Chapter 1

'A Smell of Nuclear Powder'

Early in January 1939, nine months before the outbreak of the Second World War, a letter from Paris alerted physicists in the Soviet Union to the startling news that German radiochemists had discovered a fundamental new nuclear reaction. Bombarding uranium with neutrons, French physicist Frédéric Joliot-Curie wrote his Leningrad colleague Abram Fedorovich Ioffe, caused that heaviest of natural elements to disintegrate into two or more fragments that repelled each other with prodigious energy. It was fitting that the first report of a discovery that would challenge the dominant political system of the world should reach the Soviet Union from France, a nation to which Czarist Russia had looked for culture and technology. Joliot-Curie's letter to the grand old man of Russian physics "got a frenzied going-over" in a seminar at Ioffe's institute in Leningrad, a protégé of one of the participants reports. "The first communications about the discovery of fission...astounded us," Soviet physicist Georgi Flerov remembered in old age. "...There was a smell of nuclear powder in the air."

Reports in the British scientific journalNaturesoon confirmed the German discovery and research on nuclear fission started up everywhere. The news fell on fertile ground in the Soviet Union. Russian interest in radioactivity extended back to the time of its discovery at the turn of the century. Vladimir I. Vernadski, a Russian mineralogist, told the Russian Academy of Sciences in 1910 that radioactivity opened up "new sources of atomic energy...exceeding by millions of times all the sources of energy that the human imagination has envisaged." Academy geologists located a rich vein of uranium ore in the Fergana Valley in Uzbekistan in 1910; a private company mined pitchblende there at Tiuia-Muiun ("Camel's Neck") until 1914. After the First World War, the Red Army seized the residues of the company's extraction of uranium and vanadium. The residues contained valuable radium, which transmutes naturally from uranium by radioactive decay. The Soviet radiochemist Vitali Grigorievich Khlopin extracted several grams of radium for medical use in 1921.

There were only about a thousand physicists in the world in 1895. Work in the new scientific discipline was centered in Western Europe in the early years of the twentieth century. A number of Russian scientists studied there. Abram Ioffe's career preparation included research in Germany with Nobel laureate Wilhelm Roentgen, the discoverer of X rays; Vernadski worked at the Curie Institute in Paris. The outstanding Viennese theoretical physicist Paul Ehrenfest taught in St. Petersburg for five years before the First World War. In 1918, in the midst of the Russian Revolution, Ioffe founded a new Institute of Physics and Technology in Petrograd. Despite difficult conditions -- the chemist N. N. Semenov describes "hunger and ruin everywhere, no instruments or equipment" as late as 1921 -- "Fiztekh" quickly became a national center for physics research. "The Institute was the most attractive place of employment for all the young scientists looking to contribute to the new physics," Soviet physicist Sergei E. Frish recalls. "...Ioffe was known for his up-to-date ideas and tolerant views. He willingly took on, as staff members, beginning physicists whom he judged talented....Dedication to science was all that mattered to him." The crew Ioffe assembled was so young and eager that older hands nicknamed Fiztekh "the kindergarten."

During its first decade, Fiztekh specialized in the study of high-voltage electrical effects, practical research to support the new Communist state's drive for national electrification -- the success of socialism, Lenin had proclaimed more than once, would come through electrical power. After 1928, having ousted his rivals and consolidated his rule, Josef Stalin promulgated the first of a brutal series of Five-Year Plans that set ragged peasants on short rations building monumental hydroelectric clams to harness Russia's wild rivers. "Stalin's realism was harsh and unillusioned," comments C. P. Snow. "He said, after the first two years of industrialization, when people were pleading with him to go slower because the country couldn't stand it:

To slacken the pace would mean to lag behind; and those who lag behind are beaten. We do not want to be beaten. No, we don't want to be. Old Russia was ceaselessly beaten for her backwardness. She was beaten by the Mongol khans, she was beaten by Turkish beys, she was beaten by the Swedish feudal lords, she was beaten by Polish-Lithuanian pans, she was beaten by Anglo-French capitalists, she was beaten by Japanese barons, she was beaten by all -- for her backwardness. For military backwardness, for cultural backwardness, for agricultural backwardness. She was beaten because to beat her was profitable and went unpunished. You remember the words of the pre-revolutionary poet: "Thou art poor and thou art plentiful, thou art mighty, and thou art helpless, Mother Russia."

We are fifty or a hundred years behind the advanced countries. We must make good the lag in ten years. Either we do it or they crush us.

