VIENNA, Austria -- Scientists, police commanders and government officials from more than 100 countries are converging on Vienna for the world's first "dirty bomb" conference, searching for ways to head off the threat of simple weapons that spread radiation and chaos.
Governments are concerned. A recent U.S. experts' report concludes that tens of thousands of the most dangerous radiation sources worldwide -- used to treat cancer, find oil deposits, disinfect food -- may be insufficiently protected.
A so-called dirty bomb -- conventional explosives combined with radioactive material -- has yet to be detonated anywhere. But the al-Qaida network is reported to have been interested in trying such a terror weapon.
When it comes to safeguarding cesium, strontium and other radiation sources, "what may have been sufficient in the past may or may not be now," U.S. Energy Secretary Spencer Abraham said in an interview ahead of today's conference opening.
His deputies acknowledge the dirty-bomb threat was rarely even thought of before the Sept. 11 terror attacks.
The worry is not of mass immediate deaths, as in the 2001 attacks, but of the spread of radiation that might cause immediate panic, because of fear of long-term illness, and make sections of cities uninhabitable for years.
The three-day gathering of world experts, sponsored by the U.S. and Russian governments, has an ambitious agenda in laying plans for a global defense against the bombs, known technically as radiological dispersal devices.
The more than 600 technical specialists, customs and other law enforcement officers, regulatory officials and others will explore ways to identify the most threatening forms of radiation sources. They also will discuss how to find abandoned -- or "orphaned" -- radioactive material, keep track of sources in use, combat smuggling of such material, and respond to the detonation of a dirty bomb in a congested city.
A prime concern is the former Soviet Union.
Washington, Moscow and the Vienna-based International Atomic Energy Agency last June announced a joint effort to trace and secure unknown numbers of orphan sources the Soviet military and government left in former Soviet states a decade ago after that union collapsed.
They include, for example, highly radioactive strontium-90 batteries used for remotely placed aviation beacons.
Abraham and his Russian counterpart, Atomic Energy Minister Alexander Rumyantsev, are expected to discuss what progress has been made in appearances at the conference Tuesday.
The recent experts' report, which said "several tens of thousands" of sources may be at risk worldwide, noted problems as well in the United States, particularly lack of controls on U.S. exports of radioactive material.
Researchers of the Center for Nonproliferation Studies, at California's Monterey Institute of International Studies, said such isotopes can be shipped from the United States to such terrorism-afflicted nations as Afghanistan and Colombia without any certification that the end user is legitimate and will protect the material from theft or misuse.
Such export controls in the United States and other countries "could rapidly ensure that the considerable majority of high-risk radiation sources in use around the world are properly protected against misuse," said the 73-page report issued in January, one of the first comprehensive studies of the threat.
In Washington, Sen. Hillary Rodham Clinton, D-N.Y., and Rep. Edward Markey, D-Mass., have introduced legislation to establish a nationwide system of tracking radiation sources.
The U.S. government reported last year that nearly 1,500 such items were believed lost or stolen in the United States since 1996.
Mohamed ElBaradei, the IAEA director-general, has said "cradle-to-grave" control of powerful radiation sources is needed.
Abraham said he expects a report soon from a U.S. task force investigating such questions.
"We felt a need to re-examine whether we're doing a proper job of tracking, accounting," he said in a telephone interview on a pre-conference visit to London.
The energy secretary said he proposed this week's global meeting last September "to begin the process of really drawing international focus on the kinds of things that needed to be done to better secure and account for the material that could be used for dirty bombs."