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What is Nuclear Fusion?

Nuclear Explained

Nuclear fusion is the process by which two light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy.

Fusion reactions take place in a state of matter called plasma — a hot, charged gas made of positive ions and free-moving electrons with unique properties distinct from solids, liquids or gases.

The sun, along with all other stars, is powered by this reaction. To fuse in our sun, nuclei need to collide with each other at extremely high temperatures, around ten million degrees Celsius. The high temperature provides them with enough energy to overcome their mutual electrical repulsion. Once the nuclei come within a very close range of each other, the attractive nuclear force between them will outweigh the electrical repulsion and allow them to fuse. For this to happen, the nuclei must be confined within a small space to increase the chances of collision. In the sun, the extreme pressure produced by its immense gravity creates the conditions for fusion.

Why are the scientists studying fusion energy?

Ever since the theory of nuclear fusion was understood in the 1930s, scientists — and increasingly also engineers — have been on a quest to recreate and harness it. That is because if nuclear fusion can be replicated on earth at an industrial scale, it could provide virtually limitless clean, safe, and affordable energy to meet the world’s demand.

Fusion could generate four times more energy per kilogram of fuel than fission (used in nuclear power plants) and nearly four million times more energy than burning oil or coal.

Most of the fusion reactor concepts under development will use a mixture of deuterium and tritium — hydrogen atoms that contain extra neutrons. In theory, with just a few grams of these reactants, it is possible to produce a terajoule of energy, which is approximately the energy one person in a developed country needs over sixty years.

Fusion fuel is plentiful and easily accessible: deuterium can be extracted inexpensively from seawater, and tritium can potentially be produced from the reaction of fusion generated neutrons with naturally abundant lithium. These fuel supplies would last for millions of years. Future fusion reactors are also intrinsically safe and are not expected to produce high activity or long-lived nuclear waste. Furthermore, as the fusion process is difficult to start and maintain, there is no risk of a runaway reaction and meltdown; fusion can only occur under strict operational conditions, outside of which (in the case of an accident or system failure, for example), the plasma will naturally terminate, lose its energy very quickly and extinguish before any sustained damage is done to the reactor.

Importantly, nuclear fusion — just like fission — does not emit carbon dioxide or other greenhouse gases into the atmosphere, so it could be a long-term source of low-carbon electricity from the second half of this century onwards.

Hotter than the sun

While the sun’s massive gravitational force naturally induces fusion, without that force a temperature even higher than in the sun is needed for the reaction to take place. On Earth, we need temperatures of over 100 million degrees Celsius to make deuterium and tritium fuse, while regulating pressure and magnetic forces at the same time, for a stable confinement of the plasma and to maintain the fusion reaction long enough to produce more energy than what was required to start the reaction.

While conditions that are very close to those required in a fusion reactor are now routinely achieved in experiments, improved confinement properties and stability of the plasma are still needed to maintain the reaction and produce energy in a sustained manner. Scientists and engineers from all over the world continue to develop and test new materials and design new technologies to achieve net fusion energy.

See more information in the following video:

The Future of Fusion Energy

Providing energy from nuclear fusion is widely regarded as the grand engineering challenge of the twenty-first century. What needs to be done to make fusion power commercially viable?

Where do we stand on fusion technology development?

Nuclear fusion and plasma physics research are carried out in more than 50 countries, and recently researchers have finally achieved scientific energy gain in a fusion experiment for the first time. Experts have come up with different designs and magnet-based machines in which fusion takes place, like stellarators and tokamaks, but also approaches that rely on lasers, linear devices and advanced fuels.

How long it will take for fusion energy to be successfully rolled out will depend on mobilizing resources through global partnerships and collaboration, and on how fast the industry will be able to develop, validate and qualify emerging fusion technologies. Another important issue is to develop in parallel the necessary nuclear infrastructure, such as the requirements, standards, and good practices, relevant to the realisation of this future energy source.

Following 10 years of component design, site preparation, and manufacturing across the world, the assembly of ITER in France, the world’s largest international fusion facility, commenced in 2020. ITER is an international project that aims to demonstrate the scientific and technological feasibility of fusion energy production and prove technology and concepts for future electricity-producing demonstration fusion power plants, called DEMOs. ITER will start conducting its first experiments in the second half of this decade and full-power experiments are planned to commence in 2036.

DEMO timelines vary in different countries, but the consensus among experts is that an electricity-producing fusion power plant could be built and operating by 2050. In parallel, numerous privately funded commercial enterprises are also making strides in developing concepts for fusion power plants, drawing on the know-how generated over years of publicly funded research and development, and proposing fusion power even sooner.

What is the role of the IAEA?

The IAEA has a long history of being at the core of international fusion research and development, and recently started supporting early technology development and deployment

  • The IAEA launched the Nuclear Fusion journal in 1960 to exchange information about advances in nuclear fusion. The journal is now considered the leading periodical in the field. The IAEA also regularly publishes TECDOCs and outreach and educational material on fusion.
  • The first international IAEA Fusion Energy Conference was held in 1961 and, since 1974, the IAEA convenes a conference every two years to foster discussion on developments and achievements in the field.  See a short film about the history of this conference series
  • Since 1971, the IAEA International Fusion Research Council has served as a catalyst for establishing improved international collaboration in fusion research.
  • The ITER Agreement is deposited with the IAEA Director General. Collaboration between the IAEA and the ITER Organization is formalized through a cooperation agreement in 2008, which was expanded and deepened in 2019.
  • The IAEA facilitates international cooperation and coordination on DEMO programme activities around the world.
  • The IAEA implements a series of technical meetings and coordinated research activities on topics relevant to fusion science and technology development and deployment, and organizes and supports education and training activities on fusion.
  • The IAEA maintains numerical databases of fundamental data for fusion energy research, as well as the Fusion Device Information System (FusDIS), which compiles information on fusion devices operating, under construction or being planned around the world.
  • The IAEA is carrying out a project on synergies in technology development between nuclear fission and fusion for energy production, and on the long-term sustainability – including the handling of radioactive waste – and legal and institutional issues for fusion facilities.
  • The IAEA is investigating key safety aspects covering the whole lifecycle of fusion facilities, where guidelines and specific reference documents are needed.
  • The IAEA is supporting a pre-feasibility study of a generic fusion demonstration plant.


This article was first published on on 31 March 2022.

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