaneously at a tiny fuel pellet to create the right conditions for nuclear fusion.

Impressive though they are, JET and NIF are still just lab experiments. Hopes of achieving anything like commercially viable fusion are now pinned on another international project known as ITER, currently under construction in southern France. Once complete, in 2019, it should be able to generate about 10 times more energy than it consumes for up to about 8–10 minutes. Even that, which will be a major achievement, will be far short of generating fusion power hour after hour, day after day, and year after year. But it's a start!

Can fusion ever deliver?

Nuclear fusion offers great promise for the future: huge supplies of clean energy made from readily available fuels that are effectively unlimited. It generates no nuclear waste or pollution (other than reactor equipment, which remains contaminated at the end of its life for about a century) and no greenhouse gases. There's no risk of nuclear accidents similar to those that have occurred with fission plants (in horrific incidents such as at Chernobyl, Ukraine in 1986 and Fukushima, Japan in 2011). On the negative side, scientists estimate we're still several decades away from seeing commercial fusion plants. It's only a matter of time before fusion plants are making some of our daily electricity, but can we develop them fast enough before Earth's energy crisis, and climate change, really hit home?

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Woodford, Chris. (2012/2016) Nuclear fusion. Retrieved from http://www.explainthatstuff.com/nuclear-fusion.html. [Accessed (Insert date here)]

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