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The dark heart of the bomb

Plutonium: A History of the World's Most Dangerous Element

  • Jeremy Bernstein
Joseph Henry Press: 2007. 258 pp. £16.99 $27.95 0309102960 | ISBN: 0-309-10296-0

Plutonium has either a celebrated or a tragic history, depending on your point of view. It was the core of the weapon that destroyed much of Nagasaki on 9 August 1945, and has only military uses. For those who find security standing behind a stockpile of plutonium bombs, the element is a reason to celebrate. By contrast, for those who regard the bombing of Nagasaki as a needless repetition of the Hiroshima catastrophe, plutonium is a symbol of the US–Soviet arms race that dominated the second half of the twentieth century. It now signifies the rank and status of a nation's military prowess.

In his book Plutonium, Jeremy Bernstein acknowledges that everything connected with the element is complicated, and that includes plutonium itself and its history. Its discovery in 1941 by Glenn Seaborg and Arthur Wahl is part of a much bigger story in which each part becomes a story in itself.

Credit: JOE MAGEE

Plutonium does not occur in earthen deposits, for example; it is produced instead by the radioactive decay of uranium by way of neptunium, and it is with uranium that the book begins. Then there is the story of the periodic table and the problems associated with fitting the elements into their proper places — especially the lanthanides (the elements of atomic number 58 to 71 that follow lanthanum in the periodic table) and the actinides (elements 90 to 103 following actinium).There is the story of radioactivity (and the connected story of the discovery of X-rays) and of Enrico Fermi bombarding uranium nuclei with slow neutrons. Add to these the story of fission, with various elements and isotopes complicating the plot. Los Alamos and the development of atomic bombs are also a central part of the plutonium story. Finally, there are the complications arising from the element plutonium itself that must be understood and the associated problems solved. Melding these many parts into a short book represents a daunting challenge, which Bernstein confronts head on.

One of the benefits of this multifaceted approach is the opportunity it gives the author to educate readers by means of historical information and thumbnail sketches of interesting people. In his 1903 Nobel address, for example, Henri Becquerel, who discovered radioactivity, suggested that the energy associated with radioactivity may involve the modification of atoms in the radioactive material. Two years later, Einstein showed that there was a loss of mass, which becomes energy according to his famous equation E = mc2. In 1934, Ida Noddack correctly criticized Fermi, suggesting that in his neutron-bombardment experiments he had actually discovered nuclear fission. Fermi's Nobel speech in 1938 was wrong on this point because he assumed he had discovered transuranic elements. When the Nobel Prize was awarded for the discovery of fission, the Nobel committee made so many erroneous assumptions about who did what, and when, that Lise Meitner was wrongly denied a share of the prize.

The tale of Fritz Houtermans is particularly interesting and not well known. Houtermans wrote a report in 1941 in which he considered the absorption of a neutron by uranium-238 and concluded that it would lead to plutonium via neptunium. He further concluded that plutonium would be fissionable. Perhaps generalizing from his own insights, he twice sent messages (from his native Germany) to the Allies that Germany was “on the track” to making plutonium. It would be interesting to know why he did this, but Bernstein says only that he wanted to “warn the Allies”. In any event, Houtermans was wrong: the Germans were not close to making plutonium.

In early 1943, the Los Alamos laboratory — the home of the Manhattan Project — began to take shape. By the summer of 1944, plutonium started arriving there. The element's idiosyncrasies and complexities soon became apparent. William Zachariasen discovered that plutonium had six different crystal structures, or allotropes, which he labelled α, β, γ, δ, δ' and ɛ. One of these allotropes had to be formed into a metal suitable for a bomb, which meant being stable and free of isotopes that would interfere with a chain reaction. The metallurgist Cyril Stanley Smith had the good fortune and acute intuition (there were no data) to select gallium to form an alloy with the δ allotrope of plutonium to produce the needed stability. It was still unclear whether the δ allotrope would revert to the α allotrope before explosion. And a way of bringing the two subcritical pieces of plutonium together to form the critical mass — and initiate the chain reaction that would lead to a nuclear explosion — had to be developed from scratch, as the gun trigger used for the uranium bomb that was dropped on Hiroshima was not suitable. Plutonium, then, presented challenges at every turn. As Bernstein suggests, it may have been only the fear of what the Germans were doing that kept the physicists working long into the night.

This book will make demands of readers. There are many things to hold in the mind as Bernstein repeatedly moves away from the main thrust of the book to develop one of these side stories, which enrich the story of plutonium but are also sometimes a distraction. But Bernstein's writing ability smoothes the way and makes this a successful book.

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Rigden, J. The dark heart of the bomb. Nature 447, 31–33 (2007). https://doi.org/10.1038/447031a

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