Soviet scientists felt a special burden of responsibility in the midst of such desperate struggle; the heat and light that radioactive materials such as radium generate for centuries without stint mocked their positions of privilege. Vernadski, who founded the State Radium Institute in Petrograd in 1922, wrote hopefully that year that "it will not be long before man will receive atomic energy for his disposal, a source of energy which will make it possible for him to build his life as he pleases." World leaders such as England's Ernest Rutherford, who discovered the atomic nucleus, and Albert Einstein, who quantified the energy latent in matter in his formulaE = mc2,disputed such optimistic assessments. The nuclei of atoms held latent far more energy than all the falling water of the world, but the benchtop processes then known for releasing it consumed much more energy than they produced. Fiztekh had spun off provincial institutes in 1931, most notably at Kharkov and Sverdlovsk; in 1932, when the discovery of the neutron and of artificial radioactivity increased the pace of research into the secrets of the atomic nucleus, Ioffe decided to divert part of Fiztekh's effort specifically to nuclear physics. The government shared his enthusiasm. "I went to Sergei Ordzhonikidze," Ioffe wrote many years later, "who was chairman of the Supreme Council of National Economy, put the matter before him, and in literally ten minutes left his office with an order signed by him to assign the sum I had requested to the Institute."

To direct the new program, Ioffe chose Igor Vasilievich Kurchatov, an exceptional twenty-nine-year-old physicist, the son of a surveyor and a teacher, born in the pine-forested Chelyabinsk region of the southern Urals in 1903. Kurchatov was young for the job, but he was a natural leader, vigorous and self-confident. One of his contemporaries, Anatoli P. Alexandrov, remembers his characteristic tenacity:

I was always struck by his great sense of responsibility, for whatever problem he was working on, whatever its dimensions may have been. A lot of us, after all, take a careless, haphazard attitude toward many aspects of life that seem secondary to us. There wasn't a bit of that attitude in Igor vasilievich....[He] would sink his teeth into us and drink our blood until we'd fulfilled [our obligations]. At the same time, there was nothing pedantic about him. He would throw himself into things with such evident joy and conviction that finally we, too, would get caught up in his energetic style....

We'd already nicknamed him "General."...

Within a year, justifying Ioffe's confidence in him, Kurchatov had organized and headed the First All-Union (i.e., nationwide) Conference on Nuclear Physics, with international attendance. With Abram I. Alikhanov, he built a small cyclotron that became, in 1934, the first cyclotron operating outside the Berkeley, California, laboratory, of the instrument's inventor, Ernest O. Lawrence. He directed research at Fiztekh in 1934 and 1935 that resulted in twenty-four published scientific papers.

Kurchatov was "the liveliest of men," Alexandrov comments, "witty, cheerful, always ready for a joke." He had been a "lanky stripling," his student and biographer Igor N. Golovin writes, but by the 1930s, after recovering from tuberculosis, he had developed "a powerful physique, broad shoulders and ever-rosy cheeks." "Such a nice soul," an Englishwoman who knew him wrote home, "like a teddy bear, no one could ever be cross with him." He was handsome, Sergei Frish says -- "a young, clean-shaven man with a strong, resolute chin and dark hair standing straight up over his forehead." Golovin mentions lively black eyes as well, and notes that Kurchatov "worked harder than anyone else....He never gave himself airs, never let his accomplishments go to his head."

When Igor was six, his father, a senior surveyor in government service, took a cut in pay to move west over the Urals from the rural Chelyabinsk area to Ulyanovsk, on the Volga, where the three Kurchatov children could attend a proper academicgymnasium.Three years later, in 1912, Igor's older sister Antonina sickened with tuberculosis. For her health the family moved again, to the balmier climate of Simferopol on the Crimean Peninsula. The relocation proved to be a forlorn hope; Antonina died within six months.

The two surviving Kurchatov children -- Igor and his brother Boris, two years younger -- thrived in the Crimea. Both boys did well ingymnasium,played soccer, traveled into the country with their father during the summer on surveying expeditions. Igor ran a steam threshing machine harvesting wheat the summer he was fourteen. Another summer he worked as a laborer on the railroad.

A chance encounter with Orso Corbino'sAccomplishments of Modern Engineeringencouraged the younggymnasiumstudent to dream of becoming an engineer. The Italian physicist would influence Kurchatov's career again indirectly in the 1930s when Corbino sponsored Enrico Fermi's Rome group that explored the newly discovered phenomenon of artificial radioactivity. The discoveries of the Rome group would inspire and challenge Kurchatov's Fiztekh research.

