SEED Science


Global Climate Change and Energy
Alternative Energy Sources: Fusion

What if we could take the same source of energy that powers the Sun and use it to provide energy on Earth? That’s exactly what researchers are trying to accomplish with nuclear fusion. The hope is that in the future, much of our energy needs on Earth will be supplied by fusion.

Fusion has many advantages. The process produces enormous amounts of energy. The fuel is a common form of hydrogen found in water. Most important, fusion produces no polluting emissions and a minimal amount of by-products.

 

fusion_sun

Photo courtesy of NSSDC / NASA,

The Sun gets its energy from fusion.

Right now, fusion remains far from ready for commercial development. A fusion reaction is extremely hot and extremely powerful, more than any other energy source. That means we need to manage the heat and contain the reaction, something we are still learning to do. But with solutions to these problems in place, fusion will be a great source of energy for the world.

How It Works

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).

 

fusion_isotopes

Photo courtesy of Princeton Plasma Physics Laboratory.

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).

 

 

 

fusion_deuterium

Photo courtesy of Princeton Plasma Physics Laboratory.

Hydrogen isotopes deuterium and tritium produce helium and a neutron in a fusion reaction.

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.

 

fusion_magnetic_field

Photo courtesy of EFDA-JET,

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.

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.

Controlling the Reaction

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

fusion_tokamak

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.

Advantages

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.

The Futlure

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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