Nuclear fusion

Explore nuclear fusion with the IOP

New technologies and scientific advances have propelled fusion power from a distant dream to a distinct possibility. Its potential for clean, limitless energy could be the key weapon to combat climate change.


Nuclear fusion and nuclear fission. Their names may be similar but they are in fact completely different. Though they both alter atoms to create energy, fission splits one atom into two, whereas fusion glues two lighter atoms into a larger one.

Nuclear power plants providing electricity to homes around the world rely on fission, splitting uranium atoms to heat water and ultimately produce energy. But the dream for nuclear power is to harness nuclear fusion – the same process powering all stars, including our Sun.

Fission and fusion - at a glance
  • Nuclear fusion is the process that makes stars - including our sun - so hot that they glow, giving out light and heat, releasing an enormous amount of energy.
  • During nuclear fusion, energy is released when the nuclei of light atoms like hydrogen are combined or fused together.
  • Nuclear fission is the opposite process, where heavy elements such as uranium are split apart, releasing heat in the process.
  • Currently, energy is generated in nuclear power plants through fission, not fusion.
  • Scientists want to build fusion power plants because fusion is much safer, and releases four times more energy than fission with no long-lived toxic waste.

Fusion would have a limitless supply of fuel, running on atoms distilled from water. It would release four times the energy of nuclear fission. Moreover, it would come with none of the risks of fission, with no possibility of a Fukushima-like nuclear meltdown and no long-lived radioactive waste.

Why then, are we not building nuclear fusion power plants right now? Well, fusion science is tricky. To achieve a controlled nuclear fusion reaction here on Earth, scientists attempt to combine two forms of hydrogen: deuterium and tritium. Hydrogen has just a single proton and an electron. Deuterium has an extra neutron in its nucleus, and tritium has two extra neutrons. When they are combined, they form helium and release a neutron. Crucially, the final mass is less than the initial mass. By Einstein’s famous equation E = mc2, this means a huge amount of energy is produced. The energy can then be converted to electricity via steam just like current fission power plants.

The tricky bit is bringing the deuterium and tritium together. To make it happen, scientists need extremely high temperatures (150 million °C) and pressures, and to maintain them for long enough to create a self-sustaining fusion reaction. Most importantly, they need to ensure that the fusion process releases more energy than is put in to creating these extreme conditions.

So far, scientists have been unable to sustain fusion reactions on Earth for more than a few seconds, and are not even close to breaking even in terms of energy input versus output. The closest any fusion device came was in 1997 when the Joint European Torus (JET) in the UK generated 16 MW of fusion power for 24 MW of input power.

JET is an example of a doughnut-shaped magnetic confinement device called a tokomak. Tokomaks are widely regarded as the most promising design for a practical fusion reactor. Over 200 have been built around the world since the first in 1958. Yet as early as the 1980s it had become clear that problems limiting their performance meant they needed to be much bigger, much more complicated and much more expensive.

These concerns led to the launch of an international joint experiment in fusion in 1985. Almost 35 years later, construction is underway in rural southern France on the biggest and most ambitious tokomak ever conceived – ITER. ITER is a collaboration between 35 countries to build a ~€20 billion tokomak capable of producing 10 times more energy than needed to run the machine –  500 MW of power for 20 minutes at a cost of only 50 MW input power. If successful, it will be the first fusion device to produce net energy and the first to maintain fusion for long periods of time.  

ITER is expected to start running in 2025, reaching full operations a decade later. Yet it is not designed to actually produce electricity. ITER’s planned successor DEMO (DEMOnstration Power Station) will hopefully do this around the mid-21st Century. And, if successful, DEMO will pave the way for commercial fusion power plants after.

These long timescales mean ITER-like fusion power plants will only be contributing substantially to the energy mix near the end of the century. Might there be a simpler and cheaper way of harnessing fusion energy?

Both private companies and public organizations around the world think there is. Among other businesses, Tokamak Energy, TAE Technologies, General Fusion, Lockheed Martin and Commonwealth Fusion Systems are all pursuing more compact and less expensive designs they hope will pave the way to the first commercially viable fusion power plants.

Many of these are based on a design called a ‘spherical tokomak’. Spherical tokomaks were first pioneered in the late 1990s by the UK Atomic Energy Agency with the START and MAST experiments and the Princeton Plasma Physics Laboratory’s NSTX machine. These experiments have shown that spherical tokomaks are more efficient than their more traditional counterparts like ITER and JET, requiring less magnetic field. However, they come with challenges, not least removing the intensely concentrated heat from the reactor.

Those in the industry believe these challenges can be overcome in the next few years, with commercial fusion power plants coming online as early as the 2030s. As we witness the mounting effects of climate change, the sooner fusion power can provide limitless, clean energy the better.

Learn more

Interested in learning more about nuclear fusion? Physics World and IOP’s partners have great content that will satisfy your curiosity:

  • Nuclear Fusion, published by IOP Publishing on behalf of the  International Atomic Energy Agency (IAEA), is the acknowledged world-leading journal specialising in fusion.
  • Physics World has produced news articles, magazine features, editorials, reviews and podcasts dedicated to nuclear fusion.