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Pu, A. Jonathan Buhalis
by Jonathan Buhalis


The state of chemistry around 1930 was that almost all of the elements in the periodic table had been found. Every element in the table has a number, and it was known by then that this atomic number is that element's number of protons.

The highest atomic number known was 92, uranium. But, might heavier elements exist or be created? A cyclotron, invented in 1932, is an electromagnetic machine that accelerates fundamental particles to strike a target substance. One use of the cyclotron is to accelerate a particle into an atom (skipping over some details) to raise its atomic number.

The first cyclotron was at the University of California at Berkeley. A team there bombarded atoms of uranium in 1940 and then identified element 93 and element 94 in the result. As Neptune and Pluto were the planets beyond Uranus, the elements were named neptunium and plutonium.

In ordinary times, the trace amount of the new element would be carefully analyzed over the course of years. But, this was wartime. One isotope (variant) of plutonium was capable of fission. It could power an atomic bomb. All research on plutonium became classified.

pellet of plutonium oxide, A. Jonathan BuhalisThe properties of plutonium were investigated very quickly during 1942-43 at a secret laboratory at the University of Chicago. Plutonium was chemically much like uranium. Later, it would be determined that uranium, neptunium, plutonium, and all elements from 89 actinium to not-yet-discovered 103 lawrencium formed a series resembling the series of similar elements called the lanthanides or rare earths.

Meanwhile, as part of America's Manhattan Project, nuclear reactors at Oak Ridge, Tennessee, and then Hanford, Washington, were producing large amounts of plutonium. This would be incorporated into the atomic bomb, code named "Fat Man", under construction at the Los Alamos Laboratory in New Mexico. The design of the Hanford reactor took into consideration chemical and physical properties of plutonium that had been scaled up from microgram samples. The difference was a factor of ten million, which physicist Glen Seabord called "surely the greatest scale up factor ever attempted".

The plutonium test explosion in the New Mexico desert in 1945 was a success, and Fat Man exploded over Nagasaki, Japan. Plutonium has been a part of technology and politics ever since.

Occurrence and Production

Although plutonium was first synthesized rather than mined, trace amounts of it have since been discovered in uranium ore deposits in Gabon and elsewhere. When a uranium atom undergoes radioactive decay, it occasionally and naturally emits a particle into a nearby uranium atom, mimicking what was done at the Hanford reactor.

Additionally, the longest-lived isotope of plutonium decays by 50% after 80 million years (its half-life). This is long enough that some primordial plutonium in existence when the Earth was formed is still around.

Nevertheless, the overwhelming majority of plutonium is created in nuclear reactors from uranium. Plutonium has no chemical applications, so all of the plutonium created is specific isotopes for particular nuclear uses.

Properties and Uses

Plutonium is a silvery metal that tarnishes in air. It is very dense. Plutonium has low electrical conductivity, for a metal, and it has the unusual property (shared with water) that it expands when it freezes. (Solid plutonium floats on liquid plutonium.)

All isotopes of plutonium are radioactive. Plutonium-239 (Pu- 239) undergoes self-sustaining nuclear fission. In other words, Pu-139 can power a nuclear bomb or a reactor.

Mars Rover Curiosity, A. Jonathan BuhalisPlutonium-238 generates considerable heat as it decays with a half-life of 88 years. Pu-238 is therefore used as a power source in NASA's spacecraft that must operate for years or decades unattended (Curiosity rover, right). Pu-238 was also used once to power pacemakers. A plutonium battery is good for decades before surgery is required to replace it. However, lithium batteries developed in the 1970s became a less problematic option.

Clean Tech Implications

Plutonium has good and bad implications for clean tech. On the positive side, a plutonium reactor generates much more energy for its size than a solar or wind farm. Unlike a coal-fired plant, the nuclear power plant emits no conventional pollution.

On the negative side of the ledger, plutonium is both toxic and radioactive. It is a heavy metal that accumulates in the bones if inhaled, more slowly if ingested. The chemical toxicity is very significant, but is nevertheless sometimes overstated. As a radiation source, plutonium is most dangerous when inhaled into the lungs.

Plutonium also presents a real social and political challenge in that reactors and decommissioned weapons generate waste. The problem can be reduced when waste is processed and recycled into reactors of a different design. Eventually, though, the radioactive byproducts must be confined and stored for a period of generations or more. The engineering details and political choices involved in very long-term storage are still under debate.

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