Nuclear Chemistry-State
Fusion-A
of the Rrt for Teachers
Potential Power Source
Torkil H. Jensen General Atomics, San Diego, CA 92186-9784, U.S.A. According to a n old engineering rule of thumb, a man can provide 114 horsepower for a short time. In terms of present day electricity prices, this corresponds to about 2 centsh. Our modem society is dependent on, some say addicted to, cheap energy. Most of the power used in modem society is derived from chemical energy by burning coal, oil, and natural gas. These resources were formed and stored near the surface of the earth. The energy source was light from the sun generated by fusion. I t took millions of years to accumulate these resources that we consume so ra~idlv. . Alternative energy sources must be developed and brought into operation if we are to avoid u marked decline in livine standards in the foreseeable future. Alternative energy sources that have been considered or developed include solar power, nuclear fusion, and nuclear fission. Of these, nuclear fission reactors are already developed; 70% of electric power produced in France is derived from fission reactor plants. Solar power has been developed for specialized uses, and nuclear fusion is in the research and development stage.
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Nuclear Energy Chemistry deals with how atoms of the elements are arranged in forming molecules. Because molecules can be made up of many atoms, chemistry is complicated. The energy involved in chemical reactions, that is, with rearrangement of atoms in molecules, is about 1eV per atom. I n the process of burning coal to carbon dioxide, for example, the energy liberated is about 4 eV per carbon atom. Nuclide Stability The building blocks of the nuclei of atoms are the nucleons: ~ m t o n and s neutrons. Because there are onlv a few hundred itable nuclei, the science of the structure of nuclei is in a sense much simpler than chemistry. The energy involved in nuclear reactions, that is, rearrangement of the nucleons into different nuclei, is typically 1MeV per nucleon, a million times larger than that of chemical reactions. Figure 1 shows the binding energy per nucleon versus the number of particles per nucleus. The binding energy per particle has a maximum for nuclei containing 50 to 100 particles. Thus, energy can be derived from two types of nuclear reactions: those that involve splitting heavy nuclei into lighter ones and those in which light nuclei are fused into heavier ones. The first type is used in nuclear fission reactors, and the second is the energy source for our sun and other stars and potentially for power-producing fusion reactors.
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NUMBER OF PARTICLES PER NUCLEUS
Figure 1. Binding energy per nucleon (proton or neutron)versus number of particles per nucleus Forces Two forces operate in nuclear reacuons: the nuclear force and the electrostatic force The nuclear force, wh~choverates only a t short ranges, is a n attractive force betwken neutrons and protons. The electrostatic force, which has a long range, is repulsive and operates only on protons. Two light nuclei will therefore repel each other a t large distances; only a t small distances will the nuclear forces become operative. Thus, two light nuclei must approach each other with suff~cientkinetic energy to overcome the long-distance repulsion before fusion can take place. In the sun the temperature is so high that the kinetic energy per nucleus overcomes the repulsive electrostatic force, and fusion reactions occur. The power balance of the sun is maintained by energy production from fusion of light nuclei and energy loss by radiation from the solar surface. Fusion Confining the Plasma Duplicating the fusion process of the sun and the stars for energy production on earth would present many difficulties. The temperatures needed are so large that atoms are fully ionized. The state of matter a t such temperatures-the plasma state-may be considered a gas of elec-
Magnetic Confinement Magnetic Field
Electron
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FlgLre 2. Cnarged pan c es move in he1 cal-1ke orb 1s n a magnetic the same energy. !he ion (or nJc eJs)orb I has a arger raO I ~ than S that of the ighter eectron f efd For
trons and nuclei, so one problem is the need to confine a hot, reacting plasma. Because the plasma is an electric conductor, it is subject to magnetic forces. Thus, one approach is to confine the hot plasma by a magnetic field. Another approach is to heat the matter so rapidly that the fusion reactions take place before the matter has had time to fly apart, that is, to use inertial confinement. Fauorable Reactions The most favorable nuclear fusion reactions for terrestrial power production are 'H+'H+~H+~+~.o~M~v
+ 3 ~ +en + 3.27 M ~ V 'H + 3~ + 'He + n + 17.58MeV 'H + 'He + 'He + p + 18.38MeV 'H + 'H
(1)
(2) (3) (4)
