molecule of the month: plutonium

John Emsley
Monday 21 August 1995 23:02

Fifty years ago this month, plutonium, a new, man-made element, burst on to an unsuspecting world when it was used in a bomb over Nagasaki, killing 70,000 people. (The earlier bomb on Hiroshima contained uranium.) Today, there are about 1,200 tons of plutonium, of which 200 have been made for bombs; the rest has accumulated as a by-product of the nuclear power industry. It seems we are destined to live with it for hundreds of thousands of years.

Britain has a stockpile of more than 100 tons of "civil" plutonium. Some resides in the fuel within our power-generating nuclear reactors, but most is held by British Nuclear Fuels at Sellafield, in Cumbria. BNFL stores about 85 tons, of which some 30 tons belong to overseas customers. The size of the British military stockpile of plutonium (much of it also at Sellafield) is, of course, an official secret.

Until 55 years ago, plutonium did not exist on Earth. Glen Seaborg, Arthur Wahl and Joseph Kennedy were the first to make atoms of plutonium in December 1940 at Berkeley, California, by bombarding uranium oxide with deuterium. They christened it plutonium after the outermost planet, Pluto, because it came after uranium and neptunium in the sequence of elements. These had been named after Uranus and Neptune.

Seaborg and colleagues quickly realised they had stumbled upon a remarkable metal. Its most compelling property was that it was fissile - when an atom of plutonium was hit by a neutron it split, releasing a lot of energy and expelling more neutrons. These could split more atoms, starting a chain reaction which, given a certain minimum amount of metal, could end in an explosion. This minimum, the so-called critical mass, was a surprisingly small four kilograms - about the size of an apple.

Within a year, Seaborg's group had made enough plutonium to be visible, and by the end of 1941, enough to weigh - three-millionths of a gram. By mid-1945, enough plutonium had been made for two atomic bombs, the first of which was tested at Alamogordo, New Mexico, in July. So began nuclear contamination of the planet.

Only about a quarter of the plutonium in an atomic bomb explodes; the rest vaporises. The same is true of hydrogen bombs, which have a plutonium bomb at their core. Consequently, during the Fifties, when many such bombs were tested above-ground, enough plutonium was scattered to the winds to ensure that we each now have a few thousand atoms in our body.

Plutonium is dangerous because it tends to concentrate on the surface of bones rather than being uniformly distributed throughout bone mass like other heavy metals. This is why permissible body levels of plutonium are the lowest for any radioactive element. It decays by emitting alpha- particles, feeble enough to be stopped by paper or skin, but able to damage DNA and maybe start cancers such as leukemia. In a steel can or even a plastic bag, a small piece of plutonium is safe to handle, and feels permanently warm due to its radioactivity.

Plutonium has a density of 20kg per litre - slightly higher than gold - and melts at 641C. The metal is unusual in that it can exist in six forms, and will change under its own internal heat. As it nears its melting point, it actually shrinks as it converts from one form to another. Plutonium is a relatively poor conductor of heat or electricity. The pure metal is as brittle as cast iron, but alloyed with 1 per cent aluminium, it becomes as soft as copper.

Plutonium is chemically very reactive and combines with oxygen to form the oxide PuO2. This is potentially very dangerous, as scientists at Los Alamos National Laboratory, New Mexico, discovered in 1993. A canister of the metal that had not been made air-tight split under the pressure of the metal oxidising because the oxide is 40 per cent larger in volume than the metal itself.

Accidents worry us because unwanted plutonium will have to be stored safely for hundreds of thousand of years - its half-life is 24,000 years. The Americans plan to fuse plutonium oxide with oxides of silicon, boron and gadolinium to turn it into glass. The boron and gadolinium will ensure any neutrons are safely absorbed.

Nor need we fear that these glass logs might slowly be attacked by water, which would leach out the plutonium. Plutonium oxide is one of the least soluble of oxides - a million litres of water dissolves one atom. Plutonium oxide glass is even less soluble.

The author is science writer in residence at the chemistry department of Imperial College, London.

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