Controlled Nuclear Fusion
The nuclear fusion program of the United States should seek to develop this technology sufficiently for comparison with fast breeder reactors, solar power, and other long-term sources of energy.1 The research and development program has proceeded nearly to the point of demonstrating the scientific feasibility of nuclear fusion. The technology must still be tested for engineering achievability, environmental characteristics, and reasonable cost. We recommend that the program pursue research on the principal physical concepts that have been advanced for nuclear fusion, and on the materials and design problems of fusion reactor technology, with attention to environmental characteristics and engineering and economic practicality whenever possible. The aim is to develop the most attractive forms of this technology on a timely schedule.
The following general circumstances establish the context for assessment of nuclear fusion and the program of research and development to test its possibilities.
Only four long-term sources of energy have been projected for the future: nuclear fusion, nuclear fission breeders, solar energy, and geothermal energy. The latter three are not so free of problems that fusion can be disregarded or assigned secondary importance.
Controlled fusion will require scientific and technological development for at least two decades, perhaps much longer. Its prospects as a source of power cannot be judged with confidence in advance of this development.
The safety and environmental liabilities attending the use of fusion for
power cannot be analyzed in satisfying detail at the present stage of development. Some may be far more tractable than those of fast breeders.
The complexity of problems in nuclear fusion may tend to isolate the scientific and technical community engaged in various aspects of this area of research and development. Care should be taken to ensure that this isolation does not lead to wrong decisions.
Research and development in nuclear fusion is expensive. To demonstrate the technology sufficiently for decisions about its use as a source of energy will cost $10–$20 billion. Each new possibility explored in the effort to optimize the fusion option will add to the cost.2 (The cost of developing any new long-term source of major significance, however, probably falls in the same range.)
THE FUSION REACTION
Large amounts of energy are released in the union or fusion of light nuclei. Nuclear fusion requires that two charged nuclei approach one another closely. They must approach with high enough energy to overcome their mutual electrostatic repulsion. High energy can be achieved through high temperature. The sun, for example, fuses hydrogen nuclei into helium at interior temperatures of about 20 million degrees Celsius. For the most promising earth-bound possibility—fusing deuterium and tritium into helium—temperatures about 10 times that of the sun’s core must be achieved and maintained long enough to allow a significant fraction of the fuel to react. At the high temperatures of fusion reactions, matter has decomposed into atoms whose electrons are stripped away. The result is an ionized gas, or plasma, that conducts electricity and responds readily to magnetic and electric forces.
The practical use of fusion as a source of energy, then, depends on the simultaneous achievement of high temperatures and effective containment of the plasma. Heating and containing the plasma, and operating the ancillary equipment that may be used to drive the reaction, represent a large investment in energy that the net production of energy from fusion must pay back with interest.
FUSION FUEL CYCLES AND THE ENVIRONMENT
The fusion reaction that requires the lowest temperature and the least effective containment, and offers the prospect of the highest power densities, is one of deuterium and tritium.
Most of the energy released (14.1 MeV) is carried by the escaping neutron. By being slowed down in solid or liquid materials surrounding the plasma, these neutrons have their kinetic energy converted into heat, which is then converted into electricity in a conventional heat engine such as a steam turbine.
Deuterium makes up 1/7000 of the hydrogen in water and, at a present cost per unit of energy 1/10,000 that of coal, can be considered free. Tritium is weakly radioactive and decays into helium-3 (3He) with a half-life of 12.3 years. It is almost nonexistent in nature and must be regenerated in the fusion fuel cycle after use in deuterium-tritium (D-T) reactions. The following two reactions of lithium (Li) with neutrons permit tritium breeding at a gain theoretically much greater than 1.
Here n′ represents a neutron that has lost 2.7 MeV of its kinetic energy in an inelastic collision with lithium, whose resulting excited state dissociates into tritium and 4He as indicated by the equation.
The breeding reactions can be arranged to occur by absorption of some of the neutrons released by fusion in a surrounding lithium blanket, at temperatures conventional in power plants. Thus, a fusion reactor on this fuel cycle is a “breeder” reactor fed by deuterium and lithium.
