Utopia or Armageddon

There remain the connections among the processes that the Berlin group had tied up in the problematic isomers of U²³⁹ . Recall that the long chains in figure 9.5 arise from fast and thermal neutrons respectively, and the short chain from resonance capture. Meitner and Hahn kept the third heavy uranium, of half-life 23 minutes, as a true U²³⁹ and tacitly referred both the long chains to the fission of U²³⁸ . They clung to this interpretation despite its odd consequence, that U²³⁸ would be fissionable by both fast and thermal neutrons but not by ones with moderate speeds, because the two chains seemed to be present in about equal strengths. They knew that a lighter isotope of uranium existed, but since it makes up less than 1 percent of natural uranium, it did not appear to them to be the ancestor of either long chain. Bohr's highly skeptical associate, George Placzek, adduced the ambiguity of the role ascribed to U²³⁸ as a strong argument against the possibility of fission. Thus inspired, Bohr found a way to father thermalneutron fission on U²³⁵ . It was only necessary to detach the fissions from the old decay chains: experiment could not distinguish which uranium isotope gave rise to which fragment; the

genetic connections in the first steps of the long chains were illusory.

Bohr argued from the excitation energies of the compound nuclei U²³⁹ and U²³⁶ . In the former case, the impacting neutrons must bring in the explosive energy, because, as an unpaired nucleon in the odd-numbered isotope 239, it will not excite the compound nucleus very much by its binding energy; in the latter case, the pairing of the neutron in the even isotope 236 contributes enough to the general excitation to create a chance of fission. The slower the neutron, the longer it takes to pass a nucleus and the greater the chance of its absorption.^[73] Hence the final interpretation of the "transuranic" chains: one comes from fast-neutron fission of U²³⁸ , the other from thermal-neutron fission of U²³⁵ . Placzek did not think that Bohr's reasons were very compelling.^[74] Others thought them strong enough to undertake the very difficult task of separating enough U²³⁵ from natural uranium to investigate its fissile properties directly.

Among the bold were the enterprising pair Seaborg and Kennedy. In the summer of 1939 they began construction of a tube, eventually over twenty feet in length, which they fixed to the outside of the chemistry building. Their scheme adapted a recent serendipitous invention made by Klaus Clusius and Gerhard Dickel, chemists at the University of Munich, who subjected a mixed gas to thermal gradient between a hot wire running down the tube's axis and the cooler tube walls. The resultant motion of the gas is hard to calculate but easy to describe: lighter molecules move radially inward under the thermal stress and axially upward by convection, heavier ones radially outward and axially downward. In the circulation, light molecules concentrate at the top and heavy ones at the bottom of the tube. Clusius and Dickel reported an almost complete separation of the major isotopes of chlorine.^[75] Kennedy and Seaborg planned to work with uranium hexafluoride gas, or "hex," as it came to be known from its

unsavory character, and they set up a fluorine generator to cast the spell. A little hex eventuated; a graduate student, Arthur Wahl, joined the project; Clusius's column was installed and Clusius written, in the best and most naive tradition of open science, for advice about uranium separation. Then, on January 12, 1940, the fluorine generator exploded while the three would-be hex splitters were working on it. The next day Kennedy was ill. Seaborg consulted a chemistry book: "Uranium is an extremely powerful, slow-acting poison." Kennedy stayed sick.^[76] Although his problem turned out to be mononucleosis, the incident destroyed the group's enthusiasm for hex. They put their columns—eventually they had three in operation—to separating the uncursed isotopes of hydrogen, carbon, and chlorine.^[77]

Another reason that Seaborg's group did not pursue uranium splitting was that others were doing it with greater success. By early 1940, the mass spectrograph run by Alfred Nier at the University of Michigan had collected enough U²³⁵ for the Columbia group to make possible a positive test of Bohr's conjecture about its fission by thermal neutrons. By the summer of 1940, when Kennedy and Seaborg made a tour of laboratories including Nier's, they learned about several attempts to acquire U²³⁵ in bulk: Urey and Tuve by diffusion methods, Beams by centrifugation, all starting with hex. The Columbia group had the help of Aristid von Grosse, a former collaborator of Hahn's and an expert in hex manfacture.^[78] That did not exhaust the competition. There were also Frisch, then relocated in England, where he and Otto Blüh, a refugee from Prague, set up a Clusius tube late in 1939; Clusius himself, who had declined to supply the additional information that Seaborg had requested; and Wilhelm Krasny-Ergen, who had established a Clusius distillery in Stockholm, from which, "had his activities not been suspended by the political situation," he expected a sevenfold enrichment in eighty days.^[79] Although

