The Complex World of Plutonium Science
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The Complex World of Plutonium Science Siegfried S. Hecker Introduction Plutonium symbolizes everything we associate with the nuclear age. It evokes the entire gamut of emotions from good to evil, from hope to despair, and from the salvation of humanity to its utter destruction. No other element bears such a burden. Its discovery in 1941, following the discovery of fission in 1938, unlocked the potential and fear of the nuclear age. During the Cold War, the primary interest in plutonium was to provide triggers for thermonuclear weapons that formed the basis of nuclear deterrence. Beginning in the 1950s, plutonium also became an integral part of the quest for nearly limitless electrical power. The end of the Cold War has dramatically altered the military postures of the United States and Russia, allowing each to reverse the engines fueling the nuclearweapons buildup. Now, both countries face the challenge of keeping the remaining stockpile of nuclear weapons safe and reliable without nuclear testing, as well as cleaning up nuclear contamination and preventing the spread of nuclear weapons and terrorism. Moreover, current concerns about energy availability and global warming have rekindled interest in nuclear power. Regardless of how plutonium is viewed geopolitically, it is undoubtedly the most complex element in the periodic table. As element 94, it fits near the middle of the actinide series. The 239 isotope of plutonium fissions when bombarded with neutrons over a large range of neutron energies, releasing millions of times the energy typically released in chemical explosives or in the burning of fossil fuels. However, it is the 5f electrons that make plutonium so interesting. With little provocation, the metal changes its density by as much as 25%. It can be as brittle as glass or as malleable as aluminum; it expands when it solidifies— much like water freezing to ice; and its shiny, silvery, freshly machined surface will tarnish in minutes. It is highly reactive in air and strongly reducing in solution, forming multiple compounds and complexes in the environment and during
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chemical processing. It transmutes by radioactive decay, causing damage to its crystalline lattice and leaving behind helium, americium, uranium, neptunium, and other impurities. Plutonium damages materials on contact and is therefore difficult to handle, store, or transport. Who would ever dream of making and using such a material? Physicists did—in order to take advantage of the extraordinary nuclear properties of 239Pu.1 In this article, I will focus first on the greatest of the many peculiarities of plutonium metal and alloys, namely, its instability—with temperature, pressure, chemical additions, and with time. Next, I will briefly summarize plutonium ceramics and plutonium chemistry, before examining the complications resulting from aging. Instability and surface reactions constitute the greatest limitations for extending the lifetimes of plutonium components in nuclear weapons and must be understood for long-term storage or dispo
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