by Isaac Asimov
In 1941, experiments were conducted with uranium-graphite mixtures, and enough information was gathered to lead physicists to decide that, even without enriched uranium, a chain reaction might be set up if only the lump of uranium were made large enough.
Physicists set out to build a uranium chain reactor of critical size at the University of Chicago. By that time some six tons of pure uranium were available; this amount was eked out with uranium oxide. Alternate layers of uranium and graphite were laid down one on the other, fifty-seven layers in all, with holes through them for insertion of the cadmium control rods. The structure was called a pile—a noncommittal code name that did not give away its function. (During the First World War, the newly designed armored vehicles on caterpillar treads were referred to as tanks for the same purpose of secrecy. The name tank stuck, but atomic pile fortunately gave way eventually to the more descriptive name nuclear reactor.)
The Chicago pile, built under the football stadium, measured 30 feet wide, 32 feet long, and 21½ feet high. It weighed 1,400 tons and contained 52 tons of uranium, as metal and oxide. (Using pure uranium 235, the critical size would have been, it is reported, no more than 9 ounces.) On 2 December 1942, the cadmium control rods were slowly pulled out. At 3:45 P.M. the multiplication factor reached 1: a self-sustaining fission reaction was under way. At that moment humanity (without knowing it) entered the Nuclear Age.
The physicist in charge was Enrico Fermi, and Eugene Wigner presented him with a bottle of Chianti in celebration. Arthur Compton, who was at the site, made a long-distance telephone call to James Bryant Conant at Harvard, announcing the success. “The Italian navigator,” he said, “has entered the new world.” Conant asked, “How were the natives?” The answer came at once: “Very friendly!”
It is a curious and interesting that the first Italian navigator discovered one new world in 1492, and the second discovered another in 1942.
THE NUCLEAR AGE
Meanwhile another fissionable fuel had turned up. Uranium 238, upon absorbing a thermal neutron, forms uranium 239, which breaks down quickly to neptunium 239, which in turn breaks down almost as quickly to plutonium 239.
As the plutonium-239 nucleus has an odd number of neutrons (145) and is more complex than uranium 235, it should be highly unstable. It seemed a reasonable guess that plutonium 239, like uranium 235, might undergo fission with thermal neutrons. In 1941, this was confirmed experimentally. Still uncertain whether the preparation of uranium 235 would prove practical, the physicists decided to hedge their bets by trying to make plutonium in quantity.
Special reactors were built in 1943 at Oak Ridge and at Hanford, in the State of Washington, for the purpose of manufacturing plutonium. These reactors were a great advance over the first pile in Chicago. For one thing, the new reactors were designed so that the uranium could be removed from the pile periodically. The plutonium produced could be separated from the uranium by chemical methods; and the fission products, some of them strong neutron absorbers, could also be separated out. In addition, the new reactors were water-cooled to prevent overheating. (The Chicago pile could operate only for short periods, because it was cooled merely by air.)
By 1945, enough purified uranium 235 and plutonium 239 were available for the construction of bombs. This portion of the task was undertaken at a third secret city, Los Alamos, New Mexico, under the leadership of the American physicist J. Robert Oppenheimer.
For bomb purposes it was desirable to make the nuclear chain reaction mount as rapidly as possible, This called for making the reaction go with fast neutrons, to shorten the intervals between fissions, so the moderator was omitted. The bomb was also enclosed in a massive casing to hold the uranium together long enough for a large proportion of it to fission.
Since a critical mass of fissionable material will explode spontaneously (sparked by stray neutrons from the air), the bomb fuel was divided into two or more sections. The triggering mechanism was an ordinary explosive which drove these sections together when the bomb was to be detonated. One arrangement was called the “Thin Man”—a tube with two pieces of uranium 235 at its opposite ends. Another, the “Fat Man,” had the form of a ball in which a shell composed of fissionable material was imploded toward the center, making a dense critical mass held together momentarily by the force of the implosion and by a heavy outer casing called the tamper. The tamper also served to reflect back neutrons into the fissioning mass and, therefore, to reduce the critical size.