Tuve volunteered to send enough hex for preliminary experiments, Kennedy and Seaborg wisely withdrew from uranium separation. Perhaps Lawrence's belief that centrifugation held the greatest promise for large-scale separation of heavy isotopes influenced their decision.^[80]

The confirmation that U²³⁵ has a large cross-section for fission by slow neutrons gave physicists greater confidence and worry that an explosive chain reaction could be achieved. "Physicists are anxious that there be no public alarm over the possibility of the world being blown to bits by their experiments," Science Service reported in melodramatic ignorance just after the confirmation of fission at the end of January 1939. The world was not alarmed, despite a revelation in the New York Times that a little U²³⁵ could wipe out New York City and leave a hole halfway to Philadelphia.^[81] During 1939 physicists calculated what might be possible, but in ignorance of the relevant cross-sections and reactions they could only conjecture. An effort to keep pertinent data secret assisted the progress of ignorance and speculation. Although Szilard's novel notion that physicists should censor themselves failed to persuade Joliot and so failed internationally, enough was withheld in the United States that physicists as well placed as Tuve and Frisch were "hard pressed to get some data on uranium fission."^[82] The most sanguine discussion of the future of fission came from a colleague of Hahn's at the Kaiser-Wilhelm-Institut für Chemie, Siegfried Flügge, who offered a path to a "uranium machine."

Flügge started from the obvious: for a chain to succeed, enough slow neutrons must be obtained and caused to provoke fissions before they are lost or captured ineffectually. To slow the fast

fission neutrons, it is necessary only to pass them through water, which, to be sure, also captures them; but, according to Flügge's calculations, based on the probability that a slow neutron will cause a fission as measured at Columbia and on his own estimate of the liability of a fast neutron to loss in water while slowing down, a mixture of fifteen kilograms of uranium oxide per liter of water will sustain a chain reaction. That assumed the conservative estimate that an average fission sets free two fast neutrons. It also assumed that the reaction once started could be controlled. Flügge planned a power plant, not a bomb. A method of control had been published by two members of Joliot's team. It rests on the principle that the faster the neutron, the lower its probability of provoking a fission in uranium, and on the fact that the appetite of cadmium nuclei for slow neutrons is almost independent of the temperature. Therefore sprinkle a little cadmium dust in the water along with the uranium oxide. The chain begins, the mixture heats up, the neutrons move faster, the number of fissions goes down; equilibrium will be reached at a temperature fixed by the amount of neutron-removing cadmium present. According to Flügge, with 0.2 gram of cadmium per liter as seasoning, his uranium stew would boil along at a safe and steady 350°. The reactor vessel would require a diameter of a little over a meter. From it heat could be removed and used to make steam to drive a turbine; a cubic meter of uranium thus exploited could generate electricity for eleven years at a rate equivalent to Germany's consumption on the eve of the Depression.^[83]

By the time Flügge's design was published, Fermi and his colleagues had decided that water absorbs too many neutrons to serve as decelerator, or moderator, in a uranium machine. Hopes of capitalizing quickly on fission faded; by August 1939, when Tuve managed to learn what had been learned, he concluded that "all indications are that no chain can occur but it is pretty close." At the same time, Bohr was calculating the amount of hydrogen moderator needed to slow the neutrons. He arrived at a ratio of ten atoms of hydrogen to one of uranium, which he thought would prohibit a fast chain or strong explosion. Recalculation reduced

the ratio to one to one, which allowed a possibility. The Paris group was more negative. Their experiments with a homogeneous mixture of three atoms of hydrogen to one of uranium gave evidence of fissions caused by secondary and tertiary neutrons, but not of a divergent multiplication of fissions; and by the end of October 1939, in a paper they withheld from publication, they concluded that "it is almost certainly impossible" to promote a divergent chain reaction in a homogeneous blend of naturally occurring uranium, oxygen, and hydrogen.^[84] Of course, a moderator other than ordinary water, or a heterogeneous mixture of uranium and water, might perform better. The published consensus of physicists, as expressed in the several reviews of the year's fission research composed toward the end of 1939, was that exploitation of nuclear energy would not occur in the near future and might not be possible at all.^[85]

