What Is Nuclear Fusion?

Nuclear fusion is the process in which two light atomic nuclei join together to form a heavier nucleus, releasing energy as they do so. It is the reaction that powers the Sun and the other stars, and it is the source of nearly all the light and warmth that reaches the Earth. Fusion is often described as the opposite of nuclear fission, the reaction used in today’s nuclear power stations, and researchers have spent decades trying to recreate it in a controlled way on Earth as a possible source of low-carbon electricity.

How fusion releases energy

Atomic nuclei are made of protons and neutrons, and they carry a positive electric charge because of their protons. Like charges repel, so two nuclei approaching each other are pushed apart by a strong electrical force. If they can be driven close enough together, however, a much stronger but very short-range force, the strong nuclear force, takes over and binds them into a single heavier nucleus.

The energy comes from a small difference in mass. The heavier nucleus that results weighs slightly less than the sum of the pieces that made it. That missing mass is converted into energy, in line with Einstein’s relation between mass and energy. Because each fusion event converts a tiny amount of mass, and because a fuel contains an enormous number of nuclei, the total energy released is very large for the amount of fuel used.

Fusion in the Sun

In the core of the Sun, temperatures reach around fifteen million degrees Celsius and the pressure is immense. Under these conditions hydrogen nuclei fuse in a series of steps that ultimately turn hydrogen into helium, releasing energy at each stage. This is why the Sun shines, and it has been doing so for roughly four and a half billion years.

The Sun manages fusion partly through its colossal gravity, which squeezes its core to densities and pressures that cannot be reproduced on Earth. Because we cannot match that pressure, experimental reactors compensate by using even higher temperatures, so that nuclei move fast enough to overcome their mutual repulsion.

The rate at which the Sun fuses hydrogen is, by human standards, enormous, yet it is steady and self-regulating. If the core were to heat up slightly, it would expand and cool, slowing the reaction; if it cooled, it would contract and heat up again. This balance between gravity pulling inward and the pressure of fusion pushing outward keeps a star stable for billions of years, and it is only when the available fuel begins to run low that the balance is finally disturbed.

Fusion versus fission

Fusion and fission both release nuclear energy, but they work in opposite directions. Fission splits a heavy nucleus, such as uranium, into lighter fragments, and it is the reaction used in current nuclear power plants. Fusion joins light nuclei, such as forms of hydrogen, into a heavier one.

• Fusion fuels, such as isotopes of hydrogen, are abundant and can be sourced in part from seawater.
• Fusion produces no long-lived, high-activity radioactive waste of the kind associated with fission fuel, though reactor components can become activated.
• A fusion reaction cannot run out of control in the way a fission chain reaction can; if conditions are disturbed, the reaction simply stops.

Recreating fusion on Earth

The chief difficulty is holding a fuel hot enough and dense enough for long enough. The fuel must be heated to temperatures of the order of a hundred million degrees, far hotter than the Sun’s core, at which point it becomes a plasma, a gas so hot that its electrons are stripped from its nuclei. No solid material could contain such a plasma without being destroyed or cooling it instantly.

The leading approach, magnetic confinement, uses powerful magnetic fields to hold the plasma away from the walls of a doughnut-shaped chamber called a tokamak. ITER, an international project under construction in southern France, is being built to demonstrate that a tokamak can produce far more energy than is put in to heat the plasma. A separate approach, inertial confinement, compresses tiny fuel pellets with intense lasers. Both are the subject of active research, and experiments have produced fusion reactions, but a commercial fusion power station does not yet exist.

Why fusion matters

Interest in fusion rests on its potential to generate large amounts of electricity from plentiful fuel, without the greenhouse-gas emissions of fossil fuels and without the long-lived waste or meltdown risk associated with fission. That prospect is a major reason it attracts long-term research funding from many governments. The physics is well understood and fusion reactions have been demonstrated repeatedly in laboratories; the outstanding challenge is engineering a reactor that sustains the reaction and produces surplus energy reliably and economically. Fusion is frequently discussed alongside the wider shift towards low-carbon power described in the context of the greenhouse effect, though it remains a technology for the future rather than the present.