The American scientists present on that historic December day were part
of the tremendous super-secret scientific and industrial complex that
bore the unrevealing title Manhattan District. The United States had
been at war almost a year. An uncontrolled fission reaction gave promise
of producing an explosion of untold proportions. This promise, coupled
with the possibility that enemy scientists might be nearing such a goal,
had launched a vast Allied effort.
The Manhattan Project, as it was commonly known, included a variety of
“hush-hush” facilities. Each of these installations, in New York,
Illinois, Tennessee, New Mexico, California, and Washington, had its own
experts working night and day to solve the baffling problems surrounding
development of a fission weapon.
Ordinary uranium as found in nature was not suitable for an atomic bomb
because less than one percent of the atoms in it are fissionable isotope
²³⁵U.[3] It therefore became necessary to find some means for separating
the rare ²³⁵U from the large quantity of ²³⁸U. Chemistry could not do it
since the two isotopes are identical chemically.
Several methods of achieving large-scale separation were tried. The most
successful and economical, known as “gaseous diffusion,” involves
compressing normal uranium, in the form of uranium hexafluoride gas,
against a porous barrier containing millions of holes, each smaller than
two-millionths of an inch. Since the ²³⁵U molecules are slightly lighter
than the ²³⁸U, they bounce against the barrier more frequently and have
a greater chance of penetrating. Thus, although the gas at first
contains only 0.7% ²³⁵U, the process of compression is repeated several
thousand times, and the proportion gradually increases until the
necessary concentration is reached.
For this operation an enormous plant containing a very large barrier
area, miles of piping, and countless pumps was built at Oak Ridge,
Tennessee.
At the same time that vast efforts were being made to produce a ²³⁵U
bomb, another project of equal importance was being pursued to develop a
different kind of fission bomb. Uncertainty as to whether it would be
possible to separate usable amounts of ²³⁵U led to a decision to exploit
a highly significant discovery about one of the transuranic elements.
By 1941 Glenn T. Seaborg, Edwin M. McMillan, Philip H. Abelson, and
others at the Radiation Laboratory, Berkeley, California, had identified
isotopes of two new transuranic elements developed when they bombarded
²³⁸U nuclei with neutrons. The new elements were named neptunium and
plutonium after the planets Neptune and Pluto, which lie beyond Uranus
in the solar system.[4] One isotope of plutonium, plutonium-239, which
resulted from the absorption of a neutron by a ²³⁸U nucleus and the
emission of two beta particles, was discovered to be as fissionable as
²³⁵U and hence theoretically just as feasible for a bomb. Since
plutonium is chemically different from uranium, it offered the
tremendous advantage that it could readily be concentrated by
conventional chemical techniques.
The way to manufacture usable amounts of plutonium, an element that had
never before been detected on earth, is to expose uranium to a very
intense neutron bombardment. The best-known place to find a rich supply
of neutrons was the heart of a self-sustaining chain-reacting pile of
uranium. Accordingly, very large piles, or _reactors_, were rushed to
completion near the Columbia River at Hanford, Washington, to make
plutonium.
[Illustration: _First atomic bomb explosion at Alamagordo, New
Mexico, at 5:30 a.m. on July 16, 1945._
Courtesy U. S. Army]
On July 16, 1945, a plutonium bomb, carefully assembled by another group
of scientists at “Project Y,” Los Alamos, New Mexico, was successfully
tested in the New Mexico desert. The heat from that first man-made
nuclear explosion completely vaporized a tall steel tower and melted
several acres of surrounding surface sand. The flash of light was the
brightest the earth had ever witnessed.
A ²³⁵U bomb was dropped on Hiroshima, Japan, on August 6, 1945. Three
days later a plutonium bomb was dropped on Nagasaki, Japan. Hostilities
ended on August 14, 1945.