To test such a device on a minor scale was impossible. The bomb had to be above critical size or nothing. Consequently, the first test was the explosion of a full-scale nuclear-fission bomb, usually called, incorrectly, an atom bomb or A-bomb. At 5:30 A.M. on 16 July 1945, at Alamogordo, New Mexico, a bomb was exploded with truly horrifying effect; it had the explosive force of 20,000 tons of TNT. I. I. Rabi, on being asked later what he had witnessed, is reported to have said mournfully, “I can’t tell you, but don’t expect to die a natural death.” (It is only fair to add that the gentleman so addressed by Rabi did die a natural death some years later.)
Two more fission bombs were prepared. One, a uranium bomb called “Little Boy,” 10 feet long by 2 feet wide and weighing 4½ tons, was dropped on Hiroshima on 6 August 1945; it was set off by radar echo. Days later, the second, a plutonium bomb, 11 feet by 5 feet, weighing 5 tons, and named “Fat Man,” was dropped on Nagasaki. Together, the two bombs had the explosive force of 35,000 tons of TNT. With the bombing of Hiroshima, the Nuclear Age, already nearly three years old, broke on the consciousness of the world.
For four years afterward, Americans lived under the delusion that there was a nuclear-bomb “secret” which could be kept from other nations forever if only security measures were made tight enough. Actually, the facts and theories of nuclear fission had been matters of public record since 1939, and the Soviet Union was fully engaged in research on the subject in 1940. If the Second World War had not occupied that nation’s lesser resources to a far greater extent than it occupied the greater resources of the uninvaded United States, the U.S.S.R. might have made a nuclear bomb by 1945, as we did. As it was, the Soviet Union exploded its first nuclear bomb on 22 September 1949, to the dismay and unnecessary amazement of most Americans. It had six times the power of the Hiroshima bomb and an explosive effect equal to 210,000 tons of TNT.
On 3 October 1952, Great Britain became the third nuclear power by exploding a test bomb of its own. On 13 February 1960, France joined the “nuclear club” as the fourth member, setting off a plutonium bomb in the Sahara. On 16 October 1964, the People’s Republic of China announced the explosion of a nuclear bomb and became the fifth member. In May 1974, India detonated a nuclear bomb, making use of plutonium that had been surreptitiously removed from a reactor (intended for peaceful power production) given it by Canada, and became the sixth member. Since then, a variety of powers, including Israel, South Africa, Argentina, and Iraq, have been reported to be on the edge of possessing nuclear weapons.
Such nuclear proliferation has become a source of alarm to many people. It is bad enough to live under the threat of a nuclear war initiated by one of the two superpowers who (presumably) are uncomfortably aware of the consequences and who, for forty years, have refrained. To be at the mercy of small powers, acting in anger over narrow issues, guided by petty rulers of no great mental breadth, would seem intolerable.
THE THERMONUCLEAR REACTION
Meanwhile the fission bomb had been reduced to triviality. Human beings had succeeded in setting off another energetic nuclear reaction which made much more devastating bombs possible.
In the fission of uranium, 0.1 percent of the mass of the uranium atom is converted to energy. But in the fusion of hydrogen atoms to form helium, fully 0.5 percent of their mass is converted to energy, as had first been pointed out in 1915 by the American chemist William Draper Harkins. At temperatures in the millions of degrees, the energy of protons is high enough to allow them to fuse. Thus two protons may unite a
nd, after emitting a positron and a neutrino (a process that converts one of the protons to a neutron), become a deuterium nucleus. A deuterium nucleus may then fuse with a proton to form a tritium nucleus, which can fuse with still another proton to form helium 4. Or deuterium and tritium nuclei will combine in various ways to form helium 4.
Because such nuclear reactions take place only under the stimulus of high temperatures, they are referred to as thermonuclear reactions. In the 1930s, the one place where the necessary temperatures were believed to exist was at the center of stars. In 1938, the German-born physicist Hans Albrecht Bethe (who had left Hitler’s Germany for the United States in 1935) proposed that fusion reactions were responsible for the energy that the stars radiated. It was the first completely satisfactory explanation of stellar energy since Helmholtz had raised the question nearly a century earlier.
