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Dec. 2 1942, a double milestone for nuclear research
As
you may know Canada was also putting in efforts to achieve the
sustained nuclear chain reaction during the same time period, the reason
they lost to the Americans in the race was access to large quantities
of high purity graphite. There is a very nice write up by George C.
Laurence detailing these efforts: "Experiments in Ottawa
Heavy water
was scarce and costly to produce. The 185 kilograms, that the French
scientists had obtained from a hydroelectric plant in Norway and brought
to England, was most of the world's supply. Rough calculations with the
inaccurate data then available suggested that it might be possible to
obtain a large release of energy using some form of carbon, instead of
heavy water, with the uranium. Carbon would be less suitable for the
purpose but was cheaper and easier to obtain. I decided to experiment
with carbon and uranium oxide. The experiment would have to be done
mostly in overtime because my small section was very busy assisting
Canadian industry to become proficient in the radiographic inspection of
parts for military aircraft and other equipment. Months later, I
learned without surprise that similar experiments with carbon and
uranium had been started both in England and the United States at about
the same time.
The purpose of the experiment was to determine whether
a very large release of nuclear energy would be possible in a large
bulk of the kinds of uranium and carbon which I had. It would be
possible if at least as many neutrons were released by fission as were
captured. That implied that if an independent source of neutrons is
surrounded by a small quantity (i.e. a few tonnes) of the combination of
uranium and carbon, more neutrons would reach the surrounding walls
than if the combination of materials was not present.
In our
experiments in Ottawa to test this, the source of neutrons was beryllium
mixed with a radium compound in a metal tube about 2.5 centimetres
long. Alpha particles, emitted spontaneously from the radium,
bombarded atoms of beryllium and released neutrons from them. The
carbon was in the form of ten tonnes of calcined petroleum coke, a very
fine black dust that easily spread over floors, furniture and
ourselves. The uranium was 450 kilograms of black oxide, which was
borrowed from Eldorado Gold Mines Limited. It was in small paper sacks
distributed amongst larger paper sacks of the petroleum coke.
The
sacks of uranium and coke were held in a wooden bin, so that they
occupied a space that was roughly spherical, 2.7 m in diameter. The
wooden bin was lined with paraffin wax about five centimetres thick to
reduce the escape of neutrons. The arrangement is shown above, as a
sectional view through the bin and its contents.
A thin wall metal
tube supported the neutron source at the centre of the bin, and provided
a passage for insertion of a neutron detector which could be placed at
different distances from the source. In the first tests the detector
was a silver coin, but in most of the experiments it was a layer of
dysprosium oxide on an aluminum disc.
The experimental routine was to
expose the detector to the neutrons for a suitable length of time, then
remove it quickly from the assembly and place it in front of a Geiger
counter to measure the radioactivity produced in it by the neutrons.
The Geiger counter tubes and the associated electrical instruments were
homemade because there was very little money to spend on equipment.
The
relative rates of neutron capture and neutron release by fission were
calculated from the data obtained. If the release had been greater
than the capture it would have been possible to estimate the "critical
quantity" of uranium and coke, that is the minimum quantity needed to
produce a self-sustained reaction that would release a large amount of
nuclear energy.
Prof. B. Sargent of Queen's University joined me in
these experiments during the summer university vacations of 1941 and
1942. Progress was slow because the work was interrupted by other
duties and we lacked the better equipment that would be available today.
By
late summer in 1942, our measurements had shown that the release of
neutrons by fission in our combination of materials was a few percent
less than the capture. Therefore, it would not be possible to obtain a
large release of nuclear energy in that combination of materials even
if large quantities were used. There was too much loss of neutrons by
capture in impurities in the coke and uranium oxide and in the small
quantities of paper and brass that were present. We did not then
realize how a little impurity could lead to failure.
Meanwhile in the
United States, E. Fermi, H.L. Anderson, B. Field, G. Weil and W. Zinn,
after a first attempt that was also unsuccessful, did succeed in showing
that a large release of energy would be possible using purer uranium
and very pure carbon in the form of graphite. Using the necessarily
larger quantities, the Americans then built the first nuclear reactor
and operated it on December 2, 1942. They called it an "atomic pile".
In
the summer of 1940, R.H. Fowler visited Ottawa, followed soon by J.D.
Cockroft. They had been to the United States to stimulate greater
American interest in research of military importance. They told me
about the nuclear energy research in England and that in the United
States which they had just seen.
With Prof. Fowler's introduction, I
visited L.J. Briggs, who was chairman of the committee that coordinated
the American nuclear energy research at that time, and also J.B. Conant,
E. Fermi, H.C. Urey and P.H. Abelson and learned of their work. After
my visit, we received in Ottawa copies of reports on the American
nuclear energy research for the next two years. One of them that was
particularly helpful was "A Study Concerning Uranium as a Source of
Power" by J.B. Fisk and W. Shockly, dated September 17, 1940, a
remarkable theoretical discussion of the feasibility of a nuclear
reactor to have been written so early.
In response to Cockroft's
suggestion when he returned to England we received a gift of $5,000 from
Imperial Chemical Industries, which was involved in the nuclear
research in England, in support of our experiment. It was an important
addition to our budget, but I valued it most as an expression to Dr.
Mackenzie of British confidence in our work." read more: http://media.cns-snc.ca/history/early_years/earlyyears.html
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