PILES AND PLUTONIUM
(1939-1941)
Events: Early Government
Support, 1939-1942

The Uranium Committee's first report, issued on November 1, 1939, recommended that, despite the uncertainty of success, the government should immediately obtain four tons of graphite and fifty tons of uranium oxide.  This recommendation led to the first outlay of government funds -- $6,000 in February 1940 -- and reflected the importance attached to the Fermi-Szilard pile (reactor) experiments already underway at Columbia University.  Building upon the work performed in 1934 demonstrating the value of moderators in producing slow neutrons, Enrico Fermi thought that a mixture of the right moderator and natural uranium could produce a self-sustaining fission chain reaction.  Fermi and Leo Szilard increasingly focused their attention on carbon in the form of graphite. Perhaps graphite could slow down, or moderate, the neutrons coming from the fission reaction, increasing the probability of their causing additional fissions in sustaining the chain reaction.  A pile containing a large amount of natural uranium could then produce enough secondary neutrons to keep a reaction going.  

There was, however, a large theoretical gap between building a self-generating pile and building a bomb.  Although the pile envisioned by Fermi and Szilard could produce large amounts of power and might have military applications (powering naval vessels, for instance), it would be too big for a bomb.  It would take separation of uranium-235 or substantial enrichment of natural uranium with uranium-235 to create a fast neutron reaction on a small enough scale to build a usable bomb.  While certain of the chances of success in his graphite power pile, Fermi in 1939 thought that there was "little likelihood of an atomic bomb, little proof that we were not pursuing a chimera."  

Experiments conducted in early 1941 at the Radiation Laboratory at the University of California, Berkeley, finally completed the link between pile research and bomb construction.  Edwin M. McMillan and Philip H. Abelson had been studying uranium fission fragments produced in a cyclotron there (above).  Their research led to the chemical identification of element 93, neptunium, while research by Glenn T. Seaborg (right) revealed that an isotope of neptunium decayed to yet another transuranium (man-made) element.  In February, Seaborg identified this as element 94, which he later named plutonium.  By May, he had proven that plutonium-239 was 1.7 times more likely than uranium-235 to fission.  This finding made the Fermi-Szilard experiment more important than ever, as it suggested the possibility of producing large amounts of the fissionable plutonium in a uranium pile using plentiful uranium-238, and then separating it chemically.  Surely this would be less expensive and simpler than building isotope-separation plants.  A second, perhaps easier, path to the atomic bomb now seemed possible.