Britain said in January it had evidence that al Qaeda, widely thought to be behind the attack that toppled New York City's Twin Towers, had tried to develop such a bomb in the 1990s.

Wolfgang Weiss, head of radiation hygiene at Germany's Federal Office for Radiation Protection, prepared a hypothetical case study to show what would happen if a radiation dispersal device - popularly known as a dirty bomb - exploded in Munich.

The results, based on an imaginary bomb made with weapons-grade plutonium placed in Munich's Olympic Stadium, were superficially reassuring: there would probably be no deaths and the number of severely contaminated victims would be small.

"According to the calculations I did, the radiological impact would be very limited, though the wider impact to society would be large," Weiss told Reuters during the first global conference on dirty bombs.

Severe contamination would likely occur at the centre of the explosion in the stadium, which has a capacity of almost 70,000. He said that at five km (three miles) from the stadium, radiation levels would drop by a factor of 100, resulting in only mild exposure levels.

DANGEROUS, BUT NOT DEADLY

Disregarding damage from the explosion itself, Weiss said exposure for someone near the bomb "would require emergency medical treatment, but it would not lead to death".

If the radioactive material was caesium, a common easy-to-disperse radioactive powder used in medicine and agriculture, victims would be exposed to quite low levels.

"These models tell us that you wouldn't have to evacuate a huge city. You would concentrate on an area of a few kilometres," from the explosion, Weiss said.

But the bomb would cause panic, and it would be crucial for political leaders to behave calmly, to speak honestly and in clear, easy-to-understand language about the attack.

"It's not primarily a radiological problem which we'd face, it's a psychological problem and a problem that has to do with trust in a society in their leaders," he said.

Failure to handle the situation properly could turn a manageable crisis, which emergency response teams should be capable of managing, into a disaster.

But Weiss said specific case scenarios were not a good basis for preparing a government on how to respond to an attack.

"You have to be ready to be flexible, ready for everything. Reality is always different," Weiss said. "Before the events in New York on September 11, nobody thought it was possible."

Dirty bombs hit the headlines in May, 2002, when U.S. authorities captured Jose Padilla, an American al Qaeda operative, in Chicago, and prevented a dirty bomb attack.

But there has never been a dirty bomb attack, so scientists and policymakers still have no actual case to examine.

This is why Weiss and others look closely at a tragedy in southern Brazil considered to be the benchmark dirty bomb scenario. This case shows that while the number of deaths may be low, the long-term effects of such an attack could be severe.

GOIANIA: THE DIRTY-BOMB BENCHMARK

On September 13, 1987, two men in Goiania, Brazil were looking for scrap metal at a partly demolished medical clinic.

They found a radiation therapy machine containing a small canister of highly radioactive caesium powder. Unaware of what it was, they sold it to a junkyard dealer, who took the canister apart.

Within two weeks, local children discovered the glowing blue powder. Some even used it as body paint.

This quickly led to a catastrophe that was second only to the 1986 accident at the Chernobyl nuclear reactor. A total of 249 people were exposed, 10 were seriously injured and four died.

The long-term socio-economic effects were devastating. Goiania suffered a 20 percent drop in gross domestic product, which took five years to return to normal levels.

Tourism in the tropical town dropped to zero and Goiania found itself the victim of economic discrimination, as demand for food and other products from the area plummeted.

"Imagine it would happen here in Vienna," Weiss said. "The city would never be the same."

TRAFFICKING IN RADIOACTIVE MATERIALS

"We need to take into account Murphy's Law - whatever can go wrong, will go wrong eventually," said Chris Schmitzer of the Health Physics Division from Austria's ARC research laboratories in Seibersdorf, referring to the possibility of a dirty bomb attack.

According to the United Nations' nuclear watchdog agency, there have been more than 280 confirmed cases of illicit trafficking in radioactive materials since 1993, though the agency suspects the actual number may be much higher.

Mohamed ElBaradei, head of the U.N. International Atomic Energy Agency (IAEA), said the threat of a dirty bomb attack was real and urged speedy improvements in the security of radioactive sources and border controls to keep them out of the hands of terrorists.

"The fact that you haven't seen (a dirty bomb attack) yet doesn't mean one isn't imminent," ElBaradei said.

Caesium, which ravaged Goiania, is one of many deadly radioactive sources that have fallen out of regulatory control through loss or theft across the former Soviet Union, the world's hotspot for illicit trafficking in radioactive material.

"Our database of cases of smuggling gives an indication that there is a market and there is an effort to obtain radioactive sources, and the obvious question is why," ElBaradei said.