Fusion Reactor Energy

Nuclear fusion reactions have excellent potential for the planet's energy source. Much research has been performed leading to harnessing fusion reactions for the production of power. There are today, four or five major, and a dozen secondary, research centers worldwide working on fusion technology. Few researchers doubt that energy can eventually be produced by the use of fusion reactions, but there is much disagreement about the nature of the facilities and the details of their engineering design. It would fly in the face of experience to suggest that only one reactor type will be used in the future for the production of fusion power. This discussion will show those characteristics that will be unique to the fusion energy system, properties that reactors will have in common and a preliminary sketch of the characteristics of reactors that appear likely to be used in the future. Fusion energy sources, while nuclear, are very different from the fission sources now in use. The only common thread is that both convert matter to energy by nuclear reactions. In a fusion reactor, light atoms are converted to heavier atoms. The heavy atoms weigh slightly less than the sum of the weight of the light atoms from which they are made. This mass difference is converted to energy. The most discussed fuel is a mixture of two isotopes of hydrogen: deuterium and tritium. 125 Deuterium is present in all water. Tritium is not found in nature. It is manufactured by bombarding lithium with neutrons.

123 Rose, David J. and Lester Richard K., "Nuclear Power, Nuclear Weapons and International Stability", Scientific American, Vol. 238, No. 4, April 1978, Page 45

124 Rosen, Stanley G„ "Nuclear Waste Disposal in Space" Page 131 in Macro-engineering: The Rich Potential, Edited by Salkeld, Robert, Davidson, Frank P., & Meador, Lawrence C., published by The American Institute of Aeronautics and Astronautics, 1981

125 Bromber, J. L., "Fusion: Science, Politics and the Invention of a New Energy Source", MIT Press, Cambridge, Massachusetts, 1982

Hydrogen is unique among the elements in having separate common names for its isotopes, deuterium (symbol D, mass 2) and tritium (symbol T, mass 3). This combination of isotopes is the most likely to fuel the first generation of fusion nuclear power plants, because the reaction between deuterium and tritium is the most easily ignited. When the nucleus of a deuterium atom collides at high speed with a nucleus of a tritium atom, the two atoms fuse to form a helium atom and a free neutron. When this reaction occurs a great deal of energy is released. The critical goal for the design of a fusion reactor is to force a great many deuterium and tritium atoms to collide at high speed and capture the energy released.

The temperature of a material determine by the velocity of its atoms. The temperature required for deuterium and tritium atoms to move fast enough to cause a fusion reaction is approximately 100 million degrees Kelvin. All potential construction materials vaporize at temperatures less than 4,000 degrees Kelvin. To contain the high temperature of a fusion reaction it is necessary to confine the hot hydrogen isotopes by means other than material walls. Two techniques are currently being investigated: powerful magnetic fields and inertia.

At the fusion reaction temperature, all the electrons have been stripped from the atoms. The resulting mix of negative charged free electrons and positive charged bare atomic nuclei are called plasma. The electric charges of the electrons and nuclei allow the plasma to be manipulated with strong magnetic fields. The charged particles spiral around and drift along the magnetic lines of force, but do not easily cross them. A powerful magnetic field can hold the plasma while it is heated to the fusion temperature. When the deuterium and tritium plasma reaches fusion temperatures energy is produced by the fusion reactions. Many reactions can take place in deuterium-tritium plasma, all release energy. The most important reaction is:

Deuterium + Tritium => Energy + Helium + neutron

The neutron carries much of the energy released by this reaction. The neutrons have no electric charge so they are not contained by the magnetic field. They must be captured in a blanket material. When the blanket material absorbs them, their energy is transformed into heat. The heat from the neutrons and the radiant energy emitted from the hot plasma and directly adsorbed by the walls of the reactor are the heat output of the fusion reactor. This heat can be used to produce steam for the generation of electric power by conventional steam turbines.

Only a small amount deuterium is required to fuel a fusion reactor. Natural sources of hydrogen contain 0.0156% deuterium. A metric ton (1000-kg) of hydrogen from any source contains 156 grams of deuterium. Tritium is unknown in nature; however, the neutrons produced by fusion react with lithium to produce tritium. There is sufficient deuterium and lithium to provide energy for thousands of years. 126

There are no radioactive elements produced by the fusion reaction: the product is inert helium. However, the neutrons produced by the fusion reaction are adsorbed by the atomic nuclei of the structure of the reactor and cause them to become radioactive. The tritium used as the reactant is radioactive. In total, these two sources generate far less radiation than is produced by the radioactive substances in a burner or breeder fission reactor. They are many-many times less hazardous than the radioactive waste produced by a breeder reactor.

126 Holdren, John P., "Fusion Energy In Context: Its Fitness for the Long Term", Science, Vol. 200, April 14,1978, page 168

Magnetic confinement fusion reactors use low-pressure hot plasma for the production of fusion energy. A large chamber is equipped with magnetic coils to provide the strong fields necessary to hold the plasma for a long period. The chamber has heat exchange equipment for the removal of the fusion energy. The tritium and deuterium are introduced as gas and heated to fusion temperature with auxiliary heaters to start the reaction. Neutrons from the reaction produce tritium in the heat exchange blanket. Thermal energy is extracted from the reactor and handled in the same manner as heat from a fission reactor or a coal fired power plant. This heat is used to raise steam to generate electricity.

Inertial confinement reactors use the opposite end of the density and time scale. A small pellet of deuterium and tritium is frozen at a temperature a few degrees above absolute zero, say 2 to 3 Kelvin (272 degrees Celsius). The pellet is placed in the focus of several powerful energy beams. The beams are pulsed at a high energy level for a very short time. The enormous pulse of energy heats and compresses the pellet to a temperature and density high enough that the fusion reaction occurs. The heating and compression happen so fast that much of the pellet reacts before it has time to blow up from the energy pulse.

Several types of energy beams have been tested in research efforts to ignite inertial confined fusion reactions. Multiple laser beams, electron beams and heavy ion beams have been tested. All have shown that heating and compression is possible. Thus far, none have caused the release of more energy from the deuterium tritium pellet than was present in the original laser beams.

The energy released from each pellet must be much larger than the energy of the ignition beam for inertial confinement to be useful as an energy source. The output energy will be captured in a neutron-adsorbing blanket and by the chamber walls in a manner very similar to that used in the magnetic confinement fusion reactor. As with the magnetic fusion reactor the energy output will be used to drive an essentially conventional generator to produce electric energy or the heat can be used directly for the desired purpose.

With both Magnetic and inertial confinement, there is the possibility of extracting the energy directly as electricity without the intermediate step of producing heat and using steam powered generators. Research will undoubtedly produce a reactor using this principle someday but, while potentially highly efficient, it is not essential for the use of fusion reactions as a future energy source.

The hydrogen bomb uses a similar type of fusion reaction as its source of energy. A conventional nuclear fission bomb is used as the heat source to start the fusion (thermonuclear) reaction. It may some day be possible to ignite a thermonuclear bomb reaction without a fission bomb, but at this time, no one has a practical notion as to how it might be accomplished. As a result, fusion reactors cannot lead to the production of hydrogen bombs.

A scheme has been proposed for using the neutrons from the fusion reaction to convert uranium 238 to plutonium 239 or thorium 232 to uranium 233 for the manufacture of bombs. While in theory this may be possible, it does not appear to offer an easier route to the production of bombs than the current methods of separation of uranium 235, or the production of plutonium in a conventional reactor. As a result of these factors, use of a fusion energy system will in no way add to the potential for further nuclear weapons or provide a source for the unauthorized procurement of materials that might be used to produce weapons.

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