Fusion power
Fusion power is a potential method of electric power generation from heat released by nuclear fusion reactions. In fusion, two light atomic nuclei combine to form a heavier nucleus and release energy. Devices that use this process are known as fusion reactors.
Research on fusion reactors began in the 1940s. As of 2025, the National Ignition Facility in the United States is the only laboratory to have demonstrated a fusion energy gain factor above one, but efficiencies orders of magnitude higher are required to reach engineering breakeven or economic breakeven.
Thermonuclear fusion reactions require fuel in a plasma state and a confined environment with high temperature, pressure, and sufficient confinement time. The relationship between these parameters is expressed by the Lawson criterion. In stars, gravity provides the conditions for fusing hydrogen isotopes. Experimental reactors use deuterium and tritium, heavier isotopes of hydrogen, in a process known as DT fusion. This reaction forms a helium nucleus and an energetic neutron.
Fusion fuel is extremely energy-dense, but tritium is scarce on Earth and decays with a half-life of about 12.3 years. Future reactors plan to use lithium breeding blankets that generate tritium when exposed to neutron radiation.
Fusion offers advantages compared with nuclear fission. It produces minimal high-level radioactive waste and involves lower inherent safety risks. However, the process generates intense neutron radiation that gradually damages the inner walls of a reactor. Achieving sustained energy gain beyond breakeven and converting it efficiently into electricity remain major technical challenges.
Research focuses mainly on two methods: magnetic confinement fusion and inertial confinement fusion. MCF devices use magnetic fields to contain plasma. Early concepts included the z-pinch, stellarator, and magnetic mirror, with the tokamak design becoming dominant after Soviet experiments in the 1960s. ICF compresses and heats small fuel pellets using high-energy lasers, developed primarily since the 1970s. The largest active projects are ITER in France and the National Ignition Facility in the United States. Commercial and academic teams are also studying alternatives such as magnetized target fusion and modern stellarator designs.
Terminology
The terms "fusion experiment" and "fusion device" refer to the collection of technologies used for scientific investigation of plasma, and technical advancement. Not all are capable of, or routinely used for, producing thermonuclear reactions i.e. fusion.The term "fusion reactor" is used interchangeably to mean the above experiments, or to mean a hypothetical power-producing version, at the center of a commercial power plant, requiring additions such as a breeding blanket and heat engine.
Background
Mechanism
Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei heavier than iron-56, the reaction is endothermic, requiring an input of energy. The heavy nuclei bigger than iron have many more protons resulting in a greater repulsive force. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy when they fuse. Since hydrogen has a single proton in its nucleus, it requires the least effort to attain fusion, and yields the most net energy output. Also, since it has one electron, hydrogen is the easiest fuel to fully ionize.The repulsive electrostatic interaction between nuclei operates across larger distances than the strong force, which has a range of roughly one femtometer—the diameter of a proton or neutron. The fuel atoms must be supplied enough kinetic energy to approach one another closely enough for the strong force to overcome the electrostatic repulsion in order to initiate fusion. The "Coulomb barrier" is the quantity of kinetic energy required to move the fuel atoms near enough. Atoms can be heated to extremely high temperatures or accelerated in a particle accelerator to produce this energy.
An atom loses its electrons once it is heated past its ionization energy. The resultant bare nucleus is a type of ion. The result of this ionization is plasma, which is a heated cloud of bare nuclei and free electrons that were formerly bound to them. Plasmas are electrically conducting and magnetically controlled because the charges are separated. This is used by several fusion devices to confine the hot particles.
Cross section
A reaction's cross section, denoted σ, measures the probability that a fusion reaction will happen. This depends on the relative velocity of the two nuclei. Higher relative velocities generally increase the probability, but the probability begins to decrease again at very high energies.In a plasma, particle velocity can be characterized using a probability distribution. If the plasma is thermalized, the distribution looks like a Gaussian curve, or Maxwell–Boltzmann distribution. In this case, it is useful to use the average particle cross section over the velocity distribution. This is entered into the volumetric fusion rate:
where:
- is the energy made by fusion, per time and volume
- n is the number density of species A or B, of the particles in the volume
- is the cross section of that reaction, average over all the velocities of the two species v
- is the energy released by that fusion reaction.
Lawson criterion
where:
- is the net power from fusion
- is the efficiency of capturing the output of the fusion
- is the rate of energy generated by the fusion reactions
- is the conduction losses as energetic mass leaves the plasma
- is the radiation losses as energy leaves as light and neutron flux.
Triple product: density, temperature, time
The Lawson criterion argues that a machine holding a thermalized and quasi-neutral plasma has to generate enough energy to overcome its energy losses. The amount of energy released in a given volume is a function of the temperature, and thus the reaction rate on a per-particle basis, the density of particles within that volume, and finally the confinement time, the length of time that energy stays within the volume. This is known as the "triple product": the plasma density, temperature, and confinement time.In magnetic confinement, the density is low, on the order of a "good vacuum". For instance, in the ITER device the fuel density is about, which is about one-millionth atmospheric density. This means that the temperature and/or confinement time must increase. Fusion-relevant temperatures have been achieved using a variety of heating methods that were developed in the early 1970s. In modern machines, as of 2019, the major remaining issue was the confinement time. Plasmas in strong magnetic fields are subject to a number of inherent instabilities, which must be suppressed to reach useful durations. One way to do this is to simply make the reactor volume larger, which reduces the rate of leakage due to classical diffusion. This is why ITER is so large.
In contrast, inertial confinement systems approach useful triple product values via higher density, and have short confinement intervals. In NIF, the initial frozen hydrogen fuel load has a density less than water that is increased to about 100 times the density of lead. In these conditions, the rate of fusion is so high that the fuel fuses in the microseconds it takes for the heat generated by the reactions to blow the fuel apart. Although NIF is also large, this is a function of its "driver" design, not inherent to the fusion process.
Energy capture
Multiple approaches have been proposed to capture the energy that fusion produces. The simplest is to heat a fluid. The commonly targeted D–T reaction releases much of its energy as fast-moving neutrons. Electrically neutral, the neutron is unaffected by the confinement scheme. In most designs, it is captured in a thick "blanket" of lithium surrounding the reactor core. When struck by a high-energy neutron, the blanket heats up. It is then actively cooled with a working fluid that drives a turbine to produce power.Another design proposed to use the neutrons to breed fission fuel in a blanket of nuclear waste, a concept known as a fission-fusion hybrid. In these systems, the power output is enhanced by the fission events, and power is extracted using systems like those in conventional fission reactors.
Designs that use other fuels, notably the proton-boron aneutronic fusion reaction, release much more of their energy in the form of charged particles. In these cases, power extraction systems based on the movement of these charges are possible. Direct energy conversion was developed at Lawrence Livermore National Laboratory in the 1980s as a method to maintain a voltage directly using fusion reaction products. This has demonstrated energy capture efficiency of 48 percent.