Reasonably assured terrestrial resources of lithium represent a potential source of energy for fusion comparable to world resources of coal, at a fuel cost 1/1000 that of coal. Lithium in seawater represents an energy resource 10,000 times larger; it could probably be extracted at a cost low enough to make little contribution to the cost of electricity from fusion.
The prolific release of neutrons in the D-T fusion reaction will induce radioactivity in structural and other materials in the reactor. These activation products, together with the substantial inventory of tritium, could pose a significant radiation hazard to the public in the event of a major accident, and in any case will complicate routine operation and maintenance. Tritium will be subject to some routine release from the fusion plant. The magnitude of such releases (and the corresponding public exposure) will be accurately known only after specific designs emerge and control technologies are tested at commercial scale. Based on today’s knowledge, it appears possible, at a cost, to hold routine public exposures from fusion reactors to the same low levels as those from routine operation of light water fission reactors. The neutron-induced radioactivity in fusion reactor materials can be reduced, in principle, by judicious selection of the materials used. (The radioactivity of the fission products produced in a fission reactor, by contrast, cannot be appreciably altered through reactor design.) How much of this apparent potential to reduce neutron activation in fusion can be realized in practice remains to be seen.
It depends in part on which of the low-activation materials meet the other demanding conditions of operation in a fusion reactor.
Besides the D-T cycle, other fuel cycles have been proposed; they rely on deuterium alone, on deuterium and 3He, or on ordinary hydrogen plus heavier elements such as lithium, beryllium, or boron in the plasma itself. These fuel cycles could offer various advantages. For example, more energy would be released in the kinetic energy of charged particles (and thus the potential for high efficiency would be enhanced) and less would be released in the form of neutrons. There might even be no radioactivity either induced in structural materials or in the fuel or “ashes.” These fuel cycles may eventually find application in the production of chemicals or in materials processing and perhaps ultimately in the production of electricity. They now seem unlikely to compete with the D-T-Li fuel cycle for the early generation of electricity from fusion, since they require higher temperatures and much more effective confinement.3 If an exception materializes, it will likely be the D-D reaction, catalyzed by reinjection of product tritium and 3He, which is next in difficulty after D-T and produces more neutrons than the other “advanced” reactions.
Hybrid fusion-fission fuel cycles, in which the neutrons emitted in D-T or D-D reactions are used to induce fissions or to create fissile isotopes (uranium-233 or plutonium-239) from fertile thorium-232 or uranium-238 in a blanket surrounding a fusion core, are also possible. The fissile material produced can be fissioned in place or removed to fuel nonbreeder fission reactors elsewhere. The result, in effect, is to multiply the energy release per fusion reaction by about an order of magnitude. This makes the conditions that must be achieved in the fusion core less demanding than those for a pure fusion reactor, but the additional engineering complexity of combining fusion and fission technologies in a single device will at least partly offset this advantage and may overwhelm it. The environmental and safety characteristics of hybrid devices would be substantially those of fission reactors, compounded by the addition of fusion’s tritium and activation products. The chances of some kinds of accidents might be reduced; the chances of others increased. Most proponents of hybrids argue that these fuel cycles could best be applied in the hybrid device optimized to produce fuel for fission reactors located elsewhere. The safety or antiproliferation characteristics (or both) of such a fusion-fission nuclear energy system, they argue, might be superior to those of a system consisting primarily of fission breeder reactors.
The hybrid possibility calls attention to a link between any neutron-producing fusion energy system and the potential for proliferation of fission weapons; excess fusion neutrons can be “diverted” by a reactor’s operators to producing fissile materials for bombs. The practical importance of this link may be small, however, given the difficulties of fusion
energy technology. Any group or country capable in fusion could acquire fissile materials by a number of easier means. More troublesome, perhaps, is the possibility that knowledge derived from certain aspects of research on inertial confinement approaches to fusion could be applied to the development of fusion weapons. Firm conclusions on the exact nature of this link, or its importance, cannot be reached without access to classified information.
