convoy of seven lorries rumbled past, armed police in the cabs and radioactive warning signs stuck on the shipping containers they carried

Frantic policemen, some wearing ski masks and all armed with submachineguns, flashed their headlights and leant out of their patrol car windows, shouting and waving to make the traffic pull over and stop at the side of the road as helicopters clattered overhead.Then a convoy of seven lorries rumbled past, armed police in the cabs and radioactive warning signs stuck on the shipping containers they carried.
The frightened-looking motorists and their families didn't know it but this convoy two weeks ago wasn't an emergency; it was no exercise though, and the cargo being moved through the Warsaw suburbs in a top secret operation was the stuff of nightmares.
The lorries carried enough bomb-grade uranium for terrorists to build eight nuclear devices, sealed inside thick metal flasks weighing five tons each to stop radiation leaking.
The shipment, at the beginning of a 3,500 mile journey to a Russian reprocessing plant where it will be made safe, was part of an effort to secure hundreds of tons of highly enriched uranium worldwide before terrorists can acquire it.
"The world is a safer place because of this shipment," said Andrew Bieniawski, a senior official with the US government's Global Threat Reduction Initiative, as the convoy and its sinister-looking escort of Polish special forces police started off.
American intelligence officials believe that if al Qaeda could get its hands on a piece of highly enriched uranium (HEU) the size of a grapefruit, let alone a consignment as big as the Polish one, the destruction of a city like London, New York or Washington would follow.
So far, such a nightmare has been confined to Hollywood thrillers. But the US government is so concerned at the threat of nuclear terrorism that next year the budget for making bomb-grade material secure worldwide will be increased by 67 per cent to $558 million dollars (£352 million).
The American effort, constantly expanded since the attacks of September 11th 2001, is intended to deal with weapons-grade uranium in 28 nations around the world, most of it the Cold War legacy of the Atoms for Peace programmes when America and Russia shared nuclear secrets with their allies.
In the 1950s they helped spread civilian nuclear power plants and research reactors around the world, to win friends and help mankind benefit from cheap electricity and medical isotopes; but the unforeseen result has been a stockpile of deadly spent fuel - HEU - which can be used as the raw material for the type of atom bomb used at Hiroshima.
Most of the HEU in Eastern Europe has been stored since Soviet times, often in badly maintained and poorly guarded facilities where for years underpaid staff were potentially vulnerable to bribery by well-funded terrorists
Last year a massive new effort to dramatically reduce the amount of civilian HEU worldwide was announced by President Barack Obama in a high-profile speech in Prague, his first major foreign policy speech delivered abroad.
The President has made countering nuclear terrorism a top priority and described it as "the greatest danger we face". He has committed the United States to secure the world's vulnerable civilian bomb-grade material by the end of 2013.
He has taken the threat so seriously that over the next three years the President wants to spend $7.9 billion on nuclear nonproliferation programmes, including homeland security to detect nuclear bombs or material being smuggled into America, as well as programmes like the Global Threat Reduction Initiative.
The shipment in Poland was the biggest the Americans have organised anywhere.
It started its journey at a nuclear research reactor in a forest outside Warsaw, where HEU had been stored in cooling ponds for years. The convoy, escorted by more than 100 policemen, moved rapidly to a railway yard on the outskirts of the capital where it was loaded onto a goods train for the overnight journey 200 miles north to the port of Gdansk. The route took it past villages and towns whose sleeping inhabitants had no idea of the deadly cargo passing so close to them. At every stage technicians checked that radioactivity was not leaking.
On arrival in Gdansk it was loaded onto a specially converted ship, with thick metal radiation-proof plates installed, for a sea voyage to the Russian Arctic port of Murmansk. The consequences of spreading radiation in a crash ruled out air transport.
In Murmansk it was loaded on to another train for the last stage of the journey, hundreds of miles across Russia to a reprocessing plant beyond the Ural mountains, deep in Siberia.
In the past year this journey, lasting three weeks, has been repeated five times, moving 1,000 lbs of Polish HEU in total - enough to make 18 atom bombs - at a cost to the US taxpayer of $60 million.
The Global Threat Reduction Initiative decided to forgo its usual secrecy rules and invite The Sunday Telegraph to observe the final shipment, in order to make its work in Poland public; details can now be revealed after it arrived safely at its destination.
Intelligence agencies will not reveal their reasons for being so frightened now about what for years seemed a remote and unlikely risk.
But it may be because of the deeply troubling cases of smuggling that surface from time to time in Eastern Europe, hinting at the existence of a nuclear black market.
Such attempts at illicit nuclear sales have been made at least twice this year, once in Moldova, when a gang attempted to sell a small amount of nuclear material, and once in Georgia where several smugglers were arrested with an undisclosed amount of uranium. That was a far more disturbing case, according to investigators who said it showed a worrying level of organisation.
Since the end of the Cold War the International Atomic Energy Authority, the UN's nuclear watchdog, has logged 800 incidents of radioactive material going missing or being seized by smugglers. A handful of cases have involved weapons-grade material.
Nobody knows whether gangsters or corrupt officials really could deliver enough material for a home-made bomb to terrorists or rogue states. US officials fear that anyone trying to acquire HEU on a nuclear black market will want it to destroy an American city.
"We know that terrorists are actively seeking to acquire this material to target the United States," Mr Bieniawski said. "If they acquire it, they have basically overcome the main hurdle to getting a bomb. The risk is low but we can't just trust to luck when we are talking about the catastrophic effects of a nuclear weapon." Since work started in 2004 HEU has been removed from 18 nations, including five in the past year - Romania, Libya, Taiwan, Turkey, and Chile, where the shipment was briefly delayed by February's earthquake.
Until the attacks of September 11, 2001, there was little concern about the estimated 2000 tons of HEU stockpiled around the world, much lying around half-forgotten in badly-guarded facilities in poor countries with corruption problems.
Facilities often lacked armed guards, secure fences, even locks that worked properly.
American efforts which had begun in the chaos of Russia in the 1990s to secure vulnerable nuclear material were stepped up worldwide after 2001; as long ago as 1998 Osama bin Laden spoke of his determination to acquire the bomb "to terrorise the enemies of God".
Frank Barnaby, an author and former Aldermaston nuclear physicist, said: "The really frightening thing about HEU is that it is so easy to make an atom bomb out of it. You only need a couple of PhD students and a small amount of material.
"I think we should be very frightened about the possibility of nuclear terrorism; I'm surprised it hasn't happened yet." The American experts hope their work with 130 nations will make that nightmare less likely.
In some cases they have strengthened defences at plants judged vulnerable to theft.
But their preferred method is to remove HEU for reprocessing. "That way it is made safe, permanently," Mr Bieniawski said.
Poland, like many nations with HEU stockpiles, has to send the material abroad because it has no reprocessing plant of its own.
The American officials were at pains to stress that they have complete trust in Russia to keep to its end of the deal and reprocess the uranium sent inside its borders in US-funded shipments.
Under an agreement between Mr Obama and Russian President Dmitry Medvedev last year, the USA and Russia each take back uranium they supplied to friendly countries. So Romania and Poland sent theirs to Russia, while that of Chile and Turkey has been reprocessed in the United States.
There are, however, glaring omissions in President Obama's plan; the Global Threat Reduction Initiative cannot make HEU safe in a few nuclear nations, most notably Pakistan and North Korea, which are judged to pose the greatest risk of terrorists obtaining the raw material for a bomb.
As a Polish member of the team working on the operation said: "This shipment makes nuclear nightmares less likely."