Terrorists are "all but certain" to set off a radiological weapon in the United States, since it will take authorities too many years to track and secure the radioactive materials of such "dirty bombs," (search) a team of nuclear researchers has concluded.

The U.S. and other key governments took an important step on controls this month, agreeing at the G-8 (search) summit to tighten — by the end of 2005 — restraints on international trade in highly radioactive materials.

But thousands, possibly tens of thousands, of high-risk radioactive sources are already in use worldwide, with few accurate registries for tracing them, the scientists say. They cite Iraq, where an undetermined number of such sources have gone missing in the postwar chaos.

The findings are being published in a 300-page book, "The Four Faces of Nuclear Terrorism," the result of a two-year study by the authoritative Center for Nonproliferation Studies, or CNS, of California's Monterey Institute of International Studies.

The team also examined the potential for terrorists to steal or build an actual nuclear weapon, but found that less likely than the construction of a radiological dispersal device, or dirty bomb.

Unlike warheads designed to kill and destroy through a huge nuclear blast and heat, these radiation weapons — which thus far no one has employed — would rely on conventional explosives to blow radioactive material far and wide. A successful bomb could make a section of a city uninhabitable for years.

The fear of such weapons grew in the aftermath of the Sept. 11 attacks in the United States. Al Qaeda (search) and Russia's Chechen rebels have shown an interest in highly radioactive material.

Misunderstandings persist about the threat. This month, for example, the Justice Department said Al Qaeda-linked detainee Jose Padilla (search) planned to wrap explosives in uranium to make a dirty bomb. But uranium would add nothing; it has minimal radioactivity.

Instead, specialists who study the threat focus on isotopes with millions of times more radioactivity than uranium — such as cesium-137, cobalt-60 and iridium-192. These nuclear reactor byproducts have uses ranging from radiation treatment of cancer, to sterilizing food and medical equipment, to gauging thicknesses.

The CNS study notes steps taken by the U.S. government, including:

— An order quietly sent to operators of sterilizing irradiators last year, instructing them to strengthen security against theft and attack. These large, powerful devices hold immense amounts of lethal radioisotopes.

— Research to develop a substitute for cesium chloride, a talc-like powder that could spread deadly radioactivity widely and insidiously in a blast. Experts consider it the most worrisome material in use.

— Approval of sale of Prussian blue, a drug that counteracts ingested cesium. The U.S. military is "fast-tracking" research into drugs to treat a broader array of radioactive poisons.

The United States alone has an estimated 2 million licensed radioactive sources, thousands of them high-risk materials, the CNS reports. Because of disjointed licensing by federal and state agencies, no complete registry exists. Transfers are not always noted, and sources go astray.

The Energy Department says it has already collected and secured 7,500 "disused" sources, and expects to handle thousands more in the next few years.

The CNS researchers highlighted a major loophole in radioactive commerce: U.S. and other exporters can ship high-risk sources abroad without a government review of the end user, including to such turmoil-ridden lands as Afghanistan, Saudi Arabia and Colombia.

Commercial rivalries have slowed moves to close that loophole, the study says. At the recent summit, however, the United States and seven other major industrial nations agreed to seek "effective controls" on end users before 2006.

Physicist Charles Ferguson, a lead author of the CNS book, was cautious in praising the G-8 move. "The devil is in the details," he said in an interview. "The bureaucracies will have to stay on top of this to get it done."

In many "end user" countries, the domestic regulation of radiological sources is "fragmentary" at best, the study says.

"So many potent radioactive sources are now used in medicine, industry, and research around the world, and so many have fallen outside regulatory control, that it will be many years, if ever, before secure custody of these items can be achieved," it concludes.

As a result, it says, "a radiological attack appears to be all but certain within the coming years."