molecule of the month: plutonium

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.

Suggested Topics
Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Sales Administrator - Spanish Speaking

£17000 - £21000 per annum: Recruitment Genius: This is a fantastic opportunity...

Recruitment Genius: Sales Administrator - German Speaking

£17000 - £23000 per annum: Recruitment Genius: This is a fantastic opportunity...

Recruitment Genius: Sales Administrator - Japanese Speaking

£17000 - £23000 per annum: Recruitment Genius: If you are fluent in Japanese a...

Recruitment Genius: Graphic Designer - Immediate Start

£16000 - £25000 per annum: Recruitment Genius: This is a fantastic opportunity...

Day In a Page

Is this the future of flying: battery-powered planes made of plastic, and without flight decks?

Is this the future of flying?

Battery-powered planes made of plastic, and without flight decks
Isis are barbarians – but the Caliphate is a dream at the heart of all Muslim traditions

Isis are barbarians

but the Caliphate is an ancient Muslim ideal
The Brink's-Mat curse strikes again: three tons of stolen gold that brought only grief

Curse of Brink's Mat strikes again

Death of John 'Goldfinger' Palmer the latest killing related to 1983 heist
Greece debt crisis: 'The ministers talk to us about miracles' – why Greeks are cynical ahead of the bailout referendum

'The ministers talk to us about miracles'

Why Greeks are cynical ahead of the bailout referendum
Call of the wild: How science is learning to decode the way animals communicate

Call of the wild

How science is learning to decode the way animals communicate
Greece debt crisis: What happened to democracy when it’s a case of 'Vote Yes or else'?

'The economic collapse has happened. What is at risk now is democracy...'

If it doesn’t work in Europe, how is it supposed to work in India or the Middle East, asks Robert Fisk
The science of swearing: What lies behind the use of four-letter words?

The science of swearing

What lies behind the use of four-letter words?
The Real Stories of Migrant Britain: Clive fled from Zimbabwe - now it won't have him back

The Real Stories of Migrant Britain

Clive fled from Zimbabwe - now it won’t have him back
Africa on the menu: Three foodie friends want to popularise dishes from the continent

Africa on the menu

Three foodie friends want to popularise dishes from the hot new continent
Donna Karan is stepping down after 30 years - so who will fill the DKNY creator's boots?

Who will fill Donna Karan's boots?

The designer is stepping down as Chief Designer of DKNY after 30 years. Alexander Fury looks back at the career of 'America's Chanel'
10 best statement lightbulbs

10 best statement lightbulbs

Dare to bare with some out-of-the-ordinary illumination
Wimbledon 2015: Heather Watson - 'I had Serena's poster on my wall – now I'm playing her'

Heather Watson: 'I had Serena's poster on my wall – now I'm playing her'

Briton pumped up for dream meeting with world No 1
Wimbledon 2015: Nick Bollettieri - It's time for big John Isner to produce the goods to go with his thumping serve

Nick Bollettieri's Wimbledon Files

It's time for big John Isner to produce the goods to go with his thumping serve
Dustin Brown: Who is the tennis player who knocked Rafael Nadal out of Wimbeldon 2015?

Dustin Brown

Who is the German player that knocked Nadal out of Wimbeldon 2015?
Ashes 2015: Damien Martyn - 'England are fired up again, just like in 2005...'

Damien Martyn: 'England are fired up again, just like in 2005...'

Australian veteran of that Ashes series, believes the hosts' may become unstoppable if they win the first Test