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BACKGROUND INFORMATION FUSION, A FORM OF NUCLEAR ENERGY It is commonly held today that the world's ultimate energy supplies will be provided either by the sun, by geothermal energy stored in the earth's interior, by nuclear fuels found on earth, or by some combination of these sources. There are two main variants of nuclear energy to be considered: fission and fusion. This report is concerned with the latter. Fusion denotes a class of rearrangement reactions involving the nuclei of the lighter elements in the periodic table, reactions accompanied by a net release of large amounts of energy. Fission, on the other hand, denotes exothermic reactions following the neutron bombardment of the nuclei of heavier elements in the periodic table, principally certain isotopes of uranium and plutonium. A number of elements existing abun- dantly on earth can serve as nuclear fuels for either the fission or fusion process. Consequently, these fuels represent essentially inex- haustible sources of energy for the future. Unlike fission, all fusion reactions require extremely high tempera- tures, tens to hundreds of million degrees Celsius. At these tempera- tures matter is gaseous and decomposes into atoms; the atoms, in turn, are stripped of their outer electrons and thus become ionized. We refer to this state as a plasma, that is, an ionized gas distinguished from ordinary gases by its ability to conduct electricity easily and to respond readily to electric or magnetic forces. For practical purposes, it will be necessary for a fusion reactor to achieve conditions where the appro- priate fuel is raised to these elevated temperatures and held there long enough so that a significant fraction of the fuel can undergo fusion reactions. The amount of energy recovered in the process will have to exceed the amount of energy invested, and exceed it by some measure, in order for the fusion reactor to be of practical interest.
2 FUSION FUEL CYCLES A variety of existing fusion processes, or fuel cycles, might be consid- ered for terrestrial purposes.* The fusion energy will be released in a combination of three forms: radiation, kinetic energy of charged parti- cles, and fast neutrons. The distribution of energy among these three forms depends on the fuel cycle selected and hence will affect the engi- neering aspects of a fusion plant as well as its potential applications. The reaction involving the deuterium (D) and tritium (T) isotopes of hydrogen requires by far the least stringent plasma conditions and is therefore receiving the most attention today. Deuterium is found in nature along with ordinary hydrogen in the proportion of l to 6,500, and is readily recoverable from the waters of the ea.--t-h. Tritium is radio- active, decaying with the emission of a soft beta particle, and with a half-life of slightly more than l2 years. It does not occur naturally and therefore must be bred, that is, created artificially. The fusion reaction of D-T produces a nucleus of ordinary helium plus a neutron and l7.6 million electron-volts (MeV) of energy per event. Most of the energy (about 80 percent) is carried off by the l4.l MeV neutron produced in the fusion events. In a suitably designed blanket, neutrons can be made to react with lithium (Li) to produce tritium. Consequently, the D-T fusion reaction involves a breeding fuel cycle of D-T-Li, with deu- terium and lithium as the fuels ultimately consumed. The availability of lithium thus becomes an essential factor in considering the long-term viability of fusion, when operating on the D-T-Li fuel cycle. Ideally, the D-T-Li cycle, relying entirely on the (n,a) reaction of 6Li3 to produce tritium, yields 22.4 MeV of energy per one complete fusion-breeding event and translates into 8.32 meiawatt days, thermal (MWD(th)) of energy per gram of tritium burned. The l4.l MeV fusion neutrons are able to initiate (n,an') reactions in Lij and produce tritium, as well. With natural lithium as the feed and a 40 percent conversion efficiency of thermal energy to electricity, one finds that the lithium makeup required by breeding amounts to approximately l.8 kilos per megawatt electric year (MWe-year). Current estimates indicate worldwide lithium reserves and resources on the order of l0 million tonnes. This is surely a conservative esti- mate and does not take into account the vast amount of lithium recoverable from sea water. The latter, because of the inherently low fuel-cycle costs of fusion power, could ultimately contribute to the reserve figures and increase them by many orders of magnitude. Nevertheless, the l0 mil- lion tonnes figure corresponds to about 5 billion MWe years of electrical energy, which could support a global population of l0 billion people for 500 years at a per capita electrical power demand of l kilowatt. Early design studies on fusion reactors revealed that the reactor's lithium inventory would be difficult to recycle due to the buildup of excessive chemical and radioactive contaminants. This leads one to *The well-known fusion reaction involving the common form of hydrogen, which is responsible for the energy production of most stars, including our sun, proceeds at too slow a rate to be of terrestrial interest.