Commonly, a shorthand notation is used for these reactions. The 'H nucleus is called deuterium and given the symbol D. The 3H nucleus is called tritium and given the symbol T. The letters n and p stand for the neutron and oroton. Thus. ea 3 is commonly called the DT reaction bek u s r it involves deuterium and tritium. In principle only 1) is needed as fuel for these reactions. Among these reactions, the DT reaction has the largest cross section. Of the hydrogen found on earth, 1.5%is heavy hydrogen. This represents an enormous fuel resource-so large that we need not worry about any future shortage. This abundance is one of the reasons research is being conducted to find a practical way to use this fuel. There is another advantage: Among the "ashes" of the above fusion reactions, only the neutrons produce radioactive waste. This occurs when they are absorbed in nuclei of surrounding structures, which thereby become radioactive. The neutrons can also be absorbed in lithium, producing extra tritium. Useful Energy Production From simple arguments on energy and particle balance we can narrow down the parameters for conditions under which useful energy production may take place. The cross sections for the above fusions reactions are well-known. The arguments on energy balance involve the energy produced by the fusion reactions, the energy needed to heat the initially cold fuel, and the energy lost by radiation, heat conduction, and convection. The result of such arguments is that ignition will take place in a D-T plasma if the temperature is around 10 keV (about 108K)and if the product of the particle density and the confinement is about 1014s/cm3. Here the confinement
time is the characteristic time for the plasma to lose its thermal energy by radiation, heat conduction, and convection. At these temperatures, the plasma ressure is 1atm if the particle density is about 1014 cm-P, and the needed confinement time is about 1 s. For inertial confinement, the initial mass density of the hydrogen must be very large (500 g/cm3),and the bum time is very short. Magnetic Confinement Fusion Research Research aimed at construction of an energy-producing fusion reactor using magnetic confinement began, mostly in secrecy, in the early 1950's in several countries. At the United Nations' Atoms for Peace Conference in 1958, the veils of secrecy fell, and a remarkably cooperative, international research effort began. Because a plasma is an ionized gas, it can carry electric currents. Equilibrium can be maintained if the outward force associated with the plasma pressure p is balanced against the confiningelectromagnetic force, that is, if
+.
where 3 s the current density, and B 1s the magnetic-field strength. Thus, if eq 1is satisfied, the pressure is constant along a magnetic-field line. In order to avoid contact between the hot plasma and material walls, the structure of the magnetic-field line must be closed; field lines that intersect material walls must not he connected to field lines inside the plasma. Use of External Coils Several confirmrations for mametic confinement of plasmas have been tested over the-years. Such e x p e k e n t s have typically involved creating a suitable magnetic field in a vacuum chamber by currents in external coils. Initially the vacuum chamber is filled with hydrogen gas a t a pressure of typically one-millionth of an atmosphere. An electric field may then be applied, which causes electrical breakdown of the hydrogen gas and drives a n electric current in it. The hydrogen gas becomes heated and ionized by this current: it becomes a olasma. If the maenetic-field structure is successful, it prevents the hot plasma from being cooled by contact with the walls of the vacuum chamber. Such plasmas are often heated further by applying electromagnetic waves (like heating a chicken in a microwave oven) or by beams of energetic particles.
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Plasma Motion Relative to Field Lines Charged particles move in helical-like orbits encircling a bundle of magnetic-field lines, as shown in Figure 2. By collision with another charged particle, a particle can jump from encircling one bundle of field lines to a neighboring bundle. In a high temperature plasma, such collisions are rare. Thus, charged particles can move relatively freely along field lines, but only slowly when perpendicular to field lines. This is in accord with eq 1,which shows that the pressure gradient is perpendicular to the magnetic field. It has generally been found experimentally that plasma motion perpendicular to field lines is much more rapid than expected from these simple arguments; the plasma is not as well-confined as naively expected. Understanding and finding ways to minimize this anomalous transport has been the main objective of fusion research in the past. Turbulence It has been realized that collective collisions, which involve many particles, rather than the binary collisions implied above, cause this anomalous transport. Such pheVolume 71 Number 10 October 1994
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ment times of laboratory plasmas of about 10" s. Today confinement times near 1s are common. This corresponds to a doubling of the confinement time every 1.7 years. This rate of improvement has been maintained in recent years a s we approach the critical milestone of ignition. Tokamaks The magnetic configuration t h a t i s commonly considered most promising for future fusion reactors is called by the Russian name tokamak because i t was pioneered in the former Soviet Union in the late 1960's. The vacuum chamber has the shape of a toroid (doughnut). T h e main magnetic field is toroidal and mainly generated by currents in external toroidal-field coils a s shown in Figure 3. The flow of current i n t h e plasma is also mainly in the toroidal direction. I t is mostly driven by currents in t h e poloidal-field coils, also shown in Figure 3. The plasma current may be considered the current in a oneFigure 3. The DIII-D tokamak. The coils carrying currents for producing the toroidal field are shown, as t u r n secondary winding of a well as ~oioidal-fieldcoils. Thev mav be considered the Drimarv of a transformerfor which the Plasma transformer for which the poloiforms a'one-turn secondary wiriding: dal-field coils form the primary
nomena fall into a category of problems called turbulence, which is common in physics but not well-understood. A similar example of poorly understood turbulence is the hydrodynamic turbulence governing many cases of hydrodynamic flow.
