July 27, 2006 5:35 AM

VINCA, Serbia-The Vinca reactor stands still, its decrepit innards purged of their unused weapons-grade fuel. But it remains Serbia's little shop of nuclear horrors, and a potential magnet for terrorists.

That makes it representative of the next step in the world's quest to lift the threat of nuclear material falling into the wrong hands, first by taking control of the fuel that makes atomic bombs, and now by tackling the lesser but still potent menace of a dirty bomb, meaning radiation spread by blowing up radioactive material with conventional explosives.

At the Vinca Institute of Nuclear Sciences outside Belgrade, there are only a few armed guards in sight, and the barbed-wire fence around the 48-acre facility is only as tall as a man.

For would-be terrorists, "it's almost like a candy store," says Mike Durst, the International Atomic Energy Agency's point man working to strip Vinca of its attraction to nuclear thieves.

These fears are driving international agendas. Presidents Bush and Vladimir Putin used a summit of the world's richest countries earlier this month to launch the "Global Initiative to Combat Nuclear Terrorism," which calls for better accounting and protection of the Vincas of the world, scattered around the former Soviet Union, Eastern Europe, Asia and Africa.

The new program is meant to build on others created by the Bush administration, including the 3-year old "Global Threat Reduction Initiative" to deal with a broad range of vulnerable nuclear and radiological materials around the world.

Most of the existing programs focus on unused weapons-grade fuel, nearly 100 pounds of which lay in Vinca until four years ago, when Washington, Moscow and Belgrade mounted a joint operation to remove it.

Helicopters and 1,200 heavily armed troops including snipers were deployed along with decoy trucks to thwart potential mischief-makers. Half of Belgrade was sealed off, and within six hours, the fuel, enough to make at least two simple nuclear warheads, was trucked from Vinca to the airport and onward to a Russian government plant about 470 miles east of Moscow.

But that still leaves dozens of other badly secured and dangerous nuclear facilities to deal with.

Inside the Vinca reactor building, 8,000 spent fuel rods sit in pools of brackish water. Dozens contain uranium in varying degrees of enrichment, potential dirty bomb material, not to mention the environmental hazard.

The bomb-worthy material is not uranium, but its highly radioactive byproducts. These would quickly kill any terrorist who was not equipped with protective suits, robotic arms and tons of lead to encase the stolen material.

Still, research reactors such as Vinca tend to be less heavily protected than power plants, and experts like Durst fear terrorists shown willing to sacrifice their lives in other situations might do so as well to secure the material. And while building a full-blown nuclear device is technologically daunting, terrorists could easily use the material such as that in the rods to construct a dirty bomb.

With just one dirty bomb, "you could hit Broadway, and you couldn't decontaminate it for years," says Obrad Sotic, Vinca's former operations manager.

And there are concerns other than raids on Vinca. While no nuclear material is known to have gone missing employees speak openly of the potential temptations of selling some on the black market as a way supplementing monthly incomes of less than $750.

There's a lot to steal, old medical and industrial equipment, and tons of material inside the reactor or in two rickety corrugated metal sheds. There are bags of irradiated grass, containers of depleted uranium ammunition fired by NATO during its 1999 Kosovo campaign, and several tons of yellowcake, processed uranium ore of the kind Iran plans to process and enrich in what the U.S. says is an attempt to make nuclear arms.

The Serbian Science Ministry, which is responsible for Vinca, has a budget of less than $90 million for this year. That wouldn't cover the cost of upgrading security, shipping the spent fuel back to Russia and dismantling the reactor.

A centrally monitored alarm system is being installed and police will be tasked with security under a plan being worked out under IAEA guidance.

Also foreseen is the shipment of the spent fuel to Russia and building safer storage facilities for the collected nuclear junk. The ultimate goal is to dismantle of the reactor and other parts of the facility.

But again, money is a problem.

Sending the spent fuel back to Russia will cost around $10 million, and more money is needed to reprocess the fuel in Russia. Building better storage will cost an additional $5 million. About 60 percent of that amount has been pledged by donor countries, but dismantling the facility will cost some $60 million.

For Serbia's science minister, Aleksandar Popovic, the 2002 operation to remove the weapons-grade fuel has left the job only half done.

He told The Associated Press he was "very unhappy" that help has not materialized for the other half.

"Once the spent fuel is gone, I'll be one happy guy," he said.