Energy Secretary Spencer Abraham (search) announced a $450 million plan Wednesday to rid the world of the "dirty bomb" (search) threat by keeping nuclear materials out of terrorist hands.

Abraham said the Global Threat Reduction Initiative (search) would remove and secure high-risk nuclear materials that pose a menace to the international community.

Since the Sept. 11 attacks on the United States, concerns have grown that terrorists might be trying to acquire material for a so-called "dirty bomb" — a device that uses conventional explosives to spread low-level radioactive material over several city blocks.

Abraham said the objective was to collect, secure and dispose of dangerous radioactive materials from around the world.

"Where 100 years ago authorities had to worry about the anarchist placing a bomb in the downtown square, now we must worry about the terrorist who places that bomb in the square, but packed with radiological material," Abraham told an International Atomic Energy Agency conference on nuclear safety.

Abraham said the new global program would reduce the proliferation threat by cutting off access to materials and equipment by "whatever the most appropriate circumstance may be, as quickly and expeditiously as possible."

By handling problems that require attention anywhere in the world, he said, officials will ensure that nuclear and radiological materials and equipment "will not fall into the hands of those with evil intentions."

A dirty bomb uses conventional explosives to spread radiation over several city blocks. It has no atomic chain reaction and requires no highly enriched uranium or plutonium. Both materials are normally kept under tight security, so they are difficult to obtain.

Instead, the radioactive component is of lower-grade isotopes, such as those used in medicine or research. If a dirty bomb were to be detonated, the radiation release probably would be small.

IAEA Director-General Mohamed ElBaradei described Abraham's plan as "a major initiative to adjust the nonproliferation regime" by strengthening the Nuclear Nonproliferation Treaty.

"This is clearly a key in our fight to control proliferation ... to protect ourselves from nuclear terrorists," ElBaradei said. "We need to re-examine our rules of the game. We need to adjust our defenses ... The first line of defense is having adequate protection of nuclear material."

On Tuesday, the United States provided Greek police and border officials with radiation detection equipment to help guard the Aug. 13-29 Athens Olympics against a nuclear or "dirty" bomb.

Abraham said his first priority is to bring back to the United States some 330 tons of Russian-origin, highly enriched uranium by the end of 2005.

More than 220 tons have been eliminated so far. All Russian spent fuel would be recovered by 2010.

"It has become clear that an even more comprehensive and urgently focused effort is needed to respond to emerging and evolving threats," Abraham said. "Moreover, we are prepared to spend the resources necessary to guarantee success."

"But we will need more funds, and heightened international cooperation, to finish the job," he said.

The IAEA conference was examining how to better secure nuclear and radiological materials at atomic research reactors and other facilities worldwide.

The Vienna-based U.N. nuclear watchdog agency estimates as many as 110 countries worldwide do not have adequate controls over radioactive devices that, if enough of them were obtained, could be used to build an explosive device that would spread radioactive material.