OHMiC HEATING COILS
Progress In spite of these difficulties, substantial progress has been made. In 1960 it was common to measure confine-
Figure 4. Cross sections of some of the world's large tokamak devices: JET, Joint European Torus, European Community; JT-60 Upgrade. Japan Atomic Energy Research Institute. Japan: TFTR. Princeton, U.S.A.; T-15 and T-10, Kurchatov Institute. Russia; Tore Supra. C E N , France; DIII-D, General Atomics, San Diego; TPX.Tokamak Plasma Experiment, Princeton.
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POLOIDAL FIELD COILS
ULI
Figure 5. Schematic of the DIII-D device
Figure 5 shows a schematic diagram of the DIII-D device, in which pioneering research is carried out in noncircular plasma cross-section tokamaks. Figure 6 is an inside photo from the DIIID device. The wall is covered by carbon tiles that withstand the radiation and energetic particle flux from the plasma. The inherent advantage of a shaped cross section has been demonstrated hy research conducted a t General Atomics. A measure of efficiency is the ratio p between the plasma pressure and the magnetic-field "pressure". Record values of p of 0.12 have been accomplished. A P of 0.05 i s believed necessary for economy in power-plant applications. Optimal Fuel Mixtures
Figure 6. Photo from inside the DIII-D vacuum chamber. winding. The total field-the sum of the toroidal field and the poloidal field associated with the toroidal plasma current-is such that the field lines are helical-like, falling on nested surfaces. This configuration satisfies the previously mentioned requirement of being closed; field lines populated with plasma do not intersect the material wall.
T h e TFTR device i s constructed with neutron shielding and equipped to handle the superheavy hydrogen isotope tritium, which is radioactive. This makes it possible to operate the device with t h e optimum fuel mixture: a n equal mixture of d e u t e r i u m a n d tritium. This mixture is optimal because the cross section for the DT reaction has by far the largest cross section of the fusion reactions mentioned above. Recently, TFTR was operated with this optimum mixture. The resulting power due to fusion reactions was above 6 MW.
Shaped Cross Sections
Current Trends
Initiallv. .. manv tokamaks were huilt with circular cross sections. However, shaped plasmas were shown to have clear advantages. The cross sections of some of the larger ones are illustrated in Figure 4. The two largest devices in the United States are the Tokamak Fusion Test Reactor (TFTR) a t Princeton and the Doublet III-D (DIII-D) a t General Atomics in San Diego. The toroidal-field strengths in these devices are typically 5 and 2 T (tesla), and the toroidal currents are typically 2 MA. The plasma temperatures, densities, and confinement times obtained are now very close to those required for a fusion reactor. Temperatures of 10 keV (100,000,000K) are now routinely reached in tokamak devices. In these dewces plasmahc.atmg 1s accomplished mainly by injection of beams of energetic neutral atoms that become ionized inside the plasma or by radiofrequency waves
Alarge effort is presently under way to design the International Thermonuclear Experimental Reactor (ITER). This is a joint effort by the European Community, Japan, Russia, and the United States. There are design teams in San Diego; Garching, Germany, and Naka, Japan. ITER is to be builtjointly by its partners. Goals include the production of fusion power in excess of 1000 MW for studying the physics of igniting plasmas, and the integrated demonstration of fusion-reactor technologies. Magnetic-fusion research efforts are changing their focus from physics problems to engineering prohlems. It is possible now in principle to build a fusion plant but not a n economically competitive one. Better (superconducting) magnets and better structural materials must first be developed.
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