Nuclear Fusion the Power of Tomorrow

INTRODUCTION

Our growing need of energy has always been a problem. As the population grew and cities expanded, the demand for energy increased. Primarily, wind and running water were used as a source of energy to run factories during the Industrial Revolution. Before long, we started producing and using electricity to do work by burning fossil fuels and using solar, water, wind, and other forms of energy. When Albert Einstein found the relationship between mass and energy, nuclear fission was studied and developed. Now, we use various types of energy, including chemical energy, nuclear energy, kinetic energy, and solar energy. From these sources, we make huge amounts of electricity, totaling almost four billion megawatt-hours in the United States.

Although we have many ways of producing energy, some sources of energy are now declining. Fossil fuels will soon be used up, and nuclear fission will not last, as the isotopes of uranium and plutonium that are used in fission are very scarce. Fossil fuels and nuclear energy yield well over half of the electricity produced in the nation, but these two sources cause pollution and create radioactive materials. When fossil fuels are gone, a new way to produce electricity must be developed. One possible solution, called nuclear fusion, is being studied. Nuclear fusion, the process in which the nuclei of different atoms are fused together, produces large amounts of energy, much more than both fossil fuels and nuclear fission. The most studied fusion reactions are the deuterium-deuterium (D-D) and deuterium-tritium (D-T) reactions. Both deuterium and tritium are isotopes of hydrogen. Deuterium is found on Earth in large quantities, but tritium must be artificially made and therefore, is the limiting factor of fusion. These fusion reactions can produce large amounts of energy, but it will not last very long.

RESULTS

Building a fusion reactor is extremely hard and there are a number of things to consider when making one. First, one must first test whether fusion reactions can occur and measure cross sections and the energy yields. This was already done many years ago. Second, different reactors must be tested to see whether the energy output is at least equal to the energy input. Currently, research is being done with experimental reactors. Finally, the reactor must produce energy in the range of megawatts and at the same time be economically sound. Nuclear fusion is a difficult and complicated procedure. First of all, fusing nuclei together is not easy at all. The nucleus of an atom has a positive charge. Like-charged particles repel each other, and when the nuclei come close to each other, the repulsion becomes much greater. Therefore, the nuclei must be traveling at very high speeds to overcome the Coulomb force to come close to each other. In fact, in order for the nuclei to come within 10-15 meters of each other, which is the size of a nucleus, the energies of the particles must be in the range of kiloelectron- volt (keV) or higher.

An effective way to obtain such high-energy particles is through plasma. By adding more energy to the plasma, it can reach temperatures where particles can attain speeds at which they can fuse. However, the temperature at which the plasma must be is extremely high, ranging from the tens of million Kelvin and up. For example, if the particle energy must be 10 keV in order for fusion to occur, the temperature of the plasma must be 77 million K. In order for the plasma to stay at a high temperature and for fusion to take place, the plasma must be contained. One method of confinement is through the use of electric and magnetic forces, called the magnetic confinement fusion (MCF) method. By wrapping coils around a tube and running a current through it, the charges in the plasma cause it to move along the tube in the shape of a helix. Once it reaches the end of the tube, though, the plasma will bump into the wall. Therefore, the path of the plasma must be continuous. An efficient application of the MCF method is the Tokamak. The Tokamak is in the shape of a donut, and since it doesn't have an end, the plasma continues to rotate around the tube.

Another way of confinement is the inertial confinement (ICF) method. ICF machines act like a hydrogen bomb by creating a thermonuclear reaction. NIF at the Lawrence Livermore National Laboratory is an example of an ICF machine. Small pellets of deuterium and tritium with a diameter of around 1/50 of a millimeter are hit by lasers or high-speed particles from many different directions. The surface of the pellets is evaporated and forms plasma around the pellets. The plasma receives more energy or heat from electrons, which causes the surface to be evaporated even more. Particles leaving the surface cause a shockwave that moves inward inside the plasma, causing the D-T pellets to be compressed. Energy amounting to around 1 keV is given by the driver setting off a thermonuclear reaction. Picture 1 The last step to consider is the many different fusion reactions. There are nearly 100 different fusion reactions, each with their different isotopes and energy inputs and yields. The most studied reactions that involve light isotopes are the D-D, D-T, and D-3He. Of these reactions, the most efficient reaction is the D-T reaction. As seen in Picture 1, when deuterium and tritium fuse at ignition temperature of 4.4 keV, which is around 40 million K, the results are 4He, a neutron, and an energy yield of 17.6 MeV. The D-D and D-3He reactions need a much higher ignition temperature to fuse, and the D-D reaction yields significantly less energy as well, while the D-3He reaction yields around the same.

