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,” 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 summit to tighten — by the end of 2005 — restraints on international trade in highly radioactive materials.

But 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 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.

These weapons 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.