F.D. Roosevelt,
President of the United States,
White House
Washington, D.C.
Sir:
Some recent work by E. Fermi and L. Szilard, which has been communicated
to me in manuscript, leads me to expect that the element uranium may be
turned into a new and important source of energy in the immediate
future. Certain aspects of the situation which has arisen seem to call
for watchfulness and, if necessary, quick action on the part of the
Administration. I believe therefore that it is my duty to bring to your
attention the following facts and recommendations:
In the course of the last four months it has been made probable—through
the work of Joliot in France as well as Fermi and Szilard in
America—that it may become possible to set up a nuclear chain reaction
in a large mass of uranium, by which vast amounts of power and large
quantities of new radium-like elements would be generated. Now it
appears almost certain that this could be achieved in the immediate
future.
This new phenomenon would also lead to the construction of bombs, and it
is conceivable—though much less certain—that extremely powerful bombs of
a new type may thus be constructed. A single bomb of this type, carried
by boat and exploded in a port, might very well destroy the whole port
together with some of the surrounding territory. However, such bombs
might very well prove to be too heavy for transportation by air.
[Illustration: _Albert Einstein_]
The United States has only very poor ores of uranium in moderate
quantities. There is some good ore in Canada and the former
Czechoslovakia, while the most important source of uranium is Belgian
Congo.
In view of this situation you may think it desirable to have some
permanent contact maintained between the Administration and the group
of physicists working on chain reactions in America. One possible way
of achieving this might be for you to entrust with this task a person
who has your confidence and who could perhaps serve in an inofficial
capacity. His task might comprise the following:
a) to approach Government Departments, keep them informed of the
further development, and put forward recommendations for Government
action, giving particular attention to the problem of securing a
supply of uranium ore for the United States;
b) to speed up the experimental work, which is at present being
carried on within the limits of the budgets of University
laboratories, by providing funds, if such funds be required, through
his contacts with private persons who are willing to make
contributions for this cause, and perhaps also by obtaining the
co-operation of industrial laboratories which have the necessary
equipment.
I understand that Germany has actually stopped the sale of uranium
from the Czechoslovakian mines which she has taken over. That she
should have taken such early action might perhaps be understood on the
ground that the son of the German Under-Secretary of State, von
Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where
some of the American work on uranium is now being repeated.
Yours very truly,
[Illustration: /signed/]
(Albert Einstein)
As the quantity of uranium within which the fission chain reaction was
initiated grew larger, more and more of the neutrons produced found a
mark and the fission reaction would die out more and more slowly.
Finally, at some particular size—the “critical size”—the fission
reaction did not die at all, but maintained itself, with enough of the
neutrons produced finding their mark to keep the nuclear reaction
proceeding at a steady rate. At any greater size the nuclear reaction
would accelerate and there would be an explosion.
It wasn’t even necessary to send neutrons into the uranium to start the
process. In 1941 the Russian physicist Georgii Nikolaevich Flerov
(1913- ) found that every once in a while a uranium atom would
undergo fission without the introduction of a neutron. Occasionally the
random quivering of a nucleus would bring about a shape that the nuclear
interaction could not bring back to normal and the nucleus would then
break apart. In a gram of ordinary uranium, there is a nucleus
undergoing such “spontaneous fission” every 2 minutes on the average.
Therefore, enough uranium need only be brought together to surpass
critical size and it will explode within seconds, for the first nucleus
that undergoes spontaneous fission will start the chain reaction.
First estimates made it seem that the quantity of uranium needed to
reach critical size was extraordinarily great. Fully 99.3% of the metal
is uranium-238, however, and, as soon as fission was discovered, Bohr
pointed out that there were theoretical reasons for supposing that it
was the uranium-235 isotope (making up only 0.7% of the whole) that was
the one undergoing fission. Investigation proved him right. Indeed, the
uranium-238 nucleus tended to absorb slow neutrons without fission, and
to go on to beta-particle production that formed isotopes of neptunium
and plutonium. In this way uranium-238 actually interfered with the
chain reaction.
In any quantity of uranium, the more uranium-235 present and the less
uranium-238, the more easily the chain reaction would proceed and the
lower the critical size needed. Vast efforts were therefore made to
separate the 2 isotopes and prepare uranium with a higher than normal
concentration of uranium-235 (“enriched uranium”).
Of course, there was no great desire for a fearful explosion to get out
of hand while the chain reaction was being studied. Before any bomb
could be constructed, the mechanism of the chain reaction would have to
be studied. Could a chain reaction capable of producing energy (for
useful purposes as well as for bombs) be established? To test this, a
quantity of uranium was gathered in the hope that a _controlled_ chain
reaction of uranium fission could be established. For that purpose,
control rods of a substance that would easily absorb neutrons and slow
the chain reaction were used. The metal, cadmium, served admirably for
this purpose.
Then, too, the neutrons released by fission were pretty energetic. They
tended to travel too far too soon and get outside the lump of uranium
too easily. To produce a chain reaction that could be studied with some
safety, the presence of a moderator was needed. This was a supply of
small nuclei that did not absorb neutrons readily, but absorbed some of
the energy of collision and slowed down any neutron that struck it.
Nuclei such as hydrogen-2, beryllium-9, or carbon-12 were useful
moderators. When the neutrons produced by fission were slowed, they
travelled a smaller distance before being absorbed in their turn and the
critical size would again be reduced.
