In nuclear fusion, light atomic nuclei fuse into heavier ones, just as they do in the Sun. This releases an enormous amount of energy: one gram of fusion fuel delivers as much energy as more than ten tonnes of hard coal.
What is nuclear fusion?
Nuclear fusion joins light atomic nuclei into heavier ones. Energy is released in the process, and this is also how the stars shine.
Why does fusion need millions of degrees?
Atomic nuclei are positively charged, so they repel each other. To fuse, they have to collide at very high speed. Temperature is a measure of how fast particles move, which is why the fuel has to be extremely hot.
Cold
At low temperature, the hydrogen isotopes deuterium (D) and tritium (T) move slowly and at random. They hardly ever come close to each other.
Warm
Nuclei with the same charge always repel each other, at any temperature. As the temperature rises, the particles get faster, but not yet fast enough: just before they touch, the repulsion pushes them apart again.
Extremely hot
At around 150 million degrees, ten times hotter than the core of the Sun, the particles are fast enough to overcome the repulsion and collide head-on.
Fusion
The two nuclei fuse into helium (He). This releases a neutron (n), which flies off with a great deal of energy.
What happens during fusion?
A closer look inside the atoms shows what deuterium and tritium are made of and where the energy comes from.
- Proton
- Neutron
- Electron
Two atoms
The deuterium nucleus consists of one proton and one neutron, the tritium nucleus of one proton and two neutrons. One electron orbits each nucleus.
Plasma
In the heat, the electrons break away from the nuclei. This mixture of free electrons and bare nuclei is called plasma. The nuclei can now collide directly.
Fusing
Once the nuclei get close enough, the strong nuclear force holds them together. For a tiny moment, two protons and three neutrons form a single nucleus.
Helium and neutron
This nucleus immediately splits into a helium nucleus and a free neutron. Together they are slightly lighter than the original nuclei, and the missing mass is released as energy. The fast neutron carries about four fifths of it and is meant to generate heat in a power plant.
What a fusion power plant could look like
In principle, a fusion power plant works like a conventional thermal power plant: the energy released heats water, the steam drives a turbine and a generator produces electricity. Only the heat source is new. Solid lines show hot water and steam, dashed lines the cooled water on its way back.
Plasma
Inside floats the plasma, at around 150 million degrees. Powerful magnet coils, drawn in black here, keep it away from the wall. This is where deuterium and tritium fuse.
Blanket
The fast neutrons leave the plasma and are slowed down in the surrounding wall, the blanket. Their kinetic energy turns into heat. At the same time, the blanket breeds new tritium from lithium.
Steam generator
Water cools the blanket and carries the heat to the steam generator. There it brings the water of a second circuit to the boil.
Turbine
The hot steam flows through the turbine under high pressure and sets it spinning fast.
Generator
The turbine drives the generator, which converts the rotation into electricity that flows into the grid.
Condenser
After the turbine, the steam cools down and turns back into water. It flows back to the steam generator, and the cycle starts again.
Which reactor types are there?
Around the world, research focuses on three ways of holding the hot fuel together long enough and densely enough. Two of them use magnetic fields, one relies on lasers.
Tokamak
Ring-shaped coils and an electric current in the plasma itself together create the magnetic field that holds the plasma in a ring. The tokamak is the most advanced concept.
Stellarator
Here the coils alone shape the magnetic field. They are intricately twisted, so the plasma winds around the ring like a twisted ribbon. In return, a stellarator can run without pauses.
Laser fusion
Powerful laser pulses hit a tiny fuel pellet from all sides. Within billionths of a second it is compressed and heated so strongly that the nuclei fuse.
Frequently asked questions
Nuclear fission works the other way round: it splits heavy atomic nuclei such as uranium into smaller ones. Fusion, by contrast, joins light nuclei and needs no uranium. A chain reaction is physically impossible in fusion, and it produces no spent fuel that has to be stored for thousands of years.
Development is moving faster than ever. In 2022, the National Ignition Facility in the USA became the first to get more energy out of fusion than its lasers had put into the fuel. Facilities such as Wendelstein 7-X keep setting new records, and governments and private companies around the world are investing billions. Many companies are planning their first pilot plants for the 2030s. With the right framework, Germany can be at the very front.
The Bundesverband für Kernfusion brings together companies, research institutions and other stakeholders so that cutting-edge research becomes clean energy. It informs policymakers and the public about the opportunities of nuclear fusion and the state of its development. It also represents the shared interests of its members and works to make Germany the leading hub for fusion energy.
Yes, safety is one of fusion's great strengths. A chain reaction like the one in nuclear fission is physically impossible. There are only ever a few grams of fuel in the reactor, and any disturbance cools the plasma within a fraction of a second, so the reaction simply stops. A meltdown cannot happen. Proven protection concepts exist for handling tritium and activated components.
Together we can move fusion forward faster. Companies and organisations that work on nuclear fusion or want to support its development can become members. As an individual, you can help too, by attending our events, talking with us and passing on what you know about fusion energy. Just write to us, we look forward to hearing from you.
Fusion energy combines everything a climate-friendly power supply needs. It delivers electricity around the clock, regardless of weather or time of day, and emits no CO₂ during operation. The fuel is practically inexhaustible: deuterium is found in seawater, and tritium is produced from lithium inside the power plant itself. A fusion power plant needs little land and leaves behind no long-lived fission products.