Asimov's New Guide to Science

by Isaac Asimov

  FALLOUT

  If radioactive pollution by peaceful nuclear energy is a potential danger, at least it will be kept under control, and probably successfully, by every possible means. But there is a pollution that has already spread over the world and that, indeed, in a nuclear war might be broadcast deliberately. This is the fallout from atomic bombs.

  Fallout is produced by all nuclear bombs, even those not fired in anger. Because fallout is carried around the world by the winds and brought to earth by rainfall, it is virtually impossible for any nation to explode a nuclear bomb in the atmosphere without detection. In the event of a nuclear war, fallout in the long run might produce more casualties and do more damage to living things in the world at large than the fire and blast of the bombs themselves would wreak on the countries attacked.

  Fallout is divided into three types: local, tropospheric, and stratospheric.

  Local fallout results from ground explosions in which radioactive isotopes are adsorbed on particles of soil and settle out quickly within 100 miles of the blast. Air blasts of fission bombs in the kiloton range send fission products into the troposphere. These settle out in about a month, being carried some thousands of miles eastward by the winds in that interval of time.

  The huge output of fission products from the thermonuclear superbombs is carried into the stratosphere. Such stratospheric fallout takes a year or more to settle and is distributed over a whole hemisphere, falling eventually on the attacker as well as the attacked.

  The intensity of the fallout from the first superbomb, exploded in the Pacific on 1 March 1954, caught scientists by surprise. They had not expected the fallout from a fusion bomb to be so “dirty.” Seven thousand square miles were seriously contaminated-an area nearly the size of Massachusetts. But the reason became clear when scientists learned that the fusion core was supplemented with a blanket of uranium 238 that was fissioned by the neutrons. Not only did this multiply the force of the explosion, but it gave rise to a vastly greater cloud of fission products than a simple fission bomb of the Hiroshima type.

  The fallout from the bomb tests to date has added only a small amount of radioactivity to the earth’s background radiation. But even a small rise above the natural level may increase the incidence of cancer, cause genetic damage, and slightly shorten the average life expectancy. The most conservative estimators of the hazards agree that, by increasing the mutation rate (see chapter 13 for a discussion of mutations), fallout is storing up a certain amount of trouble for future generations.

  One of the fission products is particularly dangerous for human life. This is strontium 90 (half-life, twenty-eight years), the isotope so useful in SNAP generators. Strontium 90 falling on the soil and water is taken up by plants and thereafter incorporated into the bodies of those animals (including man) that feed directly or indirectly on the plants. Its peculiar danger lies in the fact that strontium, because of its chemical similarity to calcium, goes to the bones and lodges there for a long time. The minerals in bone have a slow turnover: that is, they are not replaced nearly as rapidly as are the substances in the soft tissues. For that reason, strontium 90, once absorbed, may remain in the body for a major part of a person’s lifetime (figure 10.5).

  Figure 10.5. Decay of strontium 90 over approximately 200 years.

  Strontium 90 is a brand-new substance in our environment; it did not exist on the earth in any detectable quantity until scientists fissioned the uranium atom. But today, within less than a generation, some strontium 90 has become incorporated in the bones of every human being on earth and, indeed, in all vertebrates. Considerable quantities of it are still floating in the stratosphere, sooner or later to add to the concentration in our bones.

  The strontium-90 concentration is measured in strontium units (S.U.). One S.U. is 1 micromicrocurie of strontium 90 per gram of calcium in the body. A curie is a unit of radiation (named in honor of the Curies, of course) originally meant to be equivalent to that produced by 1 gram of radium in equilibrium with its breakdown product, radon, but is now more generally accepted as meaning 37 billion disintegrations per second. A micromicrocurie is 1 trillionth of a curie, or 2.12 disintegrations per minute. A strontium unit would therefore mean 2.12 disintegrations per minute per gram of calcium present in the body.

  The concentration of strontium 90 in the human skeleton varies greatly from place to place and among individuals. Some persons have been found to have as much as seventy-five times the average amount. Children average at least four times as high a concentration as adults, because of the higher turnover of material in their growing bones. Estimates of the averages themselves vary, because they are based mainly on estimates of the amounts of strontium 90 found in the diet. (Incidentally, milk is not a particularly hazardous food, from this point of view, because calcium obtained from vegetables has more strontium 90 associated with it. The cow’s filtration system eliminates some of the strontium it gets in its plant fodder.) The estimates of the average strontium-90 concentration in the bones of people in the United States in 1959, before atmospheric nuclear explosions were banned, ranged from less than I strontium until to well over 5 strontium units. (The maximum permissible was established by the International Commission on Radiation Protection at 67 S.U.) But the averages mean little, particularly since strontium 90 may collect in hot spots in the bones and reach a high enough level there to initiate leukemia or cancer.

  The importance of radiation effects has, among other things, resulted in the adoption of a number of types of unit designed to measure these effects. One such, the roentgen, named in honor of the discoverer of X rays, is based on the number of ions produced by the X rays or gamma rays being studied. More recently, the rad (short for “radiation”) has been introduced. It represents the absorption of 100 ergs per gram of any type of radiation.

