Fusion reactions happen in a hot gas known as plasma. Plasma is made of free moving electrons and positive ions, and has unique properties that are distinct from other gases, liquids, or solids.
Stars, including our sun, are balls of plasma made of hydrogen and helium, created by the gravitational collapse of clouds of cold gas, which is compressed and heated, becoming plasma. This all creates the perfect conditions, including the high temperatures of around ten million degrees Celsius, required for hydrogen fusion.
These high temperatures provide enough energy for light nuclei to overcome their natural electrical repulsion so that, once they come into very close range of each other, the nuclear force that attracts them outweighs the electrical repulsion (known as the Coulomb force), allowing them to fuse. As well as the heat and close proximity, fusion requires the nuclei to be confined in a small area. The extreme pressures created by the gravity of stars create this final requirement for fusion to occur.
When protons and neutrons are combined from lighter nuclei by this nuclear attraction, the nuclear reaction releases extra energy. This is not the same for heavier nuclei, which have a shorter-ranged nuclear force, which, instead of releasing energy with fusion, require energy as an input.
Stars create energy by fusing hydrogen nuclei and releasing helium. The core of the Sun fuses 620 million metric tons of hydrogen each second, making 616 million metric tons of helium per second as a result. For each two hydrogen nuclei that are fused to form helium, 0.645% of the mass is carried away as alpha particle kinetic energy and other forms of energy, such as electromagnetic radiation.
Research has investigated harnessing this natural reaction and using the energy that is created for our own needs. Humans have already used the inverse process, nuclear fission, where heavier elements like uranium and plutonium are split, to create energy. It is the binding energy between nuclear matter that measures the efficiency with which nucleons are bound together as well as determining the energy released by both fission and fusion.
With the right amount of heat, a close proximity and high levels of pressure, protons and neutrons can be forced together, releasing different levels of energy depending on their make-up of the element. Hydrogen, for example, consists of a single proton, whereas the heavy isotopes of hydrogen - deuterium (D) and tritium (T) – release more energy as they contain more parts (Deuterium has one proton and one neutron, while tritium has one proton and two neutrons). So far, we have looked at the fusion that occurs in stars, but scientists and engineers have been trying to recreate the conditions for fusion here on Earth.
The massive gravitational pull of stars help induce fusion, but without this force, higher temperatures are required to cause fusion. For example, on Earth, temperatures in excess of 100 million degrees Celsius are needed to cause fusion between deuterium and tritium.
The fusion process has been studied as a means of understanding nuclear matter, to learn about the nuclear physics of stellar objects, and to create thermonuclear weapons. However, there has been a move to develop fusion reactors for energy generation since the 1940s.
It was in 1920 that Arthur Eddington first suggested that hydrogen/helium fusion could be the main source of stellar energy, with Quantum Tunnelling being discovered in 1927 by Friedrich Hund. Following this, Robert Atkinson and Fritz Houtermans used the measured masses of light elements to demonstrate that large quantities of energy are released by fusing small nuclei.
Patrick Blackett’s early experiments in artificial nuclear transmutation led to the first laboratory-based fusion of hydrogen isotopes by Mark Oliphant in 1932. The rest of the 1930s saw Hans Bethe work out the theory of the main cycle of nuclear fusion in stars.
With the advent of the Second World War, it is no surprise that military purposes took precedence as research into fusion fed into the Manhattan Project of the early 1940s and the development of nuclear armaments. This led to the Ivy Mike hydrogen thermonuclear bomb test on 1 November 1952, where self-sustaining nuclear fusion occurred.
As research programmes in the UK, United States and Soviet Union led to a better understanding of fusion during the late 1940s and into the 1950s, investigators began to consider how the process could be used for practical energy generation.
The first research programmes for fusion reactors focused primarily on the use of magnetic fields and electromagnetic forces to contain the hot plasma needed to create nuclear fusion reactions. However, the expanding hot gases were difficult to contain as they readily escaped from the magnetic structures enclosing them.
Although the Cold War was still ongoing, the Second Geneva Conference on the Peaceful Uses of Atomic Energy in 1958 saw much of the research into fusion from the United States, UK and Soviet Union declassified, opening up the international collaboration that continues to this day.
The early 1960s saw work on inertial confinement fusion (ICF) begin, offering another potential method of creating fusion using lasers. The then-classified proposal sought to employ large pulses of laser energy to implode and shock heat matter to temperatures high enough to cause nuclear fusion. This work has since advanced and been declassified (starting in the 1970s), with design and development work still ongoing today to create short-pulse, high-power lasers and millimetre-sized targets capable of producing fusion.
Magnetic confinement still seems to be leading the charge for fusion reactors, with the necessary conditions for heat insulation and plasma temperatures having been largely achieved. While we are not quite at the point where nuclear fusion reactors are ready for use, experts believe we are getting close.
Scientists and engineers have been investigating how to replicate and harness nuclear fusion on Earth at an industrial scale.
