EARLY URANIUM RESEARCH
(1939-1941)
Events: Early Government
Support, 1939-1942
President Franklin D. Roosevelt responded
to the call for government support of uranium research quickly but
cautiously. He appointed Lyman J. Briggs, director of the National Bureau
of Standards, head of the Advisory Committee on
Uranium, which met for the first
time on October 21, 1939. The committee, including both civilian and
military representation, was to coordinate its activities with Alexander Sachs
and look into the current state of research on uranium to recommend an
appropriate role for the federal government. In early 1940, only months
after the outbreak of war in Europe, the Uranium
Committee recommended that the government fund limited research on isotope
separation as well as Enrico Fermi's and Leo
Szilard's work on fission chain reactions at
Columbia University (right).
Scientists had concluded that enriched samples of
uranium-235 were necessary for further research and that the isotope
might serve as a fuel source for an explosive device. Finding the most
effective method of isotope separation thus was a high priority. Since
uranium-235 and uranium-238 were chemically identical, they could not be
separated by chemical means. And with their masses differing by less than
one percent, separation by physical means would be extremely difficult and
expensive. Nonetheless, scientists pressed forward on several complicated
techniques of physical separation, all based on the small difference in atomic
weight between the uranium isotopes.
Many scientists initially thought
the best hope for isotope separation was the high-speed centrifuge, a device based on the same principle as the
cream separator. Centrifugal force in a cylinder spinning rapidly on its
vertical axis would separate a gaseous mixture of two isotopes since the
lighter isotope would be less affected by the action and could be drawn off at
the center and top of the cylinder. A cascade system composed of hundreds,
perhaps thousands, of centrifuges could produce a rich mixture. Centrifuge
research, being pursued primarily by Jesse W. Beams at the University of Virginia and Harold Urey at Columbia University, received much of the early isotope
separation funding.
Another possible method of
separating the uranium isotopes was gaseous
diffusion. Based on the well-known principle that molecules of a lighter
isotope would pass through a porous barrier more readily than molecules of a
heavier one, this approach proposed to produce by myriad repetitions a gas
increasingly rich in uranium-235 as the heavier uranium-238 was separated out
in a system of cascades. Theoretically, this process could achieve high
concentrations of uranium-235 but would be extremely costly. British
researchers led the way on gaseous diffusion, with John R. Dunning, Urey, and
their colleagues at Columbia University joining the effort in late 1940.
Of several other separation methods
that scientists considered in the spring and summer of 1940, liquid thermal diffusion was the most
significant. This process was being investigated by the Carnegie Institution’s Philip
Abelson, working at the National Bureau of Standards where the facilities were
better. Into the space between two concentric vertical pipes, Abelson placed
pressurized liquid uranium hexafluoride. With the outer wall cooled by a
circulating water jacket and the inner heated by high-pressure steam, the
lighter isotope tended to concentrate near the hot wall and the heavier near
the cold. Convection would in time carry the lighter isotope to the top of the
column.
In June 1940, the
President transferred the Uranium Committee to the
newly-created National Defense Research Committee (NDRC). Roosevelt
appointed Vannevar Bush (right), president of the Carnegie Foundaton, to
head the NDRC. Bush reorganized
the Uranium Committee into a scientific body and eliminated military
membership. Not dependent on the military for funds, as the Uranium
Committee had been, the NDRC would have more influence and more direct access to
money for nuclear research. In the interest of security, Bush barred
foreign-born scientists from committee membership and blocked the further
publication of articles on uranium research. Retaining programmatic
responsibilities for uranium research in the new organizational setup, the
Uranium Committee recommended that all four isotope
separation methods and the chain
reaction work continue to receive funding for the remainder of
1940. Bush approved the plan and allocated the funds. All possible
paths to the bomb would continue to be pursued until the best route was
found.
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