Now the uranium-fission bomb provided the necessary temperatures on the earth. It could serve as a match hot enough to ignite a fusion chain reaction in hydrogen. For a while it looked very doubtful that the reaction could actually be made to work in the form of a bomb. For one thing, the hydrogen fuel, in the form of a mixture of deuterium and tritium, had to be condensed to a dense mass, which meant that it had to be liquefied and kept at a temperature only a few degrees above absolute zero. In other words, what would be exploded would be a massive refrigerator. Furthermore, even assuming a hydrogen bomb could be made, what purpose would it serve? The fission bomb was already devastating enough to knock out cities; a hydrogen bomb would merely pile on destruction and wipe out whole civilian populations.
Nevertheless, despite the unappetizing prospects, the United States and the Soviet Union felt compelled to go on with it. The United States Atomic Energy Commission proceeded to produce some tritium fuel, set up a 65-ton fission-fusion contraption on a coral atoll in the Pacific, and on 1 November 1952, produced the first thermonuclear explosion (a hydrogen bomb or H-bomb) on our planet. It fulfilled all the ominous predictions: the explosion yielded the equivalent of 10 million tons of TNT (10 megatons)—500 times the puny 20-kiloton energy of the Hiroshima bomb. The blast wiped out the atoll.
The Russians were not far behind; on 12 August 1953, they also produced a successful thermonuclear explosion, and it was light enough to be carried in a plane. We did not produce a portable one until early 1954. Where we developed the fusion bomb seven and one-half years after the fission bomb, the Soviets took only five years.
Meanwhile a scheme for generating a thermonuclear chain reaction in a simpler way and packing it into a portable bomb had been conceived. The key to this reaction was the element lithium. When the isotope lithium 6 absorbs a neutron, it splits into nuclei of helium and tritium, giving forth 4.8 Mev of energy in the process. Suppose, then, that a compound of lithium and hydrogen (in the form of the heavy isotope deuterium) is used as the fuel. This compound is a solid, so there is no need for refrigeration to condense the fuel. A fission trigger would provide neutrons to split the lithium. And the heat of the explosion would cause the fusion of the deuterium present in the compound and of the tritium produced by the splitting of lithium. In other words, several energy-yielding.reactions would take place: the splitting of lithium, the fusion of deuterium with deuterium, and the fusion of deuterium with tritium.
Now besides releasing tremendous energy, these reactions would also yield a great number of surplus neutrons. It occurred to the bomb builders: Why not use the neutrons to fission a mass of uranium? Even common uranium 238 could be fissioned with fast neutrons (though less readily than U-235). The heavy blast of fast neutrons from the fusion reactions might fission a considerable number of U-238 atoms. Suppose one built a bomb with a U-235 core (the igniting match), a surrounding explosive charge of lithium deuteride, and around all this a blanket of uranium 238 which would also serve as explosive.
That would make a really big bomb. The U-238 blanket could be made almost as thick as one wished, because there is no critical size at which uranium 238 will undergo a chain reaction spontaneously. The result is sometimes called a U-bomb.
The bomb was built. It was exploded at Bikini in the Marshall Islands on 1 March 1954 and shook the world. The energy yield was around 15 megatons. Even more dramatic was a rain of radioactive particles that fell on twenty-three Japanese fishermen in a fishing boat named The Lucky Dragon. The radioactivity destroyed the cargo of fish, made the fishermen ill, eventually killed one, and did not exactly improve the health of the rest of the world.
Since 1954, thermonuclear bombs have become items in the armaments of the United States, the Soviet Union, and Great Britain. In 1967, China became the fourth member of the “thermonuclear club,” having made the transition from fission in only three years. The Soviet Union has exploded hydrogen bombs in the 50- to 100-megaton range and the United States is perfectly capable of building such bombs, or even larger ones, at short notice.