The Great War impoverished the Kurchatov family. Igor added night vocational school to his heavy schedule, qualified as a machinist and worked part-time in a machine shop while taking nothing but 5's -- straight A's -- during his final two years ofgymnasium.

After the Revolution, in 1920, when he was seventeen years old, Kurchatov matriculated in physics and mathematics at Crimean State, one of about seventy students at the struggling, recently nationalized university. None of the foreign physics literature in the university library dated past 1913 and there were no textbooks, but the rector of the school was a distinguished chemist and managed to bring in scientists of national reputation for courses of lectures, among them Abram Ioffe, theoretical physicist Yakov I. Frenkel and future physics Nobel laureate Igor E. Tamm.

In the wake of war and revolution there was barely enough to eat. After midday lectures, students at Crimean State got a free meal of fish soup thickened with barley so flinty they nicknamed it "shrapnel." The distinction of an assistantship in the physics laboratory in the summer of 1921 gratified Kurcnatov in part because it won him an additional ration of 150 grams -- about five ounces -- of daily bread.

Kurchatov finished the four-year university course in three years. He chose, to prepare a thesis in theoretical physics because the university laboratory was not adequately equipped for original experimental work; he defended his dissertation in the summer of 1923. His physics professor, who was leaving for work at an institute in Baku, invited the new graduate to join him. Drawn from childhood to ships and the sea, Kurchatov chose instead to enroll in a program in nautical engineering in Petrograd. He suffered through a winter short on resources in the bitter northern cold, eking out a living as a supervisor in the physics department of a weather station, sleeping on a table in the unheated instrument building in a huge black fur coat. "This is no life I'm living," he wrote a friend that winter, uncharacteristically depressed, "but a rusted-out tin can with a hole in it." But the station director gave him real problems to solve, including measuring the alpha-radioactivity of freshly fallen snow, and the work finally won him for physics. He returned to the Crimea in 1924 to help his family -- his father had been sentenced to three years of internal exile -- and later joined his former teacher in Baku.

In the meantime, one of Kurchatov's physics classmates, his future brother-in-law Kirill Sinelnikov, had caught Ioffe's eye and accepted his invitation to work at Fiztekh. Sinelnikov told the institute director about his talented friend. Off went another invitation. Kurchatov returned to Leningrad, this time to take up his life's work. (He married Sinelnikov's sister Marina in 1927.)

Kurchatov quickly impressed Ioffe. "It was almost routine to chase him out of the laboratory at midnight," the senior physicist recalls. In the interwar years Ioffe sent twenty of his protégés abroad "to the best foreign laboratories where [they] could meet new people and familiarize [themselves] with new scientific techniques." Like a young entrepreneur too busy to bother going to college, Kurchatov never found time for foreign study. "He kept putting off taking advantage of [this opportunity]," Ioffe adds. "Everytime it was time to leave he was on an interesting experiment that he preferred to the trip."

Others left and won international reputations. Peter Kapitza explored cryogenics and strong magnetic fields at Cambridge University and became a favorite of Ernest Rutherford, the New Zealand-born Nobel laureate who directed the Cavendish Laboratory. there; Kapitza would earn a Nobel in his turn. So would theoretician Lev Landau, who worked in Germany during this period with his young Hungarian counterpart Edward Teller. The German emigré physicist Rudolf Peierls remembers a walking tour of the Caucasus with Landau after Landau had returned home when the Soviet theoretician pointed out that a nuclear reaction that produced secondary neutrons, if it could be found, would make possible the release of atomic energy -- "remarkably clear vision in 1934," comments Peierls, "just two years after the discovery of the neutron." Less conspicuously, but with more enduring influence on Soviet history, Yuli Borisovich Khariton, the youngest son of a St. Petersburg journalist and an actress in the Moscow Art Theater -- "compact, ascetically slight and very sprightly," a friend describes him -- worked at Fiztekh on chemical chain reactions with Semenov, their discoverer, before earning a doctorate in theoretical physics at the Cavendish in 1927. Alarmed by the growing mood of fascism he found in Germany on his return passage, Khariton at twenty-four organized an explosives laboratory in the new Institute of Physical Chemistry, a Fiztekh spinoff. These were only a few of Ioffe's talented protégés.

