Yesterday, at an international meeting to mark the event at London's Science Museum, a scientist from the original team recalled that day.
Dr Harold Agnew, 71, said: 'The atmosphere was rather subdued. We were tired . . . we had been working 10-hour shifts for two months. It wasn't like making a goal at the last minute in baseball, because we knew what the results were going to be. It was a more gradual story . . . although of course it was a momentous event.'
Enrico Fermi, the Italian physicist who led the team, designed the nuclear 'pile' in a rough sphere, supported within a wooden structure. It contained 340 tons (345,440 kilograms) of graphite, 37 tons of uranium oxide, 5 tons of uranium metal and a few control rods. It was about the size of a double garage.
Professor Fermi once compared the reaction to a pile of rubbish burning through internal combustion. 'In such a fire, minute parts of the pile start to burn and in turn ignite other fragments. When sufficient numbers of these fractional parts are heated to the kindling points, the entire heap bursts into flames.'
In the Chicago pile, neutrons emitted from fission within the uranium struck neighbouring atoms. These split, in turn, and produced more neutrons, bombarding other atoms until the atomic 'fire' was going full blast, kept in check by control rods of cadmium which absorbed stray neutrons. The scientists gathered on a balcony watching the indicators measuring the neutron count - giving a clue to how fast the uranium atoms were disintegrating.
The morning experiment was cut short by the automatic control rods, but in the afternoon they tried again, more cautiously, until Professor Fermi announced that the reaction was self-sustaining. 'The event was not spectacular, no fuses burned, no lights flashed. But to us it meant that release of atomic energy on a large scale would be only a matter of time.' Nuclear power now produces around one-sixth of the world's electricity.
The Science Museum has constructed a full-size replica of the Chicago pile including parts of the original construction.
Dr Agnew has since built his career in nuclear weapons. He was one of the team of three scientists that flew on the mission to drop the atomic bomb on Hiroshima in 1945, which killed more than 150,000 people.
He finds it hard to understand public reaction to the bomb. 'It's technically so sweet . . . just a beautiful reaction. From a single molecule of uranium you can liberate 70 million times the amount of energy that you can from a molecule of carbon. The media doesn't appreciate this and tends to emphasise the bogeyman aspects . . . they like to scare people.'
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