A dirty bomb explosion would cause chaos
It wouldn't take much for terrorists to wreak havoc in London - just a simple explosive and some industrial waste. Such is the gruesome reality of the dirty bomb.
Fears of a terrorist attack on the UK by Islamic extremists are running at an all-time high.
The discovery of the deadly poison ricin in a London flat has heightened concerns and recently Tony Blair said it was not a case of "if" but "when".
One frightening possibility is the so-called dirty bomb - a crudely-made device that combines a simple explosive with any radioactive material. The idea is that the blast disperses the radioactive material willy nilly.
The dirty bomb is perhaps the least understood of all terror weapons, but new research by BBC Two's Horizon programme brings home the full horror of how a dirty bomb attack might affect London.
We'd have contaminated air moving across London with no indication it's there
The dirty bomb is sometimes called the "poor man's nuclear weapon". But whereas the aim of a nuclear bomb is instant and outright destruction, a dirty bomb would have an entirely different effect.
It would wreak panic in built-up areas, see large areas contaminated and closed off and result in long-term illnesses such as cancer, caused by the dispersed radioactive material attacking living cells.
Using sophisticated modelling, experts commissioned by Horizon constructed a scenario around a radioactive material called caesium chloride, which in the old Soviet Union was used in seed irradiating.
Much of this and other radioactive material used by the Soviets is now unaccounted for. No one knows whether it has fallen into unsafe hands.
Experts working for Enviros, a consultancy that advises nuclear authorities around the world, modelled a fictional explosion combining a handful of caesium chloride - equivalent to the contents of one Soviet seed irradiating machine - with 10lbs of explosive.
Arrested in May 2002 at Chicago airport in the US
Also known as Abdullah al-Mujahir, he is accused of planning a dirty bomb attack on America
He has been called an al-Qaeda operative
They then "placed" the fictional bomb in Trafalgar Square.
The blast itself might kill 10 people immediately. As the emergency services arrived at the scene of the incident a few minutes later, they would realise this was no ordinary blast.
"The simple buoyancy of the air that's been heated may carry the radioactive material tens of metres up into the air," says Graham Smith, of Enviros.
Almost immediately, millions of tiny flakes of caesium chloride would be floating in the breeze over London. In seconds, depending on the direction of the wind, the plume could reach Whitehall. A minute later Charing Cross, then the City and within half an hour radioactive smoke could reach London's suburbs - 10 kilometres away.
"We've got contaminated air moving across London and there would be no indication that contamination was there," says Mr Smith.
Cancer time bomb
As the air began to cool, the particles would fall on people who are completely unaware of the danger around them. They would settle on parks, gardens, pavements and cars.
A TV reconstruction of the Goiania disaster
The worry then is of a cancer time bomb. Every day we are exposed to natural radiation, and in low doses this background level is harmless.
Anyone five km from the blast would face only a tiny increased risk of cancer - one in 1,000 - as the background level would be largely unaffected.
But at one km, radiation doses would rise to six times background level, increasing the risk of cancer by about one in 100. At 500 metres downwind from the blast, the risk of dying of cancer from this radiation exposure would be about one in 50 and at 200 m radiation levels would be 80 times background level, equating to a one in seven increased risk of dying of the disease.
The next challenge would be to deal with the contaminated parts of central London. Any clean-up job would be immense and costly, but left undisturbed the particles could remain harmful for 200 years.
Swathes of the city could be cordoned off
One option might be to abandon or demolish parts of the city.
But perhaps the biggest immediate threat wrought by a dirty bomb is not the destruction or the threat to life, but its ability to stir blind panic among thousands, maybe millions, of people.
A leak of caesium chloride in the city of Goiania, southern Brazil, in 1987 contaminated 200 people. The experience gives a useful template for how other cities might cope.
When news of an attack breaks, there will be a clamour for information and help. People will want to know the extent of contamination, but it can be hard to supply answers.
In the Brazil example, medical services were swamped as a tenth of the city's population queued for radiation screening.
It is the dirty bomb's power to spread fear and spawn chaos that makes it a really effective weapon.
Horizon - Dirty Bomb was on BBC Two on Thursday 30 January at 2100 GMT.