It is mentioned in the [espionage] material that Halban and Kowarski intend to continue their experiments with larger amounts of heavy water in America, where, it is said, production of this substance is organized on a very large scale… Therefore it is extremely important to find out if Halban and/or Kowarski went from Britain to America (in 1941–1942) and if they carried out [their] experiments…
Kurchatov vigorously defended Khariton and Zeldovich's heavy-water calculations, stressing that the theoreticians could not have produced a more accurate cross-section estimate because they lacked the necessary experimental equipment. He understood that he and his colleagues were being watched. The only mistakes the Soviet leadership tolerated were its own; “wrecking,” real and metaphorical, was a crime that crowded the camps of the gulag.
Then Kurchatov played a brilliant hunch. His hunch reveals that as of March 1943, word had not yet reached the Soviet Union of the construction and successful operation of the world's first man-made nuclear reactor in the United States on December 2, 1942 — Enrico Fermi's uranium-graphite reactor, CP-1, stacked by hand in a doubles squash court under the west stands of the University of Chicago's Stagg Field. Kurchatov wrote: “All experiments with systems of uranium and moderator that have so far been conducted and published used homogeneous mixtures of these components.” But a heterogeneous system might be better, he guessed, one where “the uranium is concentrated within the mass of [moderator] in spaced blocks of appropriate dimensions.” That conclusion had come independently to Fermi and to Leo Szilard at Columbia in 1940; CP-1 was just such a three-dimensional lattice. “Kurchatov was an exceptional leader,” Khariton and Smirnov comment, “who organized a strategically correct program from the very beginning.” He had an “unerring ability to find correct ways of attaining goals… despite the scarce and incomplete initial scientific data.” As soon as Kurchatov realized that a heterogeneous arrangement might be superior to the homogeneous systems he had been sponsoring — might make possible a reactor assembled from natural uranium without enrichment — he asked his team to study the two different arrangements theoretically and experimentally and asked Pervukhin to set Soviet intelligence to find out which kind of system the British and the Americans were studying. “In the end,” Khariton and Smirnov conclude, referring to members of the Kurchatov team, “it was 1.1. Gurevich and Isaak Pomeranchuk who successfully solved Kurchatov's problem, showing the decisive advantage of a heterogeneous reactor.”
Part III of Kurchatov's report, “The Physics of the Fission Process,” primarily discussed espionage information that confirmed what Soviet scientists had already worked out on their own, but it included more questions which Kurchatov hoped further espionage might resolve. Kurchatov made a point of emphasizing that Otto Frisch in England had “confirmed the phenomenon of the spontaneous fission of uranium, discovered by Soviet physicists G. N. Flerov and K. A. Petrzhak,” that the fact of spontaneous fission made it necessary to keep a critical mass disassembled until the moment of explosion (so that a stray secondary neutron would not cause it to chain- react prematurely) and that Flerov's calculation of the necessary speed of assembly closely matched British estimates.
Finally, Kurchatov assessed the issue that had bothered Beria, and probably Stalin as well, since Vadim's first report had come in: was this harvest of espionage information or disinformation?
Naturally, the question is raised whether the materials received reflect the real status of research and development in Britain rather than a legend aimed at misdirecting our science. This question is of special importance for us because in many important areas we are not in a position to test the data (because we lack the technical base necessary to do so).
Based on a close examination of the material, I formed the conclusion that it reflects the real state of things.
Certain conclusions, even some that refer to quite important parts of the work, seem dubious to me, some of them not well founded, but for this the British scientists are responsible rather than the reliability of the information.
The most crucially important information about atomic-bomb development that the Soviet Union would acquire through espionage — information that would accelerate the Soviet program by a full two years — took up only a brief paragraph in Kurchatov's March 7 report. There he noted that he would discuss it in more detail in a separate letter. He sent Pervukhin that separate seven-page letter two weeks later, on March 22,1943-
“Fragmentary remarks” in the espionage materials he reviewed, Kurchatov wrote in his March 22 letter, referred to the possibility of using plentiful U238 as well as rare U235 to make a bomb. The documents contained “very important remarks on the use for bomb material of an element with mass 239, which should be produced in the ‘uranium pile’[7] as the result of the absorption of neutrons by uranium 238.”
The suggestion had sent Kurchatov to the library to look through the last papers that American scientists had published on transuranium elements in the
Kurchatov proceeded to review the workings of a “uranium pile,” pointing out that “it was always assumed that only the light isotope of uranium — U235, which constitutes only 1/140th part of regular uranium — would be useful in a ‘pile.’ The rest of the uranium — U238, constituting 139/l40ths — would be useless, since it does not emit large amounts of energy or produce secondary neutrons when hit by a slow neutron… This conclusion may be totally wrong.”
Kurchatov was alluding to the transmutation of U238 under neutron bombardment that Edwin McMillan and Philip Abelson at Berkeley had successfully explored in 1940. McMillan and Abelson had found, in Kurchatov's words, that “the nucleus of U238, hit by a neutron, passes through certain changes and transforms itself into uranium-239. This element is unstable and in twenty minutes (on average) transforms spontaneously into element 93 (which doesn't exist on earth) — the element called eka-rhenium.” McMillan and Abelson had a better name for the first artificial new element beyond uranium (and hence “transuranic”), but they had not published it in their
Uranium was the heaviest element to occur naturally on earth because its nucleus, densely packed with positively charged protons that repelled each other electrically, was only marginally stable. It should follow that man-made transuranic elements like element 93, with even more protons packed into their nuclei, would be even less stable. The German physicist Carl Fredrich von Weizsacker had independently worked out these consequences of U238 bombardment in the summer of 1940, had assumed that 93 might fission and chain-react and had reported the idea to his government, which failed to take up the suggestion. In fact, 93 was more like U238 than U235, and in any case its 2.3-day half-life made it unsuitable for use in a weapon. But since 93 was radioactive, spontaneously emitting beta electrons and gamma rays, it followed that it would quickly transmute itself further. And on theoretical grounds, element 94 ought to fission and chain-react even more vigorously than U235. McMillan and Anderson had mentioned this “daughter product 94239” in their paper on 93 and had measured the outside limits of several of its physical characteristics. (Glenn Seaborg and his colleagues had then taken over the research and discovered 94 — plutonium — but by then the work was classified.)
Now, in March 1943, Kurchatov realized the same possibilities. “It appears,” he wrote, “that although eka- rhenium [93] is somewhat more stable than uranium-239, it also possesses a short half-life… and by itself transforms into element 94 — this element is called eka-osmium.[8]… According to all current theoretical ideas, the collision of a neutron with a nucleus of eka-osmium [94] will be accompanied by the release of a large amount of energy and the emittance of secondary neutrons, so in this respect it should be analogous to U235”
In Part Two of this March 22,1943, letter, Kurchatov quickly explained the enormous import of his
