Fusion could become the energy source of choice early in the third millennium - it is a long-term, inexhaustible energy option that offers the possibility of generating power in an economically and environmentally attractive system, which is compact relative to renewable energy power plants, and does not emit carbon dioxide or other greenhouse gases. The small loss of mass when light nuclei fuse into heavier nuclei provides the source of energy. In one sense it is the only long-term option other than fission, since the sun, which is the source of all forms of renewable energy, is powered by fusion reactions. In this chapter, we will present our personal vision of fusion's potential, illustrated with examples of innovative concepts at an early stage of development, and grounded in the dramatic progress towards fusion energy demonstrated by the more traditional concepts.
Fusion occurs when two positively charged nuclei approach closely enough for the attractive short-range nuclear forces to overcome the repulsive Coulomb force. Getting two nuclei this close requires that they have a high energy or temperature of about ten thousand electron volts (10 keV ~ 100 million Kelvin). (At temperatures above 0.01 keV, the atoms have such a high velocity that the electrons are knocked off by collisions. This results in the mixture of equal amounts of positively charged ions and negatively charged electrons that is known as a plasma.) In addition to needing a high temperature for fusion, the hot ions must be held at a high enough density for a long enough time so that they release more energy through fusion reactions than went into heating them and than they lose by radiation and plasma-particle loss. This density-time-temperature product has a required threshold magnitude of 1021 m~3s keV for a fusion power plant fueled by deuterium-tritium. These requirements of density, confinement time, and temperature are why fusion has been difficult to demonstrate in the laboratory.
Arthur W. Molvik and John L. Perkins Table 8.1 Candidate fusion reactions
d-d 2H + 2H ^ !H + 3H + 4.0 MeV 2H + 2H ^ !n + 3He + 3.3 MeV
Note: Courtesy of the University of California, Lawrence Livermore National Laboratory, and the Department of Energy under whose auspices the work was performed.
The difficulty of fusion has a positive side: inherent-passive safety - reactions will not, and cannot, runaway. (Passive safety means that an off-normal condition will correct itself, rather than requiring positive action, as in "active safety", to prevent an accident.) The fuel in the core of a fusion power plant is sufficient, at most, for only a few seconds of operation, and needs to be continually replenished. Fusion energy is distinguished from fission in which a heavy nuclei splits or fissions into two or more lighter nuclei, that are frequently radioactive. Fusion reaction products are non-radioactive, a significant environmental advantage. Fission releases energy when a critical mass is gathered into a volume that is small enough so that a neutron released by a fission reaction will cause another fission reaction before escaping from the fissionable material. Passive safety is much more difficult to achieve with fission reactors, because the stored energy in the fuel of a fission core is sufficient for about two years of operation.
Several possible fusion fuels exist among low atomic number nuclei. These are listed in Table 8.1, along with the reactions. The deuterium-tritium reaction has a larger cross-section (i.e., larger reactivity) at lower energy than the others do, so it is the nearest term fusion fuel. However, 80% of the output energy from the fusion of deuterium and tritium is carried by 14 MeV neutrons, which can activate the power plant structure. The remaining 3.5 MeV is carried by the 4He or alpha particle. The four reactions at the bottom of Table 8.1 produce few or no neutrons, and nearly all their energy is carried by energetic ions. These reactions have the double advantage of reducing or eliminating activation of the power plant structure, and of offering the possibility of directly converting the reaction product energy to electricity by the interaction of the energetic ion reaction products with electric or magnetic fields. Direct conversion can, in principle, achieve efficiency levels greater than obtained with a thermal cycle. These ions, however, have a higher charge, and hence require not only a higher plasma temperature to overcome the Coulomb repulsive force but also a greater density-confinement time product to overcome smaller cross-sections. These considerations strongly encourage the use of deuterium-tritium in first generation fusion power plants. And, as we will discuss, concepts exist to achieve low-activation, even with high neutron production.
Significant progress towards fusion energy has been achieved: the density-time-temperature product has increased by more than eight orders of magnitude in 30 years to >1021 m~3s_1keV (Ishida, 1999). Twelve megawatts of fusion power has been achieved in the laboratory, almost equal to the 18 MW of heating power (Watkins, 1999). The progress by the 1980s was sufficient to initiate the design of the ITER (International Thermonuclear Experimental Reactor) as a joint project between the US, Russia, Europe, and Japan. ITER, a large tokamak, was intended to study plasmas producing fusion power greater than the external power used to heat the plasma, and to begin studying power plant engineering. The engineering design has been completed (Shimomura, 1999), but construction at this time is doubtful: the costs grew to exceed $10 billion, the US has withdrawn as a major player, Russia's faltering economy has forced it into a more minor role, and Europe and Japan have not committed to assuming the full load by themselves (Glanz and Lawler, 1998).
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