Saturday, November 17, 2007

The riskiest element

kw: book reviews, nonfiction, nuclear materials

Shortly after moving to South Dakota for graduate school in 1978, I attended a forum and debate about storage of nuclear waste from electric power reactors. There was quite a bit of local controversy regarding the Union Carbide mine near the Black Hills. At one point, one of my Geology professors was explaining just what a "spent power module" was, and the problems that need to be solved to safely store one (or a few thousand of them) for many generations. He said the module was physically about the size of an oil drum, produced 10 kilowatts of heat, and would continue to do so for a few thousand years.

I stood and asked, "Can I have one to put in the crawl space of my house? My floors are really cold!" It didn't break the ice as much as I'd hoped. Folks whose fear outstrips their knowledge have little humor.

Seeing pictures like this one a few years later, during the debate over using Plutonium power sources for satellites in low orbit, did little to calm their fears. This sphere is somewhat smaller than a tennis ball, contains about half the amount needed to make a bomb, and is nickel-plated to prevent spontaneous combustion because it is as hot as a stove heating element, about 500 degrees F. It is a source of a few hundred watts of heat, that will emit about the same amount of energy for thousands of years.

Plutonium is actually much better, engineering-wise, as a heat source than as a source of explosive power. This is because of its very strange metallurgy. In a steam-generating application, you use it as a source of steady heat. In a bomb, it has to collapse—prompted by a spherical, surrounding TNT explosion—readily and smoothly in a fraction of a millisecond to a "supercritical" density. Pure Plutonium metal won't do this unless it is first heated to a few hundred degrees, into its "delta" (δ) state. In its cooler "alpha" (α) state, it is brittle and shatters instead. Precise alloying is needed to stabilize the δ state at lower temperatures, and keep it that way as the metal ages due to its internal radioactivity. Think about the implications for a thirty- or forty-year-old bomb core.

In Plutonium: A History of the World's Most Dangerous Element, physicist Jeremy Bernstein takes us through the history of radioactivity and radioactive elements, particularly the transuranics, those elements with nuclei containing more than 92 protons. He also details the chemical and metallurgical dilemmas posed by Plutonium.

A nucleus of Plutonium, symbol Pu, has 94 protons. These numbers, 92 and 94, are the Atomic Numbers of Uranium and Plutonium, respectively. Transuranic elements with Atomic Numbers up to 118 have been produced, but only named up through element 111; folks are still fighting over who gets to name the most recent ones (based on my experience in academia, scientists love to argue).

Dr. Bernstein shows how a greater-than-usual number of missteps occurred on both sides of the European Theater of WWII, as neither Germans nor English-speaking scientists realized how similar the chemistry of the cluster of elements from Uranium onward would be. Early speculative articles about elements such as 94 (not named at first) declared that separation of the new element from Uranium ought to be "simple and easy." It is anything but.

By analogy, the so-called Rare Earths, which are not so rare, and are metals (but their oxides were called Earths), all have very similar chemistry, because their outer electronic configuration stays the same while added electrons (to balance the added protons) go into an inner shell that has little influence on chemical behavior.

For the uninitiated: Chemistry is all about how the outer few electrons of an atom attract another atom's electrons as atoms approach one another. This can be complex, mainly because there are 90-plus different kinds of atoms, and each has its quirks. But there are regularities. The "alkali metals", Lithium, Sodium, Potassium, Rubidium, and Cesium, behave in similar ways, including their ability to burn on contact with water; however, Lithium is the mildest, while Cesium's reaction is rather explosive. This is because all have a single loosely-bound electron that does all the chemistry for them, and the binding is looser (so reactivity is greater) for the heavier members of this set.

By contrast, Sodium is element 11, and Magnesium is #12, but while both are reactive, Magnesium is less so than Sodium, even less so than Lithium. Magnesium is more similar to Calcium (#20), which has an outer electron configuration like Magnesium's: two outer electrons, not at loosely bound as Sodium's singleton.

The politics, chemistry, physics, engineering, and metallurgical conundrums encountered as Pu was produced, first in microgram quantities, then in milligrams, grams, kilograms, and finally by the ton, have formed an undercurrent of the past two generations' history, from 1938 onward. Today there are a 150 or so tons of "weapons grade" metal stored by half a dozen countries, and perhaps 1,700 tons of "reactor grade" mixes of Pu isotopes (versions with different numbers of neutrons, but all having 94 protons). The author makes the point that reactor grade plutonium can also be used to fabricate a bomb. It just takes more of it...and another 70 tons are produced every year.

In the current world climate, some things are likely to get very, very bad before politicians have to bow to social realities and take very, very good care of the stuff!

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