consider the economic advantages of committing the entire inventory (in one design, l.l5 tonnes of natural lithium per MWe of installed capacity) over the lifetime of the plant. In the absence of recycling, lithium requirements would increase by at least an order of magnitude. Thus, one should expect that recycling and returning the plant inventory to next- generation reactors will become economically advantageous in the course of time. Compared with other fusion fuel cycles, the D-T-Li fuel cycle has several disadvantages. It will require breeding and handling of radio- active tritium, as well as disposal of radioactive material produced within the plant by neutron activation. Due to the high background radiation in the plant, some functions will need remote operation and maintenance, making it somewhat more difficult to maintain high plant availability. The advantage of the D-T-Li fuel cycle is that it re- quires the least stringent plasma conditions for an operating reactor by a wide margin and leads to power densities one to two orders of mag- nitude greater than might be achieved with other fuel cycles. Advanced fuel cycles, by relying on deuterium, 3He2, or such higher atomic weight elements as lithium, beryllium, or boron, tend to alleviate many of the above problems to varying degrees; these cycles may prove to be of prac- tical interest at some future date, but are unlikely to compete with D-T- Li in early generation reactors. Thus, the currently most promising fusion reaction, D-T, uses radio- active material in the form of tritium and produces neutrons that, in turn, will induce radioactivity in the structural and operating material contained within the reactor. Consequently, fusion reactors have their own unique radiation hazards, and these need to be evaluated carefully. There is reason to believe that the radiation safety and waste disposal problems peculiar to fusion reactors may be solved through the appropri- ate choice of materials, engineering, and design at lesser expense than for fission reactors. In addition, it must be noted that the neutrons created through fusion could be used, with some effort and redesign, to produce material for the manufacture of nuclear weapons. This raises a safeguard issue, although possibly of a less complicated nature than for fission breeders. FUSION APPLICATIONS So far, we have made no distinction between the use of a commercial fusion energy system to produce electricity and other possible applications. In fact, attention in this country has been devoted largely to developing a fusion reactor suitable for use as an electric power source by the electric utility industry. Since the principal direct output of a D-T reactor is energetic neutrons, however, other applications have been proposed. The main one is a scheme by which the neutrons, in combination with a blanket of fertile nuclear material, are used to produce fissile material for fission converters. Interest in this approach derives from the fact that each l4.l MeV neutron could, in principle, lead to the production of up to five fissile atoms while at the same time satisfying tritium breeding needs. Each of the fissile atoms could then ultimately
produce about 300 MeV consistent with light water reactor (LWR) perfor- mance and plutonium (Pu) recycling; advanced thermal converters could achieve higher yields. (See report of Reactor Resource Group.) The opportunities for energy multiplication, theoretically, are large and could lead to a significant reduction in plasma requirements within the scope of an economical commercial system. Elaboration of this concept will be given later. Other uses for the neutrons and radiation produced in D-T reactions have been proposed. For example, by radiolysis of water to produce hydrogen, or of carbon dioxide to produce carbon monoxide, both products provide combustion fuels. Similarly, disposal of radioactive fission products could be achieved by transmutation to shorter lived or stable nuclei, etc. Although these applications could be of practical interest, such concepts have been explored only to a limited extent, and not sufficiently to determine whether they should become a significant objec- tive in developing fusion for civilian applications. Fuel cycles other than D-T exist that offer the possibility of in- creasing ths proportion of fusion energy in the form of electromagnetic radiation and kinetic energy of charged particles. This could, in prin- ciple, lead to higher efficiency energy conversion options and unique applications for chemical production and materials processing. Although of potential use in fusion, such concepts must be considered highly spec- ulative at this time.
