Fermi-pile and Plutonium

A good tourist is supposed to be able (2) to build a campfire (3) even if the wood is soaking wet. This role of a good hiking tourist in the nuclear energy project was played by the Italian-American physicist, Enrico Fermi, who actually made the "wet" uranium logs "burn”. He was able to do so by utilizing the fact mentioned above, that the effectiveness of fission neutrons in producing the fission of U²³⁵ nuclei increases quite considerably when they are slowed down. If such slowing down of fission neutrons could be achieved, the presence of inactive U²³⁸ (1) would not make much difference. To slow down the original fission neutrons it was necessary to mix natural uranium with a large amount of carbon in the form of graphite. A large "pile" of graphite bricks with small pieces of natural uranium included in the structure was constructed in great secrecy under the grandstand of the University of Chicago Stadium, and on December 2, 1941, Professor A. Compton wired to Dr. Vannevar Bush in Washington, D.C.: "The Italian navigator has landed. The natives are friendly." In the secret language of the Manhattan Project this meant: "The Fermi-pile works successfully. Nuclear chain reaction is achieved."

In the Fermi-pile, the fission chain reaction could be maintained in natural uranium, but the natural uranium was so highly diluted by carbon that high efficiency in energy production could not be achieved. Owing to (6) the presence of inactive U²³⁸, the chain reaction in the pile could not possibly develop into an efficient explosion, nor could it be very useful as a power source.

So what good was the Fermi-pile, (7) except for demonstrating the purely scientific principle of the possibility of a (4) self-maintaining nuclear reaction? Of course, the demonstration of a purely scientific principle is always of very great importance, but the Fermi-pile was built at great expense in the midst of a perilous war when all expenditures were supposed to be judged on the basis of their military usefulness.

The Fermi-pile (5) stood this acid test. Although the energy released in the fission of U²³⁵ nuclei could not be utilized and (11) was literally sent down the drain by means of the water-cooling system, a new fissionable element was produced inside the pile during operation. The neutrons that were not used in the maintenance of the chain reaction in U²³⁵ nuclei were captured by U²³⁸ nuclei, producing the heavier isotope:

Having an excess of neutrons, the nuclei of ₉₂U²³⁹ (8) underwent two successive β- transformations, (9) giving rise to elements with atomic numbers 93 and 94. These two elements, which do not exist in nature but have been produced artificially by human genius, were given the names neptunium and plutonium. The reactions following the neutron capture by U²³⁸ can be written:

Being chemically different from uranium, the plutonium produced in the Fermi-pile can be separated and purified (12) with much less effort than it takes to separate a light uranium isotope from the heavy one, and this element turned out to be even more fissionable than U²³⁵. In fact, whereas U²³⁵ gives rise to 2.5 fission neutrons, the corresponding figure for Pu²³⁹ is 2.9 fission neutrons.