Their talents barely protected them from the Great Terror that began in the Soviet Union after the assassination of Central Committee member Sergei Mironovich Kirov in December 1934 as Stalin moved to eliminate all those m power whose authority preceded his imposition of one-man rule. "Stalin killed off the founders of the Soviet state," writes the high-level Soviet defector Victor Kravchenko. "This crime was only a small part of the larger blood-letting in which hundreds of thousands of innocent men and women perished." According to a Soviet official, the slaughter claimed not hundreds of thousands but millions: "From 1 January 1935 to 22 June 1941, 19,840,000 enemies of the people were arrested. Of these 7 million were shot in prison, and a majority of the others died in camp." Exiled Soviet geneticist Zhores Medvedev notes that "the full list of arrested scientists and technical experts certainly runs into many thousands." Kharkov, where Kirill Sinelnikov had moved to direct the high-voltage laboratory after studying at Cambridge, lost most of its leaders, though Kurchatov's brother-in-law himself was spared.

The British Royal Society had funded an expensive laboratory in its own dedicated building in the courtyard outside the Cavendish for Peter Kapitza. Perhaps suspecting that he intended to defect, the Soviet government detained him during a visit home in the summer of 1934 and barred him from returning abroad. His detention shocked the British, and for a time he was too depressed to work, but the Soviet government bought his Cambridge laboratory equipment and built a new institute for him in Moscow. (A frustrated Kapitza had to order such unavailable consumer goods as wall clocks, extension telephones and door locks from England.) Eventually he went back to work, as he wrote People's Commissar Vyacheslav Molotov, "for the glory of the USSR and for the use of all the people." Niels Bohr, the Danish physicist, after visiting him in Moscow in 1937, observed that "by his enthusiastic and powerful personality, Kapitza soon obtained the respect and confidence of Russian official circles, and from the first Stalin showed a warm personal interest for Kapitza's endeavors."

Kapitza's golden captivity was not yet terror, but he needed all his connections when Lev Landau was arrested in April 1938, convicted of being a "German spy" and sent to prison, where he languished for a year and became ill. Landau had been working at Kapitza's Institute for Physical Problems. Kapitza determined to save him, writes Medvedev:

After a short meeting with Landau in prison, Kapitza took a desperate step. He presented Molotov and Stalin with an ultimatum: if Landau was not released immediately, he, Kapitza, would resign from all his positions and leave the institute....It was clear that Kapitza meant business. After a short time Landau was cleared of all charges and released.

In old age, Edward Teller would cite his friend's arrest and imprisonment as one of three important early influences on his militant anti-Communism (the other two, Teller said, were the Great Terror itself and Arthur Koestler's novelDarkness at Noon):"Lev Landau, with whom I published a paper, was an ardent Communist. Shortly after he returned to Russia, he went to prison. After that he was no longer a Communist." Communist or not, Landau continued to work at Kapitza's institute in Moscow.

Not even Ioffe escaped the general harrowing. "Although the majority of [Soviet] scientists realized the importance of work in the field of nuclear physics," writes Alexandrov, "the leadership of the Soviet Academy of Sciences and of the Council of People's Commissars believed that this work had no practical value. Fiztekh and Ioffe himself were heavily criticized at the 1936 general assembly of the Academy of Sciences for 'loss of touch with practice.'" With the Great Terror destroying lives all around them, Soviet physicists understandably learned caution from such charges. "In those years," writes Stalin's daughter Svetlana Alliluyeva, "never a month went by in peace. Everything was in constant turmoil. People vanished like shadows in the night." Her father brooded over it all, reports the historian Robert Conquest: "Stalin personally ordered, inspired and organized the operation. He received weekly reports of...not only steel production and crop figures, but also of the numbers annihilated." Shot in the back of the head at Lubyanka prison, truckloads of bodies to the crematorium at the Donskoi Monastery, smoking ashes bulked into open pits and the pits paved over. That was the era when Osip Mandelstam suffered three years' exile and then five years in a gulag camp -- five years that killed him -- for writing a poem, "The Stalin Epigram," the most ferocious portrait of the dictator anyone ever devised:

Our lives no longer feel ground under them.

At ten paces you can't hear our words.

But whenever there's a snatch of talk

it turns to the Kremlin mountaineer,

the ten thick worms his fingers,

his words like measures of weight,

the huge laughing cockroaches on his top lip,

the glitter of his boot-rims.

Ringed with a scum of chicken-necked bosses

he toys with the tributes of half-men.

One whistles, another meows, a third snivels.

He pokes out his finger and he alone goes boom.

He forges decrees in a line like horseshoes,

one for the groin, one the forehead, temple, eye.

He rolls the executions on his tongue like berries.

