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Enrico Fermi (1901–1954).
Enrico Fermi (1901–1954).
Following the work of John Cockcroft and Ernest Walton (see here), the Hungarian physicist Leó Szilárd, then based in England, pointed out the possibility of a nuclear chain reaction. In such a process, if a neutron striking a nucleus caused it to release more neutrons, these neutrons in turn could trigger further nuclei to release neutrons in a self-sustaining and possibly runaway reaction. Little notice was taken of the idea until 1938, when Szilárd moved to New York. There, he heard about the work of Lise Meitner and Otto Hahn (see here) with uranium fission. He realized that it might be possible to produce a self-sustaining uranium fission reaction, which would release energy as the uranium nuclei ‘split’. A small-scale experiment proved the idea in principle, and Szilárd organized a letter to the President, Franklin D. Roosevelt, warning of the possibility that the Germans might develop a nuclear bomb using this technique. (Albert Einstein was persuaded to sign the letter, although he had nothing to do with this work.)
When the United States entered the Second World War, the fear of being attacked with nuclear weapons lead to the creation of the Manhattan Project, to produce a nuclear bomb first. One of the steps towards the bomb involved the construction of a nuclear reactor, in which a pile (literally) of uranium could be made to produce energy, in the form of heat, without the reaction running away and making an explosion.
The person in charge of the construction of this experimental reactor was Enrico Fermi, who had moved to America because of the threat to his Jewish wife of the Fascist regime in Italy. It depended on having just the right amount of uranium in one place, with a way to control the number of neutrons taking part in the reaction. In a very small lump of uranium, most of the neutrons produced by spontaneous fission escape from the surface and do not trigger the fission of other nuclei. In a very large lump of uranium, neutrons from fission that occurs inside the metal will be likely to hit other nuclei before they can escape, triggering more fission in a runaway process that causes an explosion. The reactor built under Fermi’s direction, inside a disused squash court at the University of Chicago, struck a balance between these extremes.
The reactor was known as Chicago Pile-1, or CP-1. The pile was built on a square base but with a rounded top, and was made up from alternating layers of graphite blocks and graphite blocks containing lumps of uranium metal or uranium oxide. Graphite absorbs or slows down neutrons, so it helps to ‘moderate’ the reaction. Fermi and Szilárd had calculated that placing the uranium in blocks of moderating material, forming a cubical lattice of uranium, would give the best chance for a neutron from one uranium atom to encounter the nucleus of another uranium atom. The key control mechanism, however, was a rod made of cadmium, which absorbs neutrons, that could be moved in or out of the pile as required. With the rod inserted, neutrons would be absorbed and there would be no chain reaction; with the rod removed, the neutrons could fly freely to trigger fission and keep the reaction going.
Fermi calculated that a pile of these graphite blocks 56 layers high would contain enough uranium to produce a self-sustaining nuclear reaction. So CP1 was built 57 layers high. The finished reactor had cost roughly a million dollars to manufacture, and contained 771,000 pounds (350 tons) of graphite, 80,590 pounds (37 tons) of uranium oxide and 12,400 pounds (5.6 tons) of uranium metal. It was 25 feet wide and 20 feet high (8 metres wide by 15 metres high). At 3.36 pm local time on 2 December 1942, the cadmium rod was slowly withdrawn from the pile, allowing the neutrons produced by fissioning uranium nuclei to trigger the fission of other nuclei. The reactor operated as predicted for 28 minutes before the rod was pushed back in and the reaction shut down. It was the beginning of the so-called ‘atomic’ (actually nuclear) age.
The success of CP1 showed not only that a nuclear chain reaction could be produced artificially, but, even more importantly, that it could be controlled. The maximum power output from the reactor was only some 200 watts, enough to run an incandescent light bulb, but as Fermi later said, ‘we all hoped that with the end of the war emphasis would be shifted decidedly from the weapon to the peaceful aspects of atomic energy’ including ‘the building of power plants’.43 But he did not live to see the full fruition of this dream. Built under wartime emergency conditions, Chicago Pile 1 had no radiation shielding of any kind; on 28 November 1954, at the age of 53, Enrico Fermi died of stomach cancer, possibly caused by his work with radioactive material.