  The nature of the radiation is of importance. A rad of massive particles is much more effective in inducing chemical change in tissues than a rad of light particles; hence, energy in the form of alpha particles is more dangerous than the same energy in the form of electrons.

  Chemically, the damage done by radiation is caused chiefly by the breakdown of water molecules (which make up most of the mass of living tissue) into highly active fragments (free radicals) that, in turn, react with the complicated molecules in tissue. Damage to bone marrow, interfering with blood-cell production, is a particularly serious manifestation of radiation sickness, which, if far enough advanced, is irreversible and leads to death.

  Many eminent scientists firmly believe that the fallout from the bomb tests is an important peril to the human race. The American chemist Linus Pauling has argued that the fallout from a single superbomb may lead to 100,000 deaths from leukemia and other diseases in the world, and he pointed out that radioactive carbon 14, produced by the neutrons from a nuclear explosion, constitutes a serious genetic danger. He was, for this reason, extremely active in pushing for cessation of testing of nuclear bombs; he endorsed all movements designed to lessen the danger of war and to encourage disarmament. On the other hand, some scientists, including the Hungarian-American physicist Edward Teller, minimized the seriousness of the fallout hazard. The sympathy of the world generally lies with Pauling, as might be indicated by the fact that he was awarded the Nobel Peace Prize in 1963.—

  In the fall of 1958, the United States, the Soviet Union, and Great Britain suspended bomb testing by a gentleman’s agreement (which, however, did not prevent France from exploding its first nuclear bomb in the atmosphere in the spring of 1960). For three years, things looked rosy; the concentration of strontium 90 reached a peak and leveled off about 1960 at a point well below what is estimated to be the maximum consistent with safety. Even so, some 25 million curies of strontium 90 and cesium 137 (another dangerous fission product) had been delivered into the atmosphere during the thirteen years of nuclear testing when some 150 bombs of all varieties were exploded. Only two of these were exploded in anger, but the results were dire indeed.


  In 1961, without warning, the Soviet Union ended the moratorium and began testing again. Since the U.S.S.R. exploded thermonuclear bombs of unprecedented power, the United States felt forced to begin testing again. World public opinion, sharpened and concentrated by the relief of the moratorium, reacted with great indignation.

  On 10 October 1963, therefore, the three chief nuclear powers signed a partial test-ban treaty (not a mere gentleman’s agreement) in which nuclear-bomb explosions in the atmosphere, in space, or underwater were banned. Only underground explosions were permitted since these do not produce fallout. This has been the most hopeful move in the direction of human survival since the opening of the Nuclear Age.

  Controlled Nuclear Fusion

  For more than thirty years, nuclear physicists have had in the back of their minds a dream even more-attractive than turning fission to constructive uses: it is the dream of harnessing fusion energy. Fusion, after all, is the engine that makes our world go round: the fusion reactions in the sun arc the ultimate source of all our forms of energy and of life itself. If somehow we could reproduce and control such reactions on the earth, all our energy problems would be solved. Our fuel supply would be as big as the ocean, for the fuel would be hydrogen.

  Oddly enough, this would not be the first use of hydrogen as a fuel. Not long after hydrogen was discovered and its properties studied, it gained a place as a chemical fuel. The American scientist Robert Hare devised an oxyhydrogen torch in 1801, and the hot flame of hydrogen burning in oxygen has served industry ever since.

  Liquid hydrogen has also been used as an immensely important fuel in rocketry, and there have even been suggestions about using hydrogen as a particularly clean fuel for the generation of electricity and in powering automobiles and similar vehicles. (In the latter cases, the problem of its ease of explosion in the air remains.) It is, however, as a nuclear-fusion fuel that the future beckons most glitteringly.

  Fusion power would be immensely more convenient than fission power. Pound for pound, a fusion reactor would deliver about five to ten times as much power as a fission reactor. A pound of hydrogen, on fusion, could produce 35 million kilowatt-hours of energy. Furthermore, fusion would depend on hydrogen isotopes which could be easily obtained from the ocean in large quantities, whereas fission requires the mining of uranium and thorium—a comparatively much more difficult task. Then, too, while fusion produces such things as neutrons and hydrogen 3, these are not expected to be as dangerous to control as fission products are. Finally, and perhaps most important, a fusion reaction, in the event of any conceivable malfunction, would only collapse and go out, whereas a fission reaction might get out of hand (a nuclear excursion), produce a meltdown of its uranium (though this has never yet happened), and spread radioactivity dangerously.

  If controlled nuclear fusion could be made feasible then, considering the availability of the fuel and the richness of the energy it would produce, it could provide a useful energy supply that could last billions of years—as long as the earth would last. The one dangerous result would then be thermal pollution—the general addition of fusion energy to the total heat arriving at the surface of the earth. This could raise the temperature slightly and have results similar to that of a greenhouse effect. This would also be true of solar power obtained from any source other than that of solar radiation reaching Earth in natural fashion. Solar power stations, operating in space, for instance, would add to the natural heat income of Earth’s surface. In either case, humanity would have to limit its energy use or devise methods for getting rid of heat from Earth into space at more than the natural rate.