CREATING THE CONDITIONS FOR FUSION: HEAT AND CONFINEMENT
The two main classes of schemes that have been proposed to heat and hold thermonuclear fuel for fusion are magnetic and inertial confinement. Experiments with magnetic confinement were begun in the early 1950s; serious investigation of inertial confinement began in the 1960s.
The reacting fuel to be contained by magnetic fields is a tenuous deuterium-tritium gas at a millionth of solid density or less. The plasma is initially formed and heated electrically. Further heating may be accomplished by a variety of methods, of which the injection of high-energy neutral beams is the preferred method. Thereafter, in many concepts (particularly the Tokamak), the fusion reaction itself keeps the plasma hot. In its hot, reacting state, the gas is typically at a pressure of 10 atm. It must be held together in isolation from material that could cool or contaminate it for at least several seconds to allow an appreciable fraction of the fuel to react. (Machines operating at somewhat higher fuel densities—up to 1/100,000 that of normal solids—need correspondingly shorter confinement times.)
Magnetic fields, coupled to electric currents in the hot gas, generate forces that confine the fusion gas to a prepared vacuum space. The particular forms of microturbulence that limit plasma lifetimes and density depend on temperature and details of the configuration. Understanding the physics of this complex matter and searching for the optimum configuration have absorbed most of the worldwide research effort, on which some $4 billion has been spent.4 Several geometrical arrangements have been advanced. So far, the leading contenders in design are all large and lead to large power systems (at least 500 megawatts (electric)*
(MWe)), principally because the vacuum region where fusion occurs must be designed with various inlets and outlets, must be sheathed in a 1-m-thick solid blanket for slowing the high-energy neutrons, breeding tritium, and carrying away the heat in cooling channels, and must be surrounded by superconducting coils and other electrical equipment. A few possibilities (much less thoroughly investigated) might work at the 100-MWe scale.
Details of the specific arrangements need not concern us here: Two leading examples are magnetic mirrors, analogous to the magnetic field in outer space that confines the earth’s “radiation belts” of trapped particles, and toroidal (doughnut-shaped) devices, such as stellarators or Tokamaks. The science of plasma physics and magnetic confinement has matured rapidly, aided by an unhindered program of international cooperation and exchange coordinated by the International Fusion Research Council (under the International Atomic Energy Agency). The United States, the U.S.S.R., England, France, the Federal Republic of Germany, the Netherlands, Italy, and Japan participate. Toroidal Tokamaks (illustrated in Figure 7–1), conceived in the U.S.S.R., have been the most heavily funded research instruments in the United States since the early 1970s. Of magnetic fusion experiments, those in Tokamaks are closest to achieving the combination of confinement conditions needed for a reactor Magnetic mirror experiments are being conducted in the U.S.S.R., the United States, and elsewhere, at a substantial level of funding, but well below that of Tokamaks.
At this point, the nature of uncertainty about the prospects for fusion can be understood. The Tokamaks are by virtue of their fundamental principles complicated in design, difficult to service, and probably operate on long pulses—minutes—rather than steadily. Mirror machines are simpler geometrically and can operate in steady state but seem likely for rather basic physical reasons to have poorer confinement than Tokamaks. Does something better than either exist—that is, an approach more likely to evolve into an attractive reactor—but because the research and development field is so complex it has yet to be recognized? The whole field of study is insufficiently advanced to guide selection, although the great variety of possibilities seems to give considerable grounds for optimism.
In inertial confinement schemes, small pellets of deuterium-tritium fuel are dropped one at a time into a vacuum and irradiated by high-energy beams that cause the outer layers to evaporate explosively. The resulting forces compress and heat the remaining pellet core, generating fusion
reactions. The pellets used in experiments are 1 mm or less in diameter, and the energy release is controllable. The principal difficulty is that of achieving the necessary compression—100 billion atm—while preserving the stability of the pellet’s core and preventing its premature heating. The first source of energy trained on the pellets was the laser (inertial confinement is thus often labeled “laser fusion”). More recently, energetic electron or ion beams that are focused and pulsed have been proposed to drive the reaction. An inertial confinement scheme is illustrated in Figure 7–2.