Toward the end of 1942 the initial stage of the project reached a
climax. Blocks of graphite containing uranium metal and uranium oxide
were piled up in huge quantities (enriched uranium was not yet
available) in order to approach critical size. This took place under the
stands of a football stadium at the University of Chicago, with Enrico
Fermi (who had come to the United States in 1938) in charge.[1]
The large structure was called an “atomic pile” at first because of the
blocks of graphite being piled up. The proper name for such a device,
and the one that was eventually adopted, was, however, “nuclear
reactor”.
On December 2, 1942, calculations showed that the nuclear reactor was
large enough to have reached critical size. The only thing preventing
the chain reaction from sustaining itself was the cadmium rods that were
inserted here and there in the pile and that were soaking up neutrons.
[Illustration: _Cutaway model of the West Stands of Stagg Field showing
the first pile in the squash court beneath it._]
[Illustration: _The exterior of the building._]
[Illustration: _Graphite layers form the base of the pile, left. On the
right is the seventh layer of graphite and edges of the sixth layer
containing 3¼-inch pseudospheres of black uranium oxide. Beginning with
layer 6, alternate courses of graphite containing uranium metal and/or
uranium oxide fuel were separated by layers of solid graphite blocks._]
[Illustration: _Tenth layer of graphite blocks containing pseudospheres
of black and brown uranium oxide. The brown briquets, slightly richer in
uranium, were concentrated in the central area. On the right is the
nineteenth layer of graphite covering layer 18 containing slugs of
uranium oxide._]
One by one the cadmium rods were pulled out. The number of uranium atoms
undergoing fission each second rose and, finally, at 3:45 p.m., the
uranium fission became self-sustaining. It kept going on its own (with
the cadmium rods ready to be pushed in if it looked as though it were
getting out of hand—something calculations showed was not likely).
News of this success was announced to Washington by a cautious telephone
call from Arthur Holly Compton (1892-1962) to James Bryant Conant
(1893- ). “The Italian navigator has landed in the new world”, said
Compton. Conant asked, “How were the natives?”, and the answer was,
“Very friendly”.
This was the day and moment when the world entered the “nuclear age”.
For the first time, mankind had constructed a device in which the
nuclear energy being given off was greater than the energy poured in.
Mankind had tapped the reservoirs of nuclear energy and could put it to
use. Had Rutherford lived but 6 more years, he would have seen how wrong
he was to think it could never be done.
The people of earth remained unaware of what had taken place in Chicago
and physicists continued to work toward the development of the nuclear
bomb.
Enriched uranium was successfully prepared. Critical sizes were brought
low enough to make a nuclear bomb small enough to be carried by plane to
some target. Suppose one had 2 slabs of enriched uranium, each below
critical size, but which were above critical size if combined. And
suppose an explosive device were added that, at some desired moment,
could be set off in such a way that it would drive 1 slab of enriched
uranium against the other. There would be an instant explosion of
devastating power. Or suppose the enriched uranium were arranged in
loosely packed pieces to begin with so that the flying neutrons were in
open air too often to maintain the chain reaction. A properly arranged
explosion might compress the uranium into a dense ball. Neutron
absorption would become more efficient and again, an explosion would
follow.
[Illustration: _Nuclear Fission of Uranium: A neutron hits the nucleus
of an atom of uranium. The neutron splits the nucleus into two parts and
creates huge amounts of energy in the form of heat. At the same time
other neutrons are released from the splitting nucleus and these
continue the fission process in a chain reaction._]
On July 16, 1945, a device that would result in a nuclear explosion was
set up near Alamogordo, New Mexico, with nervous physicists watching
from a safe distance. It worked perfectly; the explosion was tremendous.
By that time Nazi Germany had been defeated, but Japan was still
fighting. Two more devices were prepared. After a warning, one was
exploded over the Japanese city of Hiroshima on August 6, 1945, and the
other over Nagasaki 2 days later. The Japanese government surrendered
and World War II came to an end.
It was with the blast over Hiroshima that the world came to know it was
in the nuclear age and that the ferocious weapon of the nuclear bomb
existed. (The popular name for it at the time was “atomic bomb” or
“A-bomb”.)
During the war, German scientists may have been trying to develop a
nuclear bomb, but, if so, they had not yet succeeded at the time Germany
met its final defeat. Soviet physicists, under Igor Vasilievich
Kurchatov (1903-1960), were also working on the problem. The dislocation
of the war, which inflicted much more damage on the Soviet Union than on
the United States, kept the Soviet effort from succeeding while it was
on. However, since the Soviets were among the victors, they were able to
continue after the war.
In 1949 the Soviets exploded their first nuclear bomb. In 1952 the
British did the same; in 1960, the French; and in 1964, the Chinese.
Although many nuclear bombs have been exploded for test purposes, the
two over Hiroshima and Nagasaki have been the only ones used in time of
war.
Nor need nuclear bombs be considered as having destructive potential
only. There is the possibility that, with proper precautions, they might
be used to make excavations, blast out harbors or canals, break up
underground rock formations to recover oil or other resources, and in
other ways do the work of chemical explosives with far greater speed and
economy. It has even been suggested that a series of nuclear bomb
explosions might be used to hurl space vehicles forward in voyages away
from earth.