Quantum and Nuclear

Fermi’s fission

Stories from Physics for 11-14 14-16 16-19 IOP RESOURCES

Italian physicist Enrico Fermi is often cited as one of the most significant contributors to the development of nuclear physics and technology. He led the construction of the first artificial nuclear reactor, Chicago Pile-1, and also carried out research in statistical and particle physics.

Early fission without imagination

The achievement of induced fission of uranium, whilst often credited to Otto Hahn and Fritz Strassmann in 1938, may have been unwittingly achieved by Fermi as early as 1934. Fermi had been attempting to produce elements heavier than uranium by irradiating the metal with neutrons. He noted that the bombardment led to the production of several isotopes with a range of different half-lives but did not, at the time, link the products with fission of the uranium nucleus. Fermi later reported:

We did not have enough imagination to think that a different process of disintegration might occur in uranium from that in any other element, and we tried to identify the radioactive products with elements close to uranium in the periodic table of elements. Moreover, we did not know enough chemistry to separate the products of uranium disintegration from one another.

A nuclear reactor in a balloon

The first self-sustaining nuclear reactor was built by Fermi and his team at the University of Chicago. It was contained within a bag made of balloon cloth (a fabric used in hot air balloons and, historically, aircraft skins) so the scientists could remove neutron-absorbing air, if necessary, to increase the rate of reaction. Engineers at the Goodyear Tire and Rubber Company were reportedly bemused when they received a request for a rectangular balloon with no information on the use of the material.

Tigger, Piglet, Kanga and the nuclear reactor

Whilst he was constructing the reactor, Fermi was learning English by reading A. A. Milne’s Winnie the Pooh. He nicknamed several of the instruments used to observe the reactor after characters from the book: Tigger, Piglet, Kanga and Baby Roo.

Tigger, Piglet, Kanga and the nuclear reactor

As the risks of early fission experiments were poorly understood, Fermi put in place a number of safety measures. The reactor had a rather idiosyncratic safety feature. In addition to a set of control rods raised and lowered by a motor, a second rod, nicknamed ‘zip’, was attached to a heavy weight and tied to a rail. In the event of an emergency, Norman Hilberry (who went on to be president of the American Nuclear Society) waited with an axe, ready to release the neutron absorbing rod by cutting the rope. The third control rod was under Fermi’s direction and used to regulate the rate of reaction during experiments. The control rods consisted of sheets of neutron-absorbing cadmium, nailed to wooden strips. A final safety measure was the so-called ‘suicide squad’, a team of three men who stood on a platform above the reactor and could flood the reactor with calcium sulphate in the event of an emergency.

One theory suggests that the name of a technique for rapidly shutting down nuclear reactors, the SCRAM procedure, derives from Hilberry’s role (Safety Control Rod Axe Man). Another is simply that it is short for ‘scramble’ – ie run away very quickly.

References

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