How Things Work: Nuclear Fusion
Using a lithium laptop battery and 45 liters of water, a nuclear fusion reactor can produce an
estimated 200,000 kilowatt-hours of electricity — the amount of electricity consumed by one person over the course of 30 years.
Unfortunately, the world will probably have to wait another 30 years until this technology is available for use, since that is the time frame of current nuclear fusion projects.
Nuclear fusion is the process by which two nuclei become a single nucleus. The resulting nucleus is less massive than the combined mass of the original two nuclei. This difference in mass is released in the form of energy.
Scientists hope that one day nuclear fusion will be used in nuclear power plants as an alternative to fossil fuels for producing power.
In the natural world, though, nuclear fusion is very much a reality. In fact, without nuclear fusion, life on Earth would be very dark — not to mention cold.
Stars, including the sun, burn off energy through nuclear fusion. At the sun’s core, subject to temperatures ranging from 10 to 15 million Kelvin, hydrogen atoms combine to form helium by way of nuclear fusion.
Converting mass to energy is as easy as E = mc2, Einstein’s famous formula, where E stands for energy, m stands for mass, and c stands for the speed of light (approximately 300 million meters per second). This relationship between mass and energy is precisely why many consider nuclear fusion to be a profitable means of generating electricity.
In order for two nuclei to fuse, they must first overcome the Coulomb barrier, the quantity of energy required for the strong nuclear force to overcome the electrostatic force.
All nuclei contain protons, and nearly all atoms contain neutrons as well. Normally, the positive charges in two nuclei repel each other due to the electrostatic force between them, but atraction is not the dominant force in most nuclear fusion reactions.
When two nuclei come in contact with one another, the strong nuclear force becomes dominant and attracts the nuclei together. This is the force that binds particles in a nucleus together.
Consequently, the nuclei respond to an attractive force, allowing a fusion reaction to occur.
In nuclear fusion reactions, elements combine to produce heavier elements.
In general, when small elements fuse to form larger elements, the combination results in an exothermic reaction, which is a type of reaction that gives off energy. In the sun, for example, hydrogen atoms fuse to form helium. This reaction releases 26.7 mega electron volts (MeV) of energy.
However, nuclear fusion reactions resulting in elements more massive than iron (which has 56 atoms in its nucleus) are endothermic, meaning that they consume energy.
Heavier elements such as iron exceed the curve of binding energy. In other words, these elements have large atomic numbers, or a high number of protons in their nuclei. Consequently, the electrostatic force of repulsion overcomes the strong nuclear force.
On Earth, scientists are focusing on the nuclear fusion reaction between the isotopes deuterium and tritium. According to the Lawson criterion, a list of the conditions necessary for nuclear fusion to occur, deuterium-tritium reactions are the most promising fusion reactions.
When they react, deuterium and tritium fuse to form helium, releasing 14.1 MeV of energy in the process.
One of the advantages of this type of reaction is that the reactants, deuterium and tritium, are readily available in nature.
Deuterium, also known as “heavy hydrogen,” is an isotope of hydrogen with one proton and one neutron. The most abundant hydrogen nucleus, also called protium, contains only one proton and no neutrons.
Deuterium can be extracted from seawater, and the Earth’s oceans contain enough deuterium to provide electricity on a global level for about six billion years.
Tritium is also an isotope of hydrogen, but it contains one proton and two neutrons. Tritium is radioactive, and so, it is less available than deuterium.
Tritium is produced as a result of a reaction between an atom of lithum-6 and a neutron. Lithium is less plentiful than deuterium, but it still exists in sufficient amounts to power the planet for several hundred years.
There are several advantages to generating power using fusion reactions opposed to burning fossil fuels or implementing fission reactions.
First, fusion reactions are more environmentally friendly than conventional sources of power.
When burned, fossil fuels release carbon dioxide, which contributes to the greenhouse effect, or the entrapment of gases in the atmosphere that prevents the sun’s re-radiated energy from escaping into space. Nuclear fusion power would cause no such effect.
Second, nuclear fusion is a safer source of power than its more popular counterpart, nuclear fission.
The opposite of nuclear fusion, a nuclear fission reaction involves one heavy element splitting into two lighter elements.
Fission reactions often involve the separation of an isotope of uranium with an atomic mass of 235 into elements with atomic masses of about 100. Each reaction produces hundreds of MeV of energy.
Nuclear fission reactions also result in radioactive products, some with half-lives on the order of tens of thousands of years. These products are often relocated to remote areas to prevent contamination, as radiation exposure can be deadly.
With deuterium and tritium readily available to produce electricity from nuclear fusion, one may wonder why power plants do not simply use nuclear fusion as their main power source.
Though nuclear fusion power is much safer than that of fission and more environmentally friendly than fossil fuels, it is still far from becoming a viable source of human energy. In fact, scientists have yet to create a fusion reactor on earth that produces more energy than it consumes.
As stated earlier, nuclear fusion reactions occur at the cores of stars, including the sun, at extremely high temperatures. On the Earth, it’s very difficult to recreate this environment without using a great deal of energy.
The Joint European Torus (JET), a nuclear fusion experiment endorsed by countries throughout the world, including the United States, has run into such a problem.
As of November 2006, the JET was able to produce 16 megawatts of fusion power, though it required 25 megawatts to heat the surrounding plasma.
The JET is currently being refurbished, and scientists hope these changes will bring them one step closer to making nuclear fusion power a source of global energy.