VIENNA, June 28 (Reuters) - Nuclear and radioactive materials are still going missing and the information the United Nations atomic agency receives about such incidents may be the tip of the iceberg, said a senior U.N. official.
Any loss or theft of highly enriched uranium, plutonium or different types of radioactive sources is potentially serious as al Qaeda-style militants could try to use them to make a crude nuclear device or a so-called dirty bomb, experts say.
Khammar Mrabit, a director of the U.N.'s International Atomic Energy Agency (IAEA), said there had been progress in recent years to prevent that from happening. But he said more still needed to be done to enhance nuclear security.
"You have to improve continuously because also on the other side, the bad guys, they are trying to find ways how to evade such detection," Mrabit said in an interview.
"The threat is global because these people operate without borders," he said on Thursday before an IAEA-hosted meeting of more than 100 states in Vienna next week on how to ensure nuclear materials do not fall into the wrong hands.
The U.N. agency is helping states combat smuggling of uranium, plutonium or other items that could be used for a nuclear weapon or a dirty bomb, which uses conventional explosives to scatter radioactive material across a wide area posing health risks and massive cleanup costs.
About 150-200 cases are reported annually to the IAEA's Incident and Trafficking Database. More than 120 countries take part in this information exchange project, covering theft, sabotage, unauthorised access and illegal transfers.
While making clear that most were not major from a nuclear security point of view, Mrabit said some were serious incidents involving nuclear material such as uranium or plutonium.
These incidents mean that "material is still out of regulatory control", said Mrabit, who heads the nuclear security office of the IAEA. "Maybe this is the tip of the iceberg, we don't know, this is what countries report to us."
DIRTY BOMB DANGER
In one reported case, police in former Soviet Moldova two years ago seized highly enriched uranium carried by smugglers in a shielded container to prevent it from being detected, a sign of increased sophistication of such gangs.
But unlike in the 1990s - after the Cold War and the collapse of the Soviet Union weakened control over its nuclear arsenal - the few cases that are reported involve grammes of enriched uranium or plutonium, not kilogrammes.
"A lot has been improved," Mrabit said.
Analysts say radical groups could theoretically build a crude but deadly nuclear device if they have the money, technical know-how and the amount of fissile material needed.
Obtaining weapons-grade fissile material - highly enriched uranium or plutonium - poses their biggest challenge, so keeping it secure is vital, both at civilian and military facilities.
An apple-sized amount of plutonium in a nuclear device and detonated in a highly populated area could instantly kill or wound hundreds of thousands of people, according to the Nuclear Security Governance Experts Group (NSGEG), a lobby group.
Because radioactive material is seen as less hard to find and the device easier to manufacture, experts say a "dirty bomb" is a more likely threat than a nuclear bomb.
In dirty bombs, conventional explosives are used to disperse radiation from a radioactive source, which can be found in hospitals, factories or other places not very well protected.
George M. Moore, a former senior IAEA analyst, said in an article in the Bulletin of the Atomic Scientists last month that "many experts believe it's only a matter of time before a dirty bomb or another type of radioactive dispersal device" is used.
Mrabit said: "Statistically speaking no reasonable person will say that this will never happen. The probability is there."