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By Shaun Thomas
General Physical Science
June 22, 2001
Fusion: Our Future’s Energy?
Fusion energy seems to be the most promising energy source of the not-too-distant future. It is safe, it uses an energy supply that is so abundant that it will never run out, it gives off harmless waste, and it produces energy comparable to the Earth’s sun! But are there any problems with this hopeful energy source?
What is Nuclear Fusion?
To understand fusion, it is a good idea to know about fission. This is the splitting of the nuclei of atoms into two or more nuclei by bombarding them with neutrons of low energy. It was discovered in the 1930’s in an attempt to make transuranium elements (elements with atomic numbers greater than Uranium that do not exist in nature). They discovered that the nucleus of Uranium-235 breaks apart into two smaller nuclei after absorbing a neutron. This happens because the extra neutron made it unstable. This produces more neutrons that bombard more Uranium nuclei, causing a chain reaction that produces an enormous amount of energy.
The problem is the nuclear waste that is produces. It is very radioactive and has a half-life of about 7.13*10^8. Such a harmful substance is a great health concern and needs to be disposed of properly. Another problem is the energy it needs, it uses an element that is hard to find and which will eventually run out. Also, the reaction cannot be easily stopped, and if it can’t be stopped, a nuclear meltdown can occur, which can pose a threat to the environment.
Fusion is a different process that combines two nuclei into one, releasing an amount of energy that is far greater than that of fission. In a common type of reaction, two isotopes of hydrogen, deuterium and tritium fuse together, making helium and a neutron. A small amount of the mass produced is converted into an enormous burst of energy.
Difficulty of a fusion reaction
The main difficulty in a fusion reaction is the heat needed for it to occur. A reaction such as fusion that requires an intense amount of heat is called a thermonuclear reaction. It commonly takes place in huge machines called tokamaks. It can only occur in a special form of matter called plasma, a gas made up of free electrons and nuclei. When this plasma is heated millions of degrees, the nuclei move so fast that they fuse. The problem is in finding a container that can hold this extremely hot plasma.
This plasma has a tendency to expand and escape from its container. The walls of the container have to be very cool, or else they will melt. If the plasma touches the walls, it becomes too cool for the reaction to occur. But how can a container hold the plasma without touching the plasma? The answer is in devices known as “magnetic bottles,” which are twisted into coils. They have a metal wall that is surrounded by a magnet. Electrical current flows through the magnet, creating a magnetic field on the inside of the walls. This pushes the plasma away from the walls and toward the center of each coil.
There are problems with this reaction. All the fusion devices built so far use more energy than they produce! Another problem is the environmental concern. When neutrons bombard the walls of the reactors, the walls become radioactive. Walls that become less radioactive will have to be found.
Why fusion is still half a century away
The money needed to research fusion is getting smaller and after about four decades of research, the payoff seems to be far from now. The approach does not seem to be in the right direction. Researchers have put too much emphasis on designing a practical fusion power plant without really understanding some of the fundamental physics involved. The experimental reactors have no general research purpose. The fusion researchers will have to start rethinking their ideas. Research will have to change from a developing a new energy technology to developing a broader understanding of a fusion reaction.
Alternatives to current reactors
Some people say that the current fusion reactors, such as the Tokamak Fusion Test Reactor at Princeton University, should be shut down. But others say that these labs have been very successful. The Princeton reactor can produce 10 million watts of power in bursts of about a second each, but it relies on external power. These reactions have not yet reached a point where the power produced in each reaction can be used directly in the next reaction, allowing the external power to be turned off. The Princeton lab wanted to build a new machine to replace the current one, but the cost of 1.8 billion dollars and the unresolved technical issues caused the idea to be rejected. Instead, the current machines continue to be upgraded. A different reactor called the International Thermonuclear Experimental Reactor, a next-generation tokamak, has also been proposed and seems to be a better choice.
Some people say that a new type of reactor, besides the tokamak, should be used. The Japanese have a design called the Field Reversed Configuration Reactor. Another idea is to use lasers to trigger fusion in tiny pellets of fusion fuel.
The goals of fusion
An ideal fusion reactor of the future could use the hydrogen extracted from one gallon of water to produce the equivalent energy of 300 gallons of gasoline, eliminating the need for fossil fuels. Everything should be automatic with very little supervision. It should produce no harmful wastes and have no possibility of failure. But the technical obstacles involved to make such a device are far from being completed. Fusion will not become a commercial device until the second half of the next century. However, one thing is evident: Fusion will eventually be the leading source of energy for the future.
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