  However, all this is of theoretical interest only if controlled nuclear fusion can be brought to the laboratory and then made a practical commercial process. After a generation of work, we have not yet reached that point.

  Of the three isotopes of hydrogen, hydrogen 1 is the most common also the one most difficult to force into fusion. It is the particular fuel of the sun, but the sun has it by the trillions of cubic miles, together with an enormous gravity field to hold it together and central temperatures in the many millions of degrees. Only a tiny percentage of the hydrogen within the sun is fusing at any given moment; but given the vast mass present, even a tiny percentage is enough.

  Hydrogen 3 is the easiest to bring to fusion, but it exists in such tiny quantities and can be made only at so fearful an expenditure of energy that it is hopeless to think of it, as yet, as a practical fuel all by itself.

  That leaves hydrogen 2, which is easier to handle than hydrogen 1 and much more common that hydrogen 3. In all the hydrogen of the world, only one atom out of 6,000 is deuterium, but that is enough. There is 35 trillion tons of deuterium in the ocean, enough to supply man with ample energy for all the foreseeable future.

  Yet there are problems. That might seem surprising, since fusion bombs exist. If we can make hydrogen fuse, why can’t we make a reactor as well as a bomb? Ah, but to make a fusion bomb, we need to use a fission-bomb igniter and then let it go. To make a fusion reactor, we need a gentler igniter, obviously, and we must then keep the reaction going at a constant, controlled—and nonexplosive—rate.

  The first problem is the less difficult. Heavy currents of electricity, high-energy sound waves, laser beams, and so on can all produce temperatures in the millions of degrees very briefly. There is no doubt that the required temperature will be reached.

  Maintaining the temperature while keeping the (it is to be hoped) fusing hydrogen in place is something else. Obviously no material container can hold a gas at anything like a temperature of over 100 million degrees. Either the container would vaporize or the gas would cool. The first step toward a solution is to reduce the density of the gas to far below normal pressure, thus cutting down the heat content, though the energy of the particles remains high. The second step is a concept of great ingenuity. A gas at very high temperature has all the electrons stripped off its atoms; it is a plasma (a term introduced by Irving Langmuir in the early 1930s) made up of electrons and bare nuclei. Since it consists entirely of charged particles, why not use a strong magnetic field, taking the place of a material container, to hold it? The fact that magnetic fields could restrain charged particles and pinch a stream of them together had been known since 1907, when it was named the pinch effect. The magnetic bottle idea was tried and worked—but only for the briefest instant (figure 10.6). The wisps of plasma pinched in the bottle immediately writhed like a snake, broke up, and died out.

  Figure 10.6. Magnetic bottle designed to hold a hot gas of hydrogen nuclei (a plasma). The ring is called a torus.

  Another approach is to have a magnetic field stronger at the ends of the tube so that plasma is pushed back and kept from leaking. This is also found wanting, though it does not seem by much. If only plasma at 100 million degrees can be held in place for about a second, the fusion reaction would start, and energy would pour out of the system. That energy could be used to make the magnetic field firmer and more powerful and to keep the temperature at the proper level. The fusion reaction would then be self-sustaining, with the very energy it produced serving to keep it going. But to keep the plasma from leaking for just that little second is more than can be done as yet.

  Since the plasma leakage takes place with particular ease at the end of the tube, why not remove the ends by giving the tube a doughnut shape? A particularly useful design is a doughnut-shaped tube (torus) twisted into a figure eight. This figure-eight device was first designed in 1951 by Spitzer and is called a stellarator. An even more hopeful device was designed by the Soviet physicist Lev Andreevich Artsimovich. It is called Toroidal Kamera Magnetic, which is abbreviated Tokamak.

  American physicists are also working with Tokamaks and, in addition, with a device called Scyllac, which is designed to hold denser gas and therefore require a smaller containment period.

  For nearly twenty years, physicists have been inching toward fusion power. Progress has been slow, but as yet no signs of a definite dead
end have appeared.

  Meanwhile, practical applications of fusion research are to be found. Plasma torches emitting jets at temperatures up to 50,000° C in absolute silence can far outdo ordinary chemical torches. And it is suggested that the plasma torch is the ultimate waste-disposal unit. In its flame everything—everything—would be broken down to its constituent elements, and all the elements would be available for recycling and for conversion into useful materials again.

  PART II

  The Biological Sciences

  Chapter 11

  * * *

  The Molecule

  Organic Matter

  The term molecule (from a Latin word meaning “small mass”) originally meant the ultimate, indivisible unit of a substance; and in a sense, it is an ultimate particle, because it cannot be broken down without losing its identity. To be sure, a molecule of sugar or of water can be divided into single atoms or groups, but then it is no longer sugar or water. Even a hydrogen molecule loses its characteristic chemical properties when it is broken down into its two component hydrogen atoms.