When considering the different reactions, the availability of the fuels must also be taken into consideration. Deuterium is found in large quantities in water, amounting to one part in 5000 hydrogen atoms, which adds up to around 1018 kilograms. Furthermore, a gallon (around four liters) of seawater could produce as much energy as 300 gallons of gasoline (Nave, 2005). Although the supply of deuterium is nearly limitless, tritium is not. Tritium is a radioactive isotope of hydrogen, and must therefore be made artificially. One way is by bombarding lithium with a neutron. Another way of making tritium is through the deuterium cycle, or deuterium-deuterium reaction. The first method will only last around a thousand years, as lithium is not as abundant in Earth. The latter process has a 50% chance of resulting in tritium. The other half of the time, the reaction results in helium-3, a rare isotope of helium, which is one of the light isotopes used in fusion.

The helium-3 isotope, although not radioactive, is very rare in Earth's atmosphere. But, it is very abundant on the moon and on Jupiter and Saturn. If space mining could be done, we would have a virtually limitless amount of helium-3. For now, though, helium-3 could be made only through the deuterium cycle. With the helium-3 made from the deuterium cycle, the D-3He reaction could be used to produce much energy. Another important thing to think about is radioactivity. Many fusion reactions result in neutrons, which become radioactive waste. One of the few reactions that do not result in neutrons is the D-3He reaction. This reaction may need a much higher ignition temperature than the D-T reaction, but it makes as much energy and does not create radioactive waste, and therefore does not need frequent replacement of walls in fusion reactors.

DISCUSSION

The results of this research were not what were anticipated. It was found to be true that nuclear fusion will produce very large amounts of energy, even more than both fossil fuels and nuclear fission. But fusion will last a much longer time than expected. Although tritium, which limits the D-T reaction, is not found naturally on Earth, it can be artificially made from both lithium and the deuterium cycle for at least another 1000 years. This fact alone disproves the anticipated length of time that fusion will be used to produce electricity. In addition to the D-T reaction, other fusion reactions can be applied to make huge amounts of energy as well. Like the D-T reaction, the D-3He reaction uses deuterium and another product of the deuterium cycle. This reaction along with the D-T reaction will most likely last thousands or maybe even tens of thousands of years.

CONCLUSION

Nuclear fusion is a possible source of energy in the future, and soon, other than renewable sources, it may even be the only source of energy. Although most fusion reactions have many disadvantages, nuclear fusion will last at least many centuries, maybe even millenniums. For example, the D-T reaction alone can last at least a millennium with the available lithium. Furthermore, tritium can also be produced through the deuterium cycle. Other than the D-T reaction, there is also the deuterium cycle. The D-D reaction produces tritium half the time and helium-3 the other half. These two products can be fused with deuterium again to make large amounts of energy. Furthermore, the D-3He reaction produces no radioactive neutrons at all, unlike most fusion reactions. Finally, there is a virtually limitless supply of deuterium in the Earth's waters. In fact, there is over 1018 kilograms of deuterium. Some possible future studies include advanced space travel and space mining. If space travel is more advanced and mining is possible on other planets, we could use the nearly endless supply of helium-3 on Jupiter and Saturn and the deuterium on Earth for fusion. If this happens, great amounts of energy could be produced without resulting in any radioactive waste or pollution. This would be a great advantage in today's world, as not many power plants can produce large amounts of energy and at the same time create no radioactive waste or pollution. Another study that could be done is on different fusion reactors and the materials used to build the reactors. Some fusion reactions result in radioactive neutrons that bombard the walls of the reactors so they must be frequently replaced. If certain materials can withstand the shower of so many flying neutrons as well as the high heat of the plasma, then those materials could be used as permanent walls of reactors, lowering the maintenance costs of a fusion reactor.

REFERENCES

1. Murray, R. L. Nuclear Energy. Woburn, Massachusetts: Butterworth-Heinemann. Nave, C. R. (2005).
2. Nuclear Fusion. Retrieved July 24, 2005, from the Hyper Physics Website: https://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fusion.html (2005 July 21).
3. Nuclear Fusion. Retrieved July 27, 2005, from the Wikipedia Website: https://en.wikipedia.org/wiki/Nuclear_fusion Nuclear Fusion Basics. Retrieved July 21, 2005, from the EFDA-JET Website: https://www.jet.efda.org/pages/content/fusion1.html

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