In the 1970s, thermonuclear bombs were developed that minimized the blast effect and maximized radiation, particularly neutrons. Hence, less damage would be done to property and more to human beings. Such neutron bombs seem desirable to people who worry about property and hold life cheap.
When the first nuclear bombs were used in the last days of the Second World War, they were delivered by airplane. It is now possible to deliver them by intercontinental ballistic missiles (ICBMs), which are rocket-powered and are capable of being aimed with great accuracy from any place on Earth to any other place on Earth. Both the United States and the Soviet Union have great stores of such missiles, all capable of being equipped with nuclear warheads.
For that reason, an all-out thermonuclear war between the two superpowers, if engaged in with insane rage on both sides, can put an end to civilization (and, perhaps, to much of Earth’s power to support life) in as little as half an hour. If there was ever, in this world, a sobering thought, that is it.
The Nucleus in Peace
The dramatic use of nuclear power in the form of unbelievably destructive bombs has done more to present the scientist in the role of ogre than anything else that has occurred since the beginnings of science. In a way this portrayal has its justifications, for no arguments or rationalizations can change the fact that scientists did indeed construct the nuclear bomb, knowing from the beginning its destructive powers and that it would probably be put to use.
It is only fair to add that they did this under the stress of a great war against ruthless enemies and with an eye to the frightful possibility that a man as maniacal as Adolf Hitler might get such a bomb first. It must also be added that, on the whole, the scientists working on the bomb were deeply disturbed about it, and that many opposed its use, while some even left the field of nuclear physics afterward in what can only be described as remorse.
In 1945, a group of physicists, under the leadership of the Nobel laureate James Franck (now an American citizen), petitioned the secretary of war against the use of the nuclear bomb on Japanese cities and accurately foretold the dangerous nuclear stalemate that would follow its use. Far fewer pangs of conscience were felt by the political and military leaders who made the actual decision to use the bombs, and who, for some peculiar reason, are viewed as patriots by many people who view the scientists as demons.
Furthermore, we cannot and should not subordinate the fact that, in releasing the energy of the atomic nucleus, scientists put at our disposal a power that can be used constructively as well as destructively. It is important to emphasize this in a world and at a time in which the threat of nuclear destruction has put science and scientists on the shamefaced defensive, and in a country like the United States, which has a rather strong Rousseauan tradition against book learning as a corrupter of the simple integrity of human beings in a state of nature.
Even the explosion of an atomic bomb need not be purely destructive. Like the lesser chemical explosives long used in mining and in the construction of dams and highways, nuclear explosives could be vastly helpful in construction projects. All
kinds of dreams of this sort have been advanced: excavating harbors, digging canals, breaking up underground rock formations, preparing heat reservoirs for power-even the long-distance propulsion of spaceships. In the 1960s, however, the furor for such far-out hopes died down. The prospects of the danger of radioactive contamination or of unlocked-for expense, or both, served as dampers.
Yet one constructive use of nuclear power that was realized lay in the kind of chain reaction that was born under the football stadium at the University of Chicago. A controlled nuclear reactor can develop huge quantities of heat, which, of course, can be drawn off by a coolant, such as water or even molten metal, to produce electricity or heat a building (figure 10.4).
Figure 10.4. A nuclear power plant of the gas-cooled type, shown in a schematic design. The reactor’s heat here is transferred to a gas, which may be a vaporized metal circulating through it, and the heat is then used to convert water to steam.
NUCLEAR-POWERED VESSELS
Experimental nuclear reactors that produced electricity were built in Great Britain and the United States within a few years after the war. The United States now has a Heet of well over 100 nuclear-powered submarines, the first of which, the U.S.S. Nautilus (having cost 50 million dollars), was launched in January 1954. This vessel, as important for its day as Fulton’s Clermont was in its, introduced engines with a virtually unlimited source of power that permits submarines to remain underwater for indefinitely long periods, whereas ordinary submarines must surface frequently to recharge their batteries by means of diesel generators that require air for their working. Furthermore, where ordinary submarines travel at a speed of eight knots, a nuclear submarine travels at twenty knots or more.