He wishes he could hug them like big friends from home.

Igor Kurchatov organized the initial Soviet study of nuclear fission at Fiztekh in the early months of 1939, following Joliot-Curie's letter to Ioffe and confirmation of the discovery in scientific journals. Landau's remark to Peierls in 1934 about secondary neutrons points to one universal line of inquiry: examining whether the fission reaction, which a single neutron could initiate, would release not only hot fission fragments but additional neutrons as well. If so, then some of those secondary neutrons might go on to fission other uranium atoms, which might fission yet others in their turn. If there were enough secondary neutrons, the chain reaction might grow to be self-sustaining. Joliot-Curie's team in Paris set up an experiment to look for secondary neutrons in late February; in April the French reported 3.5 secondary neutrons per fission and predicted that uranium would probably chain-react. Enrico Fermi, now at Columbia University in flight from anti-Semitic persecution (his wife Laura was Jewish), and emigré Hungarian physicist Leo Szilard, also temporarily working at Columbia, soon independently confirmed fission's production of secondary neutrons. At a Fiztekh seminar in April, two young members of Kurchatov's Fiztekh team, Georgi Flerov and Lev Rusinov, reported similar results -- between two and four secondary neutrons per fission. (In 1940, Flerov and Konstantin A. Petrzhak would make a world-class discovery, the spontaneous fission of uranium, a consequence of uranium's natural instability and a phenomenon that would prove crucial to regulating controlled chain reactions in nuclear reactors. Before the young Russians succeeded, the American radiochemist Willard F. Libby, later a Nobel laureate, had tried two different ways unsuccessfully to demonstrate spontaneous fission.)

Down the street at the Institute of Physical Chemistry, Yuli Khariton and an outstanding younger colleague, theoretician Yakov B. Zeldovich, began exploring fission theory. "Yuli Borisovich notes a curious detail," Zeldovich recalled: "we considered the work on the theory of uranium fission to be apart from the official plan of the Institute and we worked on it in the evenings, sometimes until very late." Zeldovich was a brilliant original -- "not a university graduate," comments Andrei Sakharov; "...in a sense, self-educated" -- who had earned a master's degree and a doctorate "without his ever bothering about a bachelor's degree." "We immediately made calculations of nuclear chain-reactions," Khariton remembers, "and we soon understood that on paper, at least, a chain-reaction was possible, a reaction which could release unlimited amounts of energy without burning coal or oil. Then we took it very seriously. We also understood that a bomb was possible." Khariton and Zeldovich reported their first calculations in a seminar at Fiztekh in the summer of 1939, describing the conditions necessary for a nuclear explosion and estimating its tremendous destructive capacity -- one atomic bomb, they told their colleagues, could destroy Moscow.

Theoretical physicist J. Robert Oppenheimer at Berkeley, Fermi, Szilard, Peierls in England, all quickly came to similar conclusions. "These possibilities were immediately obvious to any good physicist," comments Robert Serber. But it was also soon obvious from work by Niels Bohr that a formidable obstacle stood in the way of making bombs: only one isotope of uranium, U235, would sustain a chain reaction, and U235 constituted only 0.7 percent of natural uranium; the other 99.3 percent, chemically identical, was U238, which captured secondary neutrons and effectively poisoned the reaction. There were then two difficult technical questions that needed to be resolved by any nation that proposed to explore building an atomic bomb: whether it might be possible to achieve a controlled chain reaction -- to build a nuclear reactor -- using natural uranium in combination with some suitable moderator, or whether the U235 content of the uranium would have to be laboriously enriched; and how to separate U235 from U238 on an industrial scale for bomb fuel when the only exploitable distinction between the two isotopes was a slight difference in mass. Enrichment and separation were essentially identical processes ("separated" bomb-grade uranium is natural uranium enriched to above 80 percent U235) and would use the same massive, expensive machinery that no one yet knew how to build; while a reactor fueled with natural uranium, if such would work, might be a straightforward enterprise.

Khariton and Zeldovich approached these questions from first principles, as it were, carefully calculating what was not possible as well as what might be. In the first of three pioneering papers they published in the RussianJournal of Experimental and Theoretical Physicsin 1939 and 1940 (papers that went unnoticed outside the Soviet Union) they demonstrated that a fast-neutron chain reaction was not possible in natural uranium. Isotope separation would therefore be necessary to build a uranium bomb.