Development of inertial confinement reactors depends on the beam system: In about one billionth of a second or less, enough energy must be
delivered to the pellet to compress and heat the core to a state sufficient for fusion reactions. The performance of available laser systems is far below that required for reactor purposes, although lasers adequate for proposed scientific breakeven experiments (creation of conditions that, in a reactor, could lead to net energy output) are nearing completion.
TIMETABLE FOR DEVELOPMENT AND CRITERIA FOR DEPLOYMENT
Strategies to supply energy in the future—new coal technology, solar power, perhaps local geothermal energy, accelerated and advanced nuclear fission technology—all take long times to develop and put to use. Technologies anticipated for extensive use in the first quarter of the next century would thus have to be fully developed by the end of this century. The appropriate course for further development and resolution of problems must be clear by 1985 or 1990. Advanced fission power systems, including breeder reactors, are being developed around the world in approximate accord with this schedule.
In the competition among potential long-term energy sources, the main criteria for choices among alternatives are likely to be timing of availability, cost and reliability of delivered energy, versatility of application, and environmental and social acceptability. A technology with major liabilities by any of these criteria, compared to the competition, presumably will have to offer major advantages by other criteria if it is to be extensively deployed.
With respect to timing, fusion is far behind the fission breeder reactor, and it is behind at least some forms of solar energy technology.* Three stages of feasibility must be considered in judging fusion’s position in this regard.
Scientific feasibility for fusion means achieving, in the laboratory, simultaneous conditions of fuel temperature, density, and confinement time that would lead, if they occurred in a reactor, to output power exceeding the input power.
Technological feasibility means building a device that actually produces a net output in usable form, in essentially continuous operation. Such a device must incorporate sophisticated fuel-handling and energy-conversion equipment not needed to establish scientific feasibility and demonstrate long-term operation of magnets, lasers, vacuum pumping systems, and so on. Central to solving problems of technological feasibility for fusion is finding and testing combinations of materials that can perform adequately in the fusion environment of intense high-energy neutron flux, extreme temperature gradients, intense magnetic fields or laser pulses (or both), and other stresses.
Commercial feasibility means translating technological feasibility into a device that can produce continuous and reliable power under conditions and costs that are attractive to users.
Neither magnetic confinement nor inertial confinement fusion has yet passed even the threshold of scientific feasibility, although both are widely considered likely to do so ultimately; with luck, by the early to middle 1980s. Some work related to technological feasibility is under way in advance of the expected demonstration of scientific feasibility, but the engineering and materials problems are so formidable that it is hard to imagine passing through technological feasibility to commercial feasibility in less than 15 years after that demonstration—that is, before the turn of the century. Substantial effects on electricity budgets, then, could not be expected before 2020.
With respect to the cost of delivered energy, fusion will share with the other long-term sources the characteristic that its costs are dominated by the capital costs of the power plants, with fuel costs a minor factor. Simply on the grounds of technological complexity, it seems likely that the capital costs of fusion power plants—and hence delivered energy costs—will be higher than those of fission breeder reactors. With respect to versatility of application, first-generation fusion reactors (those based on the D-T reaction) are not likely to be much different from fission breeders. Both will produce electricity in large blocks.
