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
Sport
world cup 2014A history of the third-place play-offs
News
Tommy Ramone performing at The Old Waldorf Nightclub in 1978 in San Francisco, California.
peopleDrummer Tommy was last surviving member of seminal band
Life and Style
Swimsuit, £245, by Agent Provocateur
fashion

Diving in at the deep end is no excuse for shirking the style stakes

Sport
The Mexico chief finally lets rip as his emotions get the better of him
world cup 2014
PROMOTED VIDEO
Voices
Spectators photograph the Tour de France riders as they make their way through the Yorkshire countryside
voicesHoward Jacobson: Line the streets for a cycling race? You might just as well watch a swarm of wasps
Life and Style
lifeHere's one answer to an inquisitive Reddit user's question
Life and Style
Several male celebrities have confessed to being on a diet, including, from left to right, Hugh Grant, Benedict Cumberbatch and Ryan Reynolds
...and the weight loss industry is rubbing its hands in glee
News
peopleDave Legeno, the actor who played werewolf Fenrir Greyback in the Harry Potter films, has died
Sport
Mario Balotelli, Divock Origi, Loic Remy, Wilfried Bony and Karim Benzema
transfersBony, Benzema and the other transfer targets
Sport
Yaya Touré has defended his posturing over his future at Manchester City
News
Detail of the dress made entirely of loom bands
news
News
ebookA unique anthology of reporting and analysis of a crucial period of history
Life and Style
beauty
Sport
There were mass celebrations across Argentina as the country's national team reached their first World Cup final for 24 years
transfersOne of the men to suffer cardiac arrest was 16 years old
Arts and Entertainment
Armando Iannucci, the creator of 'The Thick of It' says he has
tvArmando Iannucci to concentrate on US show Veep
News
A mugshot of Ian Watkins released by South Wales Police following his guilty pleas
peopleBandmates open up about abuse
Sport
Basketball superstar LeBron James gets into his stride for the Cleveland Cavaliers
sportNBA superstar announces decision to return to Cleveland Cavaliers
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
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

Information Security Manager (ISO 27001, Accreditation, ITIL)

£70000 per annum: Harrington Starr: Information Security Manager (ISO 27001, A...

C# Developer (HTML5, JavaScript, ASP.NET, Mathematics, Entity)

£30000 - £45000 per annum + Benefits + Bonus: Harrington Starr: C# Developer (...

C# Integration Developer (.NET, Tibco EMS, SQL 2008/2012, XML)

£60000 - £80000 per annum + Benefits + Bonus: Harrington Starr: C# Integration...

Biztalk - outstanding opportunity

£75000 - £85000 per annum + ex bens: Deerfoot IT Resources Limited: Biztalk Te...

Day In a Page

A History of the First World War in 100 Moments: Peace without magnanimity - the summit in a railway siding that ended the fighting

A History of the First World War in 100 Moments

Peace without magnanimity - the summit in a railway siding that ended the fighting
Scottish independence: How the Commonwealth Games could swing the vote

Scottish independence: How the Commonwealth Games could swing the vote

In the final part of our series, Chris Green arrives in Glasgow - a host city struggling to keep the politics out of its celebration of sport
Out in the cold: A writer spends a night on the streets and hears the stories of the homeless

A writer spends a night on the streets

Rough sleepers - the homeless, the destitute and the drunk - exist in every city. Will Nicoll meets those whose luck has run out
Striking new stations, high-speed links and (whisper it) better services - the UK's railways are entering a new golden age

UK's railways are entering a new golden age

New stations are opening across the country and our railways appear to be entering an era not seen in Britain since the early 1950s
Conchita Wurst becomes a 'bride' on the Paris catwalk - and proves there is life after Eurovision

Conchita becomes a 'bride' on Paris catwalk

Alexander Fury salutes the Eurovision Song Contest winner's latest triumph
Pétanque World Championship in Marseilles hit by

Pétanque 'world cup' hit by death threats

This year's most acrimonious sporting event took place in France, not Brazil. How did pétanque get so passionate?
Whelks are healthy, versatile and sustainable - so why did we stop eating them in the UK?

Why did we stop eating whelks?

Whelks were the Victorian equivalent of the donor kebab and our stocks are abundant. So why do we now export them all to the Far East?
10 best women's sunglasses

In the shade: 10 best women's sunglasses

From luxury bespoke eyewear to fun festival sunnies, we round up the shades to be seen in this summer
Germany vs Argentina World Cup 2014: Lionel Messi? Javier Mascherano is key for Argentina...

World Cup final: Messi? Mascherano is key for Argentina...

No 10 is always centre of attention but Barça team-mate is just as crucial to finalists’ hopes
Siobhan-Marie O’Connor: Swimmer knows she needs Glasgow joy on road to Rio

Siobhan-Marie O’Connor: Swimmer needs Glasgow joy on road to Rio

18-year-old says this month’s Commonwealth Games are a key staging post in her career before time slips away
The true Gaza back-story that the Israelis aren’t telling this week

The true Gaza back-story that the Israelis aren’t telling this week

A future Palestine state will have no borders and be an enclave within Israel, surrounded on all sides by Israeli-held territory, says Robert Fisk
A History of the First World War in 100 Moments: The German people demand an end to the fighting

A History of the First World War in 100 Moments

The German people demand an end to the fighting
New play by Oscar Wilde's grandson reveals what the Irish wit said at his trials

New play reveals what Oscar Wilde said at trials

For a century, what Wilde actually said at his trials was a mystery. But the recent discovery of shorthand notes changed that. Now his grandson Merlin Holland has turned them into a play
Can scientists save the world's sea life from

Can scientists save our sea life?

By the end of the century, the only living things left in our oceans could be plankton and jellyfish. Alex Renton meets the scientists who are trying to turn the tide
Richard III, Trafalgar Studios, review: Martin Freeman gives highly intelligent performance

Richard III review

Martin Freeman’s psychotic monarch is big on mockery but wanting in malice