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Monday, November 26, 2007

Reading List: Plutonium

Bernstein, Jeremy. Plutonium. Washington: Joseph Henry Press, 2007. ISBN 0-309-10296-0.
When the Manhattan Project undertook to produce a nuclear bomb using plutonium-239, the world's inventory of the isotope was on the order of a microgram, all produced by bombarding uranium with neutrons produced in cyclotrons. It wasn't until August of 1943 that enough had been produced to be visible under a microscope. When, in that month, the go-ahead was given to build the massive production reactors and separation plants at the Hanford site on the Columbia River, virtually nothing was known of the physical properties, chemistry, and metallurgy of the substance they were undertaking to produce. In fact, it was only in 1944 that it was realised that the elements starting with thorium formed a second group of “rare earth” elements: the periodic table before World War II had uranium in the column below tungsten and predicted that the chemistry of element 94 would resemble that of osmium. When the large-scale industrial production of plutonium was undertaken, neither the difficulty of separating the element from the natural uranium matrix in which it was produced nor the contamination with Pu-240 which would necessitate an implosion design for the plutonium bomb were known. Notwithstanding, by the end of 1947 a total of 500 kilograms of the stuff had been produced, and today there are almost 2000 metric tons of it, counting both military inventories and that produced in civil power reactors, which crank out about 70 more metric tons a year.

These are among the fascinating details gleaned and presented in this history and portrait of the most notorious of artificial elements by physicist and writer Jeremy Bernstein. He avoids getting embroiled in the building of the bomb, which has been well-told by others, and concentrates on how scientists around the world stumbled onto nuclear fission and transuranic elements, puzzled out what they were seeing, and figured out the bizarre properties of what they had made. Bizarre is not too strong a word for the chemistry and metallurgy of plutonium, which remains an active area of research today with much still unknown. When you get that far down on the periodic table, both quantum mechanics and special relativity get into the act (as they start to do even with gold), and you end up with six allotropic phases of the metal (in two of which volume decreases with increasing temperature), a melting point of just 640° C and an anomalous atomic radius which indicates its 5f electrons are neither localised nor itinerant, but somewhere in between.

As the story unfolds, we meet some fascinating characters, including Fritz Houtermans, whose biography is such that, as the author notes (p. 86), “if one put it in a novel, no one would find it plausible.” We also meet stalwarts of the elite 26-member UPPU Club: wartime workers at Los Alamos whose exposure to plutonium was sufficient that it continues to be detectable in their urine. (An epidemiological study of these people which continues to this day has found no elevated rates of mortality, which is not to say that plutonium is not a hideously hazardous substance.)

The text is thoroughly documented in the end notes, and there is an excellent index; the entire book is just 194 pages. I have two quibbles. On p. 110, the author states of the Little Boy gun-assembly uranium bomb dropped on Hiroshima, “This is the only weapon of this design that was ever detonated.” Well, I suppose you could argue that it was the only such weapon of that precise design detonated, but the implication is that it was the first and last gun-type bomb to be detonated, and this is not the case. The U.S. W9 and W33 weapons, among others, were gun-assembly uranium bombs, which between them were tested three times at the Nevada Test Site. The price for plutonium-239 quoted on p. 155, US$5.24 per milligram, seems to imply that the plutonium for a critical mass of about 6 kg costs about 31 million dollars. But this is because the price quoted is for 99–99.99% isotopically pure Pu-239, which has been electromagnetically separated from the isotopic mix you get from the production reactor. Weapons-grade plutonium can have up to 7% Pu-240 contamination, which doesn't require the fantastically expensive isotope separation phase, just chemical extraction of plutonium from reactor fuel. In fact, you can build a bomb from so-called “reactor-grade” plutonium—the U.S. tested one in 1962.

Posted at November 26, 2007 21:18