The benefits of fusion fuel include providing a fully renewable, clean, safe and affordable source of energy. Fusion can generate around four times the amount of energy than fission per kilogram and nearly four million times the amount of energy produced from burning coal or oil.
Many of the fusion reactor concepts under development will use deuterium and tritium; hydrogen atoms containing extra neutrons and able to produce a terajoule of energy from just a few grams, which is enough to provide the energy for one person in a developed country for sixty years.
Another advantage of fusion is the ease with which the raw materials can be gathered. Deuterium can be extracted from seawater and tritium can be produced from the reaction of neutrons with lithium. Both of these supplies will last for millions of years.
Not only does fusion have the potential to provide nearly limitless clean energy, but it is also a safe source of energy that, unlike fission, doesn’t produce long lived nuclear waste. Because of the difficulties with starting and maintaining a fusion reaction, there is no danger of the reaction running away and creating a meltdown. Should there be an accident, the plasma will terminate and lose its energy very quickly before any sustained damage is done to the reactor.
Finally, fusion doesn’t emit any carbon dioxide or other harmful greenhouse gases into the atmosphere, providing a source of low carbon energy too.
There are two main methods being investigated for containing the high temperature plasma required for fusion reactions on Earth. These are magnetic confinement and inertial confinement. As well as these main methods, there has also been investigations into catalysing fusion through the use of muons as well as cold fusion and bubble fusion.
We will examine each of these processes in turn:
1. Magnetic Confinement:
This method uses magnetic fields to hold the plasma in place. The plasma is usually held inside a ring-shaped chamber called a torus, with powerful magnets placed around the inside edges. The magnetic field keeps the hot plasma in the middle of the chamber and away from the edges. The plasma can also create its own magnetic fields as it flows, which can also be used to further contain the plasma itself. This method has been successful within machines known as Tokamaks that can create the required heat, particle density and energy confinement to create a fusion reaction.
2. Inertial Confinement Fusion (ICF):
ICF uses compression rather than high temperatures to begin the fusion reaction. Compressing the fuel for short periods of time (measured in nanoseconds) increases the implosion speed and creates shockwaves that heat the centre of the cooled plasma. This shock heat initiates the fusion reaction that releases energy. This method uses the same processes of collapse, compression heating and subsequent nuclear fusion as used in thermonuclear weapons. Stars also use a similar set of processes, except these are caused by gravity, causing the start to collapse, heat and expand once more to reach an equilibrium between size and temperature. The most common technology used to create this type of fusion on Earth are high powered lasers, although particle accelerators have also been used. Research into this process, using lasers in very short bursts to initiate fusion, has been taking place at facilities including the Laser MegaJoule in Bordeaux, France and the National Ignition Facility at the Lawrence Livermore National Laboratory in Livermore, California, U.S.
3. Muon Catalysed Fusion:
This process is able to create fusion at much lower temperatures and involves substituting muons for the electrons that normally surround the nucleus of a fuel atom. Muons are negatively charged subatomic particles that are similar to electrons but far less stable. These muons can be created and then immediately injected into a mixture of deuterium and tritium, where it can join with a deuteron or a triton, forming an atom of D+-μ or T+-μ. This atom is now in an excited state, allowing the muon to be transferred between deuterons and tritons, or vice versa. The muons can also join with deuterons and tritons at the same time, forming a muonic molecule (D+-μ-T+). These muonic molecules create fusion between the deuteron and triton particles, releasing energy and allowing the muon to move on and join with more deuterons and tritons. However, this process is complex and needs a series of atomic, molecular and nuclear processes to take place before the muon decays. Generating the muons themselves also takes energy (around five billion volts per muon), so creating enough energy to offset the energy expended in creating fusion is a challenge, requiring at least 300 D-T fusion reactions within the half-life of each muon.
4. Cold Fusion:
Cold fusion was first announced in 1989, when two chemists claimed that they had created fusion reactions at room temperature using electrolytic cells containing heavy water (deuterium oxide, D2O) and palladium rods that absorbed the deuterium from the heavy water. However, there has been no theoretical explanation to back up the claims and global efforts to subsequently reproduce cold fusion failed.
5. Bubble Fusion:
In 2002, scientists claimed that they created fusion reactions during acoustic cavitation experiments involving chilled acetone that was bombarded with deuterium (deuterated). This used a technique called sonoluminescence, where a gas bubble is imploded with high-pressure sound waves. As these bubbles implode the conditions for high density and temperature are created, leading to the emitting of light. The scientists said they had used larger, millimetre-sized bubbles that had been deuterated in an acetone liquid to create densities and temperatures and capable of inducing fusion reactions before the bubbles broke up. However, as with cold fusion, subsequent attempts to replicate the results have failed.
Nuclear fusion research has been conducted in over 50 countries over the past decades with varying levels of success.
The future of fusion technology development and speeding up the roll-out of nuclear fusion as a viable energy source relies on global collaboration. Emerging technologies need to be developed, validated and qualified alongside the development of supporting infrastructure and standards.
