| Fusion occurs when the nuclei of two atoms join together to become one. The results: energy, a heavier nucleus, and a free nucleon. A nucleon is an elementary particle that can exist in either a neutral (as a neutron) or a positively charged (as a proton) state.
Researchers have found that the most useful elements for fusion are the hydrogen isotopes deuterium (one proton and one neutron) and tritium (one proton and two neutrons).
The three isotopes of hydrogen are hydrogen (one electron, one proton), deuterium (one electron, one proton, one neutron), and tritium (one electron, two neutrons, one proton).
Photo courtesy of Princeton Plasma Physics Laboratory.
Under normal circumstances the nuclei repulse each other electrostatically. But certain conditions make fusion possible. One way is to push the nuclei to a very high energy, through the use of a particle accelerator. At this high energy, the nuclei collide and join together. However, this method consumes power rather than producing it. This happens because the nuclei lose their energy during the collisions. Thus, further reactions do not take place.
The other method heats the nuclei to very high temperatures to enable fusion. This is called a thermonuclear reaction, and it’s what happens in the Sun and other stars. In a thermonuclear reaction the energy from the collision gets transferred to the particle that was hit. That particle can then perform its own fusion on another particle. In a thermonuclear reaction, the hydrogen exists as a plasma. A plasma is a hot, electrically charged gas consisting of ions and electrons rather than atoms and molecules. The plasma needs to be heated to temperatures of 100 million Kelvin or more for the reaction to occur. The Kelvin temperature scale begins at absolute zero (−273.15°C or −459.67°F). It is named for the physicist who first proposed the scale, William Thomson, 1st baron of Kelvin.
Confining the plasma forces the particles to spiral within the magnetic field. It also prevents heat loss, allowing the reaction to reach the extremely high temperatures needed for the reaction.
Photo courtesy of EFDA-JET, |
Fusion is an extremely powerful technology, capable of producing tremendous amounts of energy. Uncontrolled, it can be a very destructive force capable of vaporizing anything it touches. The hydrogen bomb is an example of an uncontrolled fusion reaction. If we can control the heat and the energy of fusion, we will have a fantastic source of power for the world.
In outer space, the Sun and stars use the force of gravity to hold the plasma together. But on the Earth, that won’t work—the planet does not have sufficient mass to generate a gravity field strong enough to contain the plasma. Researchers have had to find a way to mimic the conditions of stars to create fusion. The big problem: to work, the reaction must be hotter than the interior of the Sun. Once this heat is achieved, two other issues arise. First, the fusion reactor must be capable of retaining the extreme heat without melting. Second, the plasma created by the extreme heat must somehow be confined or held together. Remember, an unconfined fusion reaction consumes energy and loses heat, so beyond a certain point reactions do not take place. By confining the plasma, the energy produced is held in. The fusion reactions thus produce more energy than what is used to heat up the gas.
One solution is to confine the plasma with powerful magnetic fields. The magnetic field serves a number of functions. It separates the plasma from the walls of the fusion containment vessel. The plasma can then be heated to the extremely hot temperatures needed. It also prevents the walls from conducting heat away from the plasma, and itprevents contamination of the plasma by impurities from the wall.
Magnetic confinement systems are designed in a number of shapes. An open system uses stronger magnetic fields at each end as “mirrors” to keep the plasma contained. The closed system undergoing the most research is one that uses a torus shape, similar to a doughnut. Both systems keep the plasma flowing within the confinement system.
Tokomak Nuclear Fusion Reactor
Two sets of magnetic fields contain the plasma in a Tokamak.
The toroidal magnets are the main confinement system.
The poloidal magnets push the plasma away from the reactor walls.
Another method to hold the plasma together still under experimentation is inertial confinement. This process compresses a capsule of deuterium and tritium to very high temperatures and densities. The fusion occurs before the atoms can fly apart, generating energy.This process is similar to what occurs in a hydrogen bomb. Laser light, ions, electrons, X-rays, or electricity can be used to compress the pellet. Current research focuses on X-rays or electricity to compress the fuel.
Fusion creates a lot of energy, a huge benefit in a world increasingly demanding more power to operate both low-tech and high-tech machines.
The product of fusion is helium gas, which is not radioactive. This means that there are no radioactive wastes to store, or wastes that can be used to make weapons. There may be some very short-term radioactivity as the fusion products react with the container walls, but any fusion power plant would be designed to meet safety standards. Researchers believe that a decommissioned plant, one that no longer operates, would remain radioactive for 50 to 100 years. This is a much shorter period of time than for nuclear fission reactors.
Unlike nuclear fission, fusion reactions cannot run out of control. Any unusual event in a fusion reaction causes the plasma to extinguish itself.
Fusion power plants remain in the developmental stages. At this time, researchers have not been able to create a plant that runs on its own heat.
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