Fusion’s disadvantage in timing and its likely disadvantage in energy cost or versatility place considerable weight on the magnitude of any advantages it may enjoy over its competitors in environmental and social acceptability. Here it is important to understand that fusion is not one potential technology but many and that these imply a substantial range of characteristics pertinent to environmental hazards, safety, and (perhaps) links to nuclear weaponry. It seems almost certain that even the least attractive of the fusion possibilities will present lower risks than fission breeder reactors in most of these characteristics, but it is quite possible that the margin of superiority would not be judged great enough to offset fusion’s disadvantages in time of availability and perhaps cost. Unfortunately, the approaches to fusion that appear to offer the greatest environmental advantages over fission—e.g., tritium inventories and neutron activation so small that the difference in accident risk and waste management problems compared to fission is many orders of magnitude—are the approaches that now appear to be the most difficult to achieve. They tend to entail advanced reactions, advanced materials, and advanced designs. Herein, then, lies a dilemma: Pushing fusion too rapidly toward early commercialization is likely to favor fusion technologies whose environmental advantages over fission are not great enough to be worth the trouble, but delaying fusion too long in pursuit of the more difficult approaches promising theoretically attractive characteristics takes the
chance that the fusion option will not be available in time to offer an alternative to fission breeders.*
The program that is moving forward in the face of this dilemma combines vigorous pursuit of demonstrating scientific feasibility, of both magnetic confinement and inertial confinement schemes, with study of reference designs of fusion reactors as a first step toward technological and commercial feasibility. The first reference designs were begun in 1967. These were not intended to serve as blueprints, but as preliminary exercises to ensure that researchers faced every problem associated with reactors designed around any given concept. Reference designs were problem-seeking rather than problem-solving exercises and, as such, registered considerable success. Long lists emerged of the scientific and technical questions demanding attention: more radiation-resistant alloys, different systems for cooling the reactor, and higher energy-handling ability per unit area of reactor wall. As those involved in the elaboration of reference designs turn their attention to these problems, the designs themselves are discarded in favor of better ones. To date, the problems look difficult but not insurmountable. Reference designs have looked progressively more attractive as conceptual solutions for various problems have emerged. New reference designs will appear in due course. The reference design exercises and experimental work proceed more or less continuously. If the fusion program succeeds, the designs for practical fusion reactors may be unlike any envisaged to date.
But will the program succeed? Scientific successes in raising temperatures and improving confinement must be balanced against the accumulating evidence that the technical
One of the formidable difficulties for the nuclear fusion program to overcome is the tendency of groups working on some phase of complicated technology to use up design flexibility in the solution of their problems and leave impossible tasks for others. Technology and engineering cannot simply be ordered to the specifications of plasma physics, nor can the laws of physics and materials science be ordered to the specification of fusion researchers. The program must continuously seek to balance the competing demands of plasma dynamics, materials science, technology, engineering, and power at a reasonable cost.
Although the development of nuclear fusion faces considerable uncertainties, it should be pursued and reevaluated in 5 years. By that time, large scientific breakeven experiments in both magnetic and inertial confinement will have been attempted. More realistic engineering designs and guidance for further research on technological obstacles should then emerge naturally.
Principal attention should be directed first to the problems of pure fusion reactors, before the question of fusion-fission hybrids is considered.
The immature state of fusion research and development offers the opportunity to given attention to the environmental and safety characteristics in the earliest stages of design. Consideration of these characteristics is so important to decisions on major investments in fusion that the opportunity should not be wasted.
A small effort should be directed to fuel cycles other than deuteriumtritium. Pure deuterium has a much lower reaction rate but no critical tritium regeneration problem, and it wreaks less structural damage from high-energy neutrons. In the so-called neutronless fuel cycles, all particles and products are electrically charged, and in theory there is no radioactivity. Smaller devices might be built, but the required plasma temperatures are much higher, and the energy balance is probably unfavorable.
High priority should be given to study and testing of structural materials, and assessments of their availability must be undertaken.
Research and development in nuclear fusion has enjoyed singularly fruitful international cooperation. This cooperation should be encouraged and extended to speed progress and reduce the cost to each individual country.
1. For an overview of fusion’s long-term prospects, see J.P.Holdren, “Fusion Energy in Context: Its Fitness for the Long Term,” Science 200 (1978):168–180.
2. D.J.Rose and M.Feirtag, “The Prospects for Fusion,” Technology Review, December 1976, pp. 20–43. See also the report of the Fusion Assessment Resource Group of the Supply and Delivery Panel: National Research Council, Supporting Paper 3: Controlled Nuclear Fusion: Current Research and Potential Progress, Committee on Nuclear and Alternative Energy Systems, Supply and Delivery Panel, Fusion Assessment Resource Group (Washington, D.C.: National Academy of Sciences, 1978).
3. Uses have also been proposed for the neutrons and radiation produced by D-T fusion; for example, radiolysis of water to yield hydrogen, or transmutation of radioactive fission products to shorter-lived or stable isotopes.
4. From the report of the Fusion Assessment Resource Group of the Supply and Delivery Panel (see note 2).