The announcement on May 1, 1940, of the details of Columbia's fissioning of Nier's latest sample of U²³⁵ transformed the discussion. William L. Laurence, a science writer for the New York Times , informed its readers on May 5 that five or ten pounds of U²³⁵ could drive an ocean liner or submarine indefinitely. So much light uranium might not be hard to procure. Nier had been able to enlarge his sample two hundred times in a matter of weeks. "It is not impossible that a few months or a year hence may see the realization of this quest." Nothing would be simpler than exploiting the new fuel. "All that is needed to put it to work running motors and steamships is to place it in a tank of water. . . . The water would be turned into steam. . . . New water supplied would keep the process going indefinitely." Furthermore, the business would be automatic and self-regulating, since the

heating of the water speeds up the neutrons and slows or stops the fission process until more water, "the colder the better," is added. Thus utopia. But evil men were trying to suborn the natural good behavior of U²³⁵ . "Every German scientist in this field, physicists, chemists, and engineers, it was learned, have been ordered to drop all other researches and devote themselves to this work alone." Fortunately they lacked the essential machine for further investigation, the cyclotron. A race had started, a race with stakes that were incalculable, or almost so. According to Laurence, a pound of U²³⁵ improperly treated would have the same explosive power as 15,000 tons of TNT. An effective separation plant, therefore, was "a secret to be given only to the United States government."^[86]

The physicists were not pleased with Laurence's mixture of fact and fiction. Nier declared that his handiwork had little present commercial or military value, the amount of U²³⁵ so far isolated being "hardly enough to spring a mousetrap." S.K. Allison rated producing utilizable atomic energy as "just as feasible as getting gold out of the ocean." George Pegram, who, as chairman of Columbia's physics department, was constantly pressed by the press, urged his colleagues to "stress what seems to be the fact, namely, that energy from uranium, even if it became available, would apparently not be cheap energy by any means and would not be very explosive energy." Pegram sold this point of view to the informed and responsible chief of the Times 's science section, Waldemar Kaempfert, who squared accounts as follows. To make a pound of U²³⁵ would cost more than the expenses of the federal government for an entire year; to make a gram by Nier's improved technique would take over a century. "The prospect of using U²³⁵ in the present war is zero."^[87]

While Kaempfert calculated, Krasny-Ergen's plan was published in Nature . It set Laurence off again. According to him, 10,000 of Krasny-Ergen's units could make a pound of U²³⁵ in forty days; at $100 per unit, a uranium factory would cost only $10 million. "It may be expected, therefore, that Germany will take measures at

once to install such a plant." Kaempfert again flew to the rescue: Krasny-Ergen's method of thermal diffusion would require 17 million kWh to separate a gram of U²³⁵ ; to make a kilogram, 34 million tons of coal would have to be burnt, at a cost of $68 million. "The more we think this over the more we are convinced that we would not invest ten cents in a uranium public utility company. . . . We doubt if the Germans have the time or the stupidity to bother much about isolating uranium-235 in large quantities."^[88]

Meanwhile Lawrence was examining a plan for a German power plant driven by light uranium. He had this information from Clifford Williams of Shell Development, who had it from Peter Debye, who had recently left the directorship of the Kaiser-Wilhelm-Institut für Physik in Berlin. According to Debye, all his former staff were engaged in developing U²³⁵ as a power source (fig. 9.8); Germany was "frantically mining uranium in Czechoslovakia;" and the native metal was to be separated by diffusion. It was perhaps indirectly from this disclosure that the author of the article on Lawrence in Scientific American obtained the information that "the Nazis are trying to lay hands on all the uranium they can find." The least secret bit of science in the United States in the summer of 1940 was that (as the San Francisco Chronicle put it) atomic power "will transform the face of the earth the moment the production of the magic element U-235 can be cheapened."^[89]

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Fig. 9.8
Design for a German U²³⁵  plant. Williams to Lawrence, 23 May 1940 (18/26).