The metal cylinders the three were trying to peddle, however, was revealed by a preliminary security service analysis to contain the radioactive substance americium produced in the Soviet Union.

The SBU said in a statement that the substance had been produced on Russian territory in the Soviet era and could have been transferred to Ukraine from a neighbouring state.

The men – identified as a member of the Ternopil regional parliament and two businessmen – believed they were selling 3.672kg of radioactive plutonium-239, a material used a the core of a nuclear weapon.

Ukraine has no official possibilities to produce plutonium, though dangerous isotopes can be extracted from spent nuclear fuel. There have been no reports that spent nuclear fuel has been seized.

The men have been charged with illegal handling of radioactive material and face from eight to 15 years in prison. Their identities have not bee released.

Although the plot to sell the material – which exceeded normal background radiation levels by 250 times – was foiled, it is a jarring wake-up call that there are still massive holes through which radioactive materials can pass from the territory of the former Soviet Union, despite efforts by Russia and the international community.

Marina Ostapenko, a spokeswoman for the Ukrainian Security Service, told Bellona Web in a telephone interview from Kiev that the material had turned out to be americium, a far more common but less potent radioactive material.

She said americium could be deployed in a dirty bomb but not in a nuclear weapon.

“They wanted to sell it as plutonium,” she said. “They were asking for $10 million for it because they thought that it was plutonium.”

Joseph Cirincione, president of the Ploughshares Foundation, a public grant-making foundation focused on nuclear weapons policy and conflict resolution, and former director of the Carnegie Endowment for International Peace confirmed that the Americium would have been ideal for a dirty bomb.

“Radioactive materials, like those seized in the Ukraine, are the deadly makings for dirty bombs,” wrote Cirincione in the Huffington Post.

“A terrorist could scatter a few ounces of americium with a conventional explosive in it any of the world’s cities, contaminating a ten-block radius with radioactive poison.”

The International Atomic Energy Agency (IAEA) reports that governments caught 827 nuclear smuggling incidents between 1993 and 2005 – several involving plutonium or highly enriched uranium.

“Luckily the Ukraine incident did not involve material for a nuclear explosive weapon. But it easily could have,” Cirincione said.

Matt Bunn, head of Harvard’s Managing the Atom Project, has said that despite US and Russian efforts, less than two-thirds of the nuclear weapons materials in the former Soviet Union have been fully secured since 1991.

The American government has spent hundreds of millions of dollars in recent years to try to help governments in Russia, Ukraine and other former republics protect nuclear materials, most of which has been furnished by the Cooperative Threat Reduction programme and other affiliated efforts through the US Energy Department and Department of State.

In May of 2006, a US Government Accountability Office investigation was able to send a team carrying enough radioactive material to make two dirty bombs into the United States, even after it set off alarms on radiation detectors installed at border checkpoints.

In 2008, a worker at Ukraine’s embassy in Germany was arrested on charges of attempted smuggling of million of dollars worth of radioactive materials in the Ukrainian city of Cherkassy.

In 2006, officials in the former Soviet republic of Georgia arrested a Russian man who was offering to sell 100 grams of highly refined uranium, about 3.5 ounces, for $1 million. The Russian was made to believe by undercover agents that the radioactive material was to be delivered to a Muslim organization.

In 2005, Two men were arrested in Istanbul while trying to sell uranium of Russian origin in a sting operation conducted by Turkish special police.

The 2008 conflict between Georgia and Russia did little to help stem the smuggling of nuclear materials from Russia. A key US Energy Department programme set up to identify possible smugglers of nuclear bomb components across the porous borders of South Ossetia and Georgia was severaly crippled by fighting.

The National Nuclear Security Administration (NNSA) a semi-autonomous division of the DOE, which oversees the department’s nuclear nonproliferation efforts in the former Soviet Union, reported that in 2006 Georgian authorities detected and seized small amounts of plutonium and cesium-137 during two separate smuggling attempts, according to the Stanford University Database on Nuclear Smuggling, Theft and Orphan Radiation Sources, one of the world’s most comprehensive nuclear smuggling tracking projects.

In 2002, five men in Belarus were arrested for attempting to sell 1.5 kilograms of uranium oxide that was smuggled from Chernobyl for a black market price of $800,000.

Over the past decade other radioactive materials, including plutonium, have also been intercepted on the black market, said Georgian officials in statements that were confirmed by *a database at Stanford University that tracks reports of nuclear smuggling attempts.

Russia’s former nuclear oversight body, Gosatomnadzor, stated in 2006 that nuclear materials had been disappearing from Russian nuclear sites for 10 years.

Cirincione wrote that smugglers exploiting still lax security, but that there is much that can be done to prevent it.

In Prague earlier this month, President Obama announced a new global initiative to secure all vulnerable nuclear material around the world within four years. The president plans to host a global summit on nuclear security within the year.

"We must act with a sense of purpose without delay," said Obama.

“A truly scary aspect of this latest incident is that these Ukrainian businessmen thought they had plutonium and still were willing to sell it,” wrote Cirincione. “This time we averted a nuclear catastrophe. Next time we might not be so lucky.”