nuclear fusion energy tokamak

Nuclear fusion is a reaction whereby two or more atomic nuclei fuse to become different atomic nuclei, neutrons or protons. The net change in mass results in the release or absorption of energy.  Fusion is the process that maintains main sequence stars, releasing tremendous amounts of energy. Fusion of lighter nuclei is an exothermic process, while fusion of heavier nuclei results in an endothermic reaction. Hydrogen and helium, are more suitable for fusion, whilst heavier elements, such as uranium and plutonium undergo nuclear fission. 



What is nuclear fusion?

The Sun is a main-sequence star, creating its own energy via nuclear fusion of hydrogen nuclei into helium. The Sun fuses 500 million metric tons of hydrogen per second.


Discovery of nuclear fusion

Eddington proposed that hydrogen-helium fusion could be the source of stellar energy. Quantum tunneling was also suggested in 1929. Atkinson and Houtermans showed that tremendous energy could be released by fusing small nuclei. Fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. The military prioritised fusion research in the early 1940s, as part of the infamous Manhattan Project. Nuclear fusion on a large scale was first carried out on 1 November 1952, as part of the Ivy Mike ‘hydrogen bomb’ test. Global research into controlled fusion has been ongoing since the 1940s. The technology is progressing but remains at an early stage.

ITER objectives in the future

Produce 500 MW of fusion power for pulses of 400 s

In 1997, JET produced 16 MW of fusion power from 24 MW of power injected into its heating systems (Q=0.67). ITER is designed for much higher fusion power gain, or Q ≥ 10. For 50 MW of injected heating power it will produce 500 MW of fusion power for long pulses of 400 to 600 seconds.

Demonstrate the operation for a fusion power plant

Scientists will be able to study plasmas under conditions similar to those expected in a future power plant and test technologies such as heating, control, diagnostics and cryogenics.

Create plasma in which the reaction is sustained through internal heating

In a burning plasma, the energy of the helium nuclei produced when hydrogen isotopes fuse becomes large enough to exceed the plasma heating that is injected from external sources. As the first such burning plasma device in the world, ITER will break new territory in controlled nuclear fusion.

Test tritium breeding

A later goal of ITER is to demonstrate producing tritium within the vacuum vessel. ITER will test mockup in-vessel tritium breeding blankets in a real fusion environment.

Demonstrate the safety characteristics of a fusion device

One of the primary goals of ITER is to demonstrate control of the plasma and fusion reactions with negligible consequences to the environment.

Nuclear fusion reaction

The fusion of deuterium with tritium produces helium-4, releasing 17.59 MeV as kinetic energy, while mass disappears, ,kinetic E = ∆mc2, Δm is the decrease in the total rest mass of particles. 


Fusion and Coulomb force

Two opposing forces determine the energy released, the nuclear force, which combines together protons and neutrons and the Coulomb force, where repel each other. Light nuclei allow the nuclear force to overcome repulsion. All nucleons feel the short-range attractive force at least as strongly as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles.


Nucleosynthesis and fusion

Fusion in the stars is called nucleosynthesis. The Sun liberates approximately 610 million metric tons of helium per second. The fusion of lighter elements in stars releases energy and mass. In the fusion of two hydrogen nuclei, 0.645% of the mass is lost as kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation.

When accelerated to high speeds, nuclei can overcome the electrostatic repulsion. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, releases more energy than it takes to bind the nuclei. This is an exothermic process.

Energy released in most nuclear reactions is larger than in chemical reactions. The ionisation energy gained by adding an electron to a hydrogen nucleus is 13.6 eV, which is less than one-millionth of the 17.6 MeV released in the deuterium–tritium fusion reaction. Fusion reactions produce far greater energy per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones.


Nuclear fusion research

Currently, controlled fusion reactions have been unable to produce net zero energy. The two most advanced approaches for it are magnetic confinement and inertial confinement designs. A prototype toroidal reactor that will deliver ten times more fusion energy than the amount needed to heat plasma are in development ITER. The ITER facility is expected to finish its construction phase in 2025. General Fusion (Canada), is developing a magnetised target fusion nuclear energy system. The US National Ignition Facility, is approaching the challenge via laser-driven inertial confinement fusion.


Nuclear fusion – stars

Stellar nucleosynthesis powers stars. The energy released from nuclear fusion reactions accounts for the longevity of stellar heat and light. The fusion of nuclei in a star provides that energy and synthesises new nuclei. Different reaction chains are involved, depending on the mass of the star.


Eddington’s theory on fusion

Approximately 1920, Eddington’s published his paper The Internal Constitution of the Stars. Eddington stated that the concept was fusion of hydrogen into helium, liberating enormous energy according to Einstein’s equation E = mc2.

Eddington’s papers suggested that the theory of stellar energy, the contraction hypothesis, should cause stars’ rotation to visibly speed up due to conservation of angular momentum. Additionally, energy was liberated from the conversion of matter to energy. Aston proposed that the mass of a helium atom was about 0.8% less than the mass of the four hydrogen atoms. He concluded that if fusion could occur, it would release considerable energy. In conclusion, if a star contained just 5% of fusible hydrogen, this explained how stars retained their energy.


Nuclear fusion conditions

This proton-proton chain reaction occurs at a solar-core temperature of 14 million kelvin. The net result is the fusion of four protons into one alpha particle, with the release of two positrons and two neutrinos (which changes two of the protons into neutrons), and energy. As a star uses up a fraction of its hydrogen, it begins to synthesise heavier elements. The heaviest elements are synthesised by fusion that occurs when a more massive star undergoes a violent supernova at the end of its life, a process known as supernova nucleosynthesis.


Nuclear fusion reactor – what is a tokamak?

A tokamak is designed to harness the energy of fusion. The donut shape chamber, the name derived from a Russian acronym meaning toroidal chamber with magnetic coils. As per any other type of power station, a fusion power plant will use the heat generated to produce steam and then electricity via turbines and generators.

Through an environment of extreme heat and pressure, gaseous hydrogen fuel becomes a plasma. The charged particles can be shaped and controlled by the magnetic coils. The coils prevent the hot plasma from contacting the vessel walls.


How does a nuclear fusion tokamak work?

Air and impurities are first evacuated from the vacuum chamber. The gaseous fuel is then introduced and a powerful electrical current is passed through the vessel, and the gas becomes ionised resulting in the plasma. As the plasma particles become energised and collide they also begin to heat up. The plasma is modulated to fusion temperatures 150 and 300 million °C. Particles overcome their electromagnetic repulsion and fuse, releasing tremendous energy.


Types of nuclear fusion

Thermonuclear fusion

Thermonuclear weapons lead to an uncontrolled release of fusion energy. Controlled thermonuclear fusion concepts use magnetic fields to confine the plasma, as per the tokamak design.

Inertial confinement fusion

Inertial confinement fusion (ICF) is a method aimed at releasing fusion energy by heating and compressing a fuel target, typically a pellet containing deuterium and tritium.

Inertial electrostatic confinement

Inertial electrostatic confinement is a set of devices that use an electric field to heat ions to fusion conditions. The most well known is the fusor. Starting in 1999, a number of amateurs have been able to do amateur fusion using these homemade devices. Other IEC devices include: the Polywell, MIX POPS and Marble concepts.


Recent category posts


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