A second, longer paper, delivered a few weeks later on October 22, 1939, developed important basic principles of reactor physics. Khariton and Zeldovich correctly identified the crucial bottleneck that experimenters would have to bypass to build a natural-uranium reactor that worked. Visualize a stray neutron in a mass of natural uranium finding a U235 nucleus, entering it and causing it to fission. The two resulting fission fragments fly apart; a fraction of a second later they eject two or three secondary neutrons. If these fast secondary neutrons encounter other U235 nuclei they will continue and enlarge the chain of fissions. But there is much more U238 than U235 in the mass of natural uranium, making an encounter with a U238 nucleus more likely, and U238 tends to capture fast neutrons. It is particularly sensitive to neutrons moving at a critical energy, twenty-five electron volts (eV), a sensitivity which physicists call a "resonance." On the other hand, U238 is opaque to slow neutrons. To make a reactor, then, Khariton and Zeldovich realized, it would be necessary to slow the fast secondary neutrons from U235 fission quickly below U238's twenty-five eV resonance. The way to do that, they proposed, was to make the neutrons give up some of their energy by bouncing them off the nuclei of light atoms such as hydrogen. "In order to accomplish [a chain] reaction [in natural uranium]," they wrote, "strong slowing of the neutrons is necessary, which may be practically accomplished by the addition of a significant amount of hydrogen."

The simplest way to mix uranium with hydrogen would be to make a slurry -- a homogeneous mixture -- of natural uranium and ordinary water. But Khariton and Zeldovich demonstrated in this second paper that such a mixture would not sustain a chain reaction, because hydrogen and oxygen also capture slow neutrons, and in a reactor fueled with natural uranium such capture would subtract too many neutrons from the mix. Important consequences followed from this conclusion. One was that instead of hydrogen in ordinary water it would apparently be necessary to use heavy hydrogen -- deuterium, H2or D, an isotope of hydrogen with a smaller appetite for neutrons than ordinary hydrogen -- perhaps in the form of rare and expensive heavy water. (In a review article published in 1940, Khariton and Zeldovich proposed carbon and helium as other possible moderators, both materials that later proved to work.) Alternatively, wrote the two Soviet physicists, "another possibility lies in the enrichment of uranium with the isotope 235." They calculated that natural uranium enriched from 0.7 percent U235 to 1.3 percent U235 would work in a homogeneous solution with ordinary water.

In a third paper submitted in March 1940, Khariton and Zeldovich identified two natural processes that would make it easy and "completely safe" to initiate and control a chain reaction in a nuclear reactor. The fissioning process would heat the mass of uranium and cause it to expand, which in turn would increase the distance the neutrons would have to travel to cause additional fissioning and would therefore slow down the chain reaction, allowing the mass of uranium to cool and the chain reaction to accelerate. This natural oscillation could be controlled by increasing or decreasing the volume of uranium. Another natural process -- delayed neutrons released in fission which would "significantly increase" the oscillation period -- subsequently proved more significant for reactor control. (Apparently critics within the Soviet scientific community had made safety a point of attack; in this third paper Khariton and Zeldovich vigorously disputed what they called "hasty conclusions...on the extreme danger of experiments with large masses of uranium and the catastrophic consequences of such experiments." Because of the natural processes they had identified, they scoffed, such conclusions "do not correspond to reality.")

Khariton and Zeldovich summarized these early and remarkable insights in the introduction to their third paper:

It would appear (the lack of experimental data precludes any categorical assertions) that by applying some technique, creating a large mass of metallic uranium either by mixing uranium with substances possessing a small capture cross-section (e.g., with heavy water) or by enriching the uranium with the U235isotope...it will be possible to establish conditions for the chain decay of uranium by branching chains in which an arbitrarily weak radiation by neutrons will lead to powerful development of a nuclear reaction and macroscopic effects. Such a process would be of much interest since the molar heat of the nuclear fission reaction of uranium exceeds by 5 · 107[i.e., 5,000,000] times the heating capacity of coal. The abundance and cost of uranium would certainly allow the realization of some applications of uranium.

Therefore, despite the difficulties and unreliability of the directions indicated, we may expect in the near future attempts to realize the process.

At the annual All-Union Conference on Nuclear Physics, held in 1939 in November at Kharkov in the Ukraine, Khariton and Zeldovich reported their conclusion that carbon (graphite) and heavy water were possible neutron moderators. They also reported that a controlled chain reaction even with heavy water would be possible in a homogeneous reactor only with uranium enriched in U235. Since uranium enric

Excerpted from Dark Sun: The Making of the Hydrogen Bomb by Richard Rhodes
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