By Nathalie Malinarich and Nick Caistor BBC News Online

The possibility that terrorists might acquire a nuclear device has been a nightmare scenario to defence analysts and Western governments even before 11 September.

Although it is widely believed that Osama Bin Laden's al-Qaeda network - which the US blames for the 11 September attack - does not have the capability to build a conventional atomic bomb, experts fear it could use radioactive material to make a "dirty bomb".

Such a device - which spreads radioactive material using conventional explosives - has been described by some experts as "the poor man's nuclear device."

Radiation could be scattered from the top of a building by detonating explosives wrapped with the radioactive material or by piloting an aircraft into a nuclear reactor.

Such a device could be used in a car bomb, driven right into the heart of an urban population centre. Thousands could be exposed, causing both short and long-term deaths and rendering areas uninhabitable for years.

But propably the most important aspect of a "dirty bomb" explosion in an American city would be as propaganda for those who want to attack the US.

Call to arm

Pakistan says its nuclear materials are "in safe hands"

"We call for the Muslim brothers to imitate Pakistan as to the possession of nuclear, chemical and biological weapons," read a letter alleged to have been written by Bin Laden and seized in London three years ago.

In October 2001, Bin Laden reportedly told a Pakistani newspaper that al-Qaeda did possess chemical and nuclear weapons.

The Dawn newspaper quoted him as saying: "If America used chemical or nuclear weapons against us, then we may retort with chemical and nuclear weapons. We have the weapons as a deterrent."

In December the US said it had found documents in a building in Kabul believed to have been used as a safe house by al-Qaeda militants containing instructions on how to build a nuclear device.

US special forces who raided former al-Qaeda facilities in Afghanistan also spoke of evidence that they were planning to create weapons of mass destruction, including nuclear devices.

Nuclear trail

Where al-Qaeda may have got nuclear materials - if they have them - is not known, but many fingers are pointing to ex-Soviet republics or Pakistan.

Independent nuclear consultant John Large thinks that al-Qaeda could well have enriched uranium bought in a former Soviet republic, possibly Kazakhstan or Uzbekistan.

A "dirty bomb" could leave areas uninhabitable for years

"With the break up of the Soviet Union there were many leaks, typically about 30 cases of smuggling of nuclear materials a year. It is like an open sieve.

"You get a superpower like the Soviet Union break up and of course its materials will come out."

David Kyd, a spokesman for the United Nations Atomic Energy Authority told the BBC that there were some 175 cases of seizure of nuclear material being smuggled from the former Soviet republics.

"We cannot exclude the possibility that Bin Laden's group could get hold of this material to make a radioactive weapon rather that buying a nuclear weapon off the shelf or making one themselves," he said.

However, experts insist that al-Qaeda does not have the technical competence to build a sophisticated nuclear weapon.

"It is difficult to think of a sub-national group doing it without the help of a nuclear state," says John Large.

"A nuclear bomb is difficult to manufacture and requires a lot of industrial infrastructure, materials, machines and tools. It also takes a long time to develop the capability."

Israel, for example, took about 15 years to achieve nuclear status.

Dangerous targets

But not being able to build an atomic bomb does not mean that radioactive materials cannot be used to cause extensive damage, as many specialists have warned.

According to some experts, it would be sufficient to explode an old X-ray machine containing cobalt 60 to produce radiation poisoning.

Alternatively, a nuclear power station might be a tempting target for terrorists to attack.

Crashing an aircraft into the cooling pool holding spent fuel would have a devastating effect on the nearby population and environment.

Recognising the threat, the French military has stationed surface-to-air missiles at key nuclear processing sites in western France as a precaution against airborne suicide attacks.

Both the UK and US governments have reviewed security measures at nuclear plants since the 11 September attacks.