The First Pile
On December 2, 1942, man first initiated a
self-sustaining nuclear chain reaction, and controlled it.
Beneath the West Stands of Stagg Fieldı,
Chicago, late in the afternoon of that day, a small group of scientists
witnessed the advent of a new era in science. History was made in what had
been a squash-rackets court.
Precisely at 3:25 p.m.,² Chicago time,
scientist George Weil withdrew the cadmium-plated control rod and by his
action man unleashed and controlled the energy of the atom.
As those who witnessed the experiment
became aware of what had happened, smiles spread over their faces and a
quiet ripple of applause could be hear. It was a tribute to Enrico Fermi,
Nobel Prize winner, to whom, more than to any other person, the success of
the experiment was due.
Fermi, born in Rome, Italy, on September
29, 1901, had been working with uranium for many years. In 1934 he
bombarded uranium with neutrons and produced what appeared to be element
93 (uranium is element 92) and element 94. However, after closer
examination it seemed as if nature had gone wild; several other elements
were present, but none could be fitted into the periodic table near
uranium where Fermi knew they should have fitted if they had been the
transuranic elements 92 and 94. It was not until five years later that
anyone, Fermi included, realized he had actually caused fission of the
uranium and that these unexplained elements belonged back in the middle
part of the periodic table.
Fermi was awarded the Nobel Prize in 1938
for his work on transuranic elements. He and his family went to Sweden to
receive the prize. The Italian Fascist press severely criticized him for
not wearing a Fascist uniform and failing to give the Fascist salute when
he received the award. The Fermis never returned to Italy.
From Sweden, having taken most of his
personal possessions with him, Fermi proceeded to London and thence to
America where he has remained ever since. ³
The modern Italian explorer of the unknown
was in Chicago that cold December day in 1942. An outsider looking into
the squash court where Fermi was working would have been greeted by a
strange sight. In the center of the 30- by 60-foot room, shrouded on all
but one side by a gray balloon cloth envelope, was a pile of black bricks
and wooden timbers, square at the bottom and a flattened sphere on top. Up
to half of its height, its sides were straight. The top half was domed,
like a beehive. During the construction of this crude appearing but
complex pile (the name which has since been applied to all such devices)
the standing joke among the scientists working on it was: "If people
could see what we're doing with a million-and-a-half of their dollars,
they'd think we are crazy. If they knew why we are doing it, they'd know
In relation to the fabulous atomic bomb
program, of which the Chicago Pile experiment was a key part, the
successful result reported on December 2nd formed one more piece for the
jigsaw puzzle which was atomic energy. Confirmation of the chain reactor
studies was an inspiration to the leaders of the bomb project, and
reassuring at the same time, because the Army's Manhattan Engineer
District had moved ahead on many fronts. Contract negotiations were under
way to build production-scale chain reactors, land had been acquired at
Oak Ridge, Tennessee, and millions of dollars had been obligated.
Three years before the December 2nd
experiment, it had been discovered that when an atom of uranium was
bombarded by neutrons, the uranium atom sometimes was split, or fissioned.
Later, it had been found that when an atom of uranium fissioned,
additional neurons were emitted and became available for further reaction
with other uranium atoms. These facts implied the possibility of a chain
reaction, similar in certain respects to the reaction which is the source
of the sun's energy. The facts further indicated that if a sufficient
quantity of uranium could be brought together under the proper conditions,
a self-sustaining chain reaction would result. This quantity of uranium
necessary for a chain reaction under given conditions is known as the
critical mass, or more commonly, the "critical size" of the
For three years the problem of a
self-sustaining chain reaction had been assiduously studied. Nearly a year
after Pearl Harbor, a pile of critical size was finally constructed. It
worked. A self-sustaining nuclear chain reaction was a reality.
Construction of the Pile
Construction of the main pile at Chicago
started in November. The project gained momentum, with machining of the
graphite blocks, pressing of the uranium oxide pellets, and the design of
instruments. Fermi's two "construction" crews, one under Zinn
and the other under Anderson, worked almost around the clock. V.C. Wilson
headed up the instrument work.
Original estimates as to the critical size
of the pile were pessimistic. As a further precaution, it was decided to
enclose the pile in a balloon cloth bag which could be evacuated to remove
the neutron-capturing air.
This balloon cloth bag was constructed by
Goodyear Tire and Rubber Company. Specialists in designing gasbags for
lighter-than-air craft, the company's engineers were a bit puzzled about
the aerodynamics of a square balloon. Security regulations forbade
informing Goodyear of the purpose of the envelope and so the Army's new
square balloon was the butt of much joking.
The bag was hung with one side left open;
in the center of the floor a circular layer of graphic bricks was placed.
This and each succeeding layer of the pile was braced by a wooden frame.
Alternate layers contained the uranium. By this layer-on-layer
construction a roughly spherical pile of uranium and graphite was formed.
Facilities for the machining of graphite
bricks were installed in the West Stands. Week after week this shop turned
out graphite bricks. This work was done under the direction of Zinn's
group, by skilled mechanics led by millwright August Knuth. In October,
Anderson and his associates joined Zinn's men.
Describing this phase of the work, Albert
Wattenberg, one of Zinn's group, said" "We found out how coal
miners feel. After eight hours of machining graphite, we looked as we were
made up for a minstrel. One shower would remove only the surface graphite
dust. About a half-hour after the first shower the dust in the pores of
your skin would start oozing. Walking around the room where we cut
graphite was like walking on a dance floor. Graphite is a dry lubricant,
you know, and the cement floor covered with graphite dust was
Before the structure was half complete,
measurements indicated that the critical size at which the pile would
become self-sustaining was somewhat less than had been anticipated in the