Currently, full-power experiments are due to start at the ITER project in France in 2036, with a date of 2050 for the realisation of an operational electricity-producing fusion power plant.
Alongside government-led research there is also a growing number of privately-funded commercial enterprises that are drawing on and expanding the decades of publicly funded research into fusion. These privately funded organisations are hinting at a date earlier than 2050 for the first operational fusion nuclear energy plant.
As the world population grows there will be a growing need for energy. This demand is likely to be met through a mix of different energy sources, including hydrokinetic energy, wind power, solar and a new generation of nuclear fission plants.
Although it is still some years away, nuclear fusion could prove to be an important addition to the future energy mix, given the potential to produce clean and renewable energy on a large scale.
To reach this point there is still work to be done to ensure that large nuclear fusion plants are effective, fully tested and safe to go into operation. Nuclear fusion has already been achieved at experimental fusion reactors, so it is surely just a matter of time until it becomes part of the clean global energy solution.
You can find out more about nuclear energy in our dedicated FAQ and a comparison of nuclear fusion and nuclear fission, here.
Can nuclear fusion go on forever?
Nuclear fission has the potential to be an almost limitless source of energy due to the abundance of hydrogen in the universe. With regards to radioactive waste, there is some produced by the process but it has a lifetime of around 100 years, as opposed to the thousands of years from fission waste.
As for the process itself, fusion is difficult to achieve and therefore also to maintain so, far from going on for a long period of time, nuclear fusion will burn itself out faster than fission should something go wrong.
Is nuclear fusion possible?
Nuclear fusion is possible and has already successfully provided an energy yield during tests. While the latest amount of energy produced by nuclear fusion on Earth is still relatively small, it is a good breakthrough after decades of research.
What are the three conditions needed for nuclear fusion?
The three conditions needed for nuclear fusion are heat, proximity and pressure. The high heat (at least 100 million degrees Celsius) allows the ions to overcome the Coulomb barrier and fuse together, the proximity of the ions to each other allow them to fuse and the pressures keep the ions close and prevents plasma cooling.
What is the basic concept of nuclear fusion?
The basic concept of nuclear fusion is when two light atomic nuclei join together to form a single heavier one. This produces large amounts of energy while also sometimes releasing other substances – such as with hydrogen fusion releasing helium.
Are nuclear fusion reactors safe?
The technologies and fundamental physics behind nuclear fusion reactors make them safe. While fission plants can suffer meltdowns or runaway reactions, this is not possible with fusion, particularly as only a small amount of fuel is required for the process (less than four grams at any given time).
Is nuclear fusion renewable?
Nuclear fusion has the potential to provide renewable energy. The main challenge is the power required to create the fusion process, but if this is sourced from renewable sources, then fusion energy has the potential to provide almost limitless, renewable energy.
Can nuclear fusion be controlled?
Research shows that fusion can be controlled, but given the difficulty in achieving nuclear fusion, the problem is not as much of control than of sustaining fusion.
Can nuclear fusion be used to generate electricity?
Yes, nuclear fusion can be used to generate electricity. However, we are not yet at the point where the technology has progressed enough to produce large enough quantities of electricity to be a viable resource. The challenges around creating more energy than is used and to create nuclear fusion at scale are being investigated around the world.
Can nuclear fusion be dangerous?
Nuclear fusion is inherently safe, emitting no harmful greenhouse gases and creating no long-lived, high activity nuclear waste. There is also no chance of a reactor meltdown or a runaway reaction, meaning that fusion is safer than nuclear fission.
Can nuclear fusion replace fossil fuels?
There is potential for nuclear fusion to replace fossil fuels, once fully developed as an energy resource. However, it is more likely that nuclear fusion will be just one part of a clean energy mix alongside other methods like wind and solar power.
Do nuclear fusion reactors exist?
Currently, only pilot and experimental fusion reactors exit, but the first operational, commercial nuclear fusion reactor is expected to be in use by 2050.
Has nuclear fusion ever been achieved?
Nuclear fusion was first achieved in laboratory experiments in the 1930s and has since been demonstrated at pilot nuclear fusion plants, but we have still not managed to harness the energy produced to generate large quantities of electricity.
Where is nuclear fusion used?
Nuclear fusion exists naturally in stars including the Sun, where hydrogen nuclei fuse and create helium while releasing the energy that lights and heats the Earth. Nuclear fusion has also been used in nuclear weapons, but research to harness fusion power for electricity generation is still ongoing.
Will nuclear fusion solve the energy crisis?
Nuclear fusion has the potential to produce energy without greenhouse gases and minimal nuclear waste, creating a resource that provides large amounts of energy without adding to global warming. Also, the fuels used for fusion are relatively easy to resource and virtually inexhaustible.
However, fusion as a viable energy source is still some years off…
When will nuclear fusion be available?
Most estimates say that the first nuclear fusion energy plant will be operative by 2050. However, others argue that nuclear fusion will be available much sooner, with some – mainly privately funded - operations saying that reactors could be available for sale in the early 2030s.