Smash hit

A "dirty bomb" is not a nuclear bomb but instead uses conventional explosives to blast radioactive material over a wide area, unleashing panic and making the area unusable.

Michael Evans, the Defence Editor of The Times said: "There have been numerous attempts over recent years to smuggle enriched uranium and other nuclear material from out of the former Soviet republics.

"The majority of the seizures by law enforcement agencies have involved radioactive isotopes or low-grade uranium. The latest seizure of smuggled uranium is reported to be 98.7 per cent enriched which makes it a more serious issue, although the quantity would not be sufficient to make an atomic device.

"A pound of enriched uranium, however, could be used to produce a radiological device, or "dirty bomb" that would have the potential to scatter radioactive particles and cause panic if exploded in a public place.

"The main concern for the authorities will be to trace the origins of the enriched uranium, to ensure that any leak of such material is plugged to prevent further smuggling attempts."

Uranium exists naturally as an ore. To make it usable for civilian or military use, scientists have to separate out two of its isotopes, uranium-238 and uranium-235.

Uranium-235 is the fissile isotope used in civilian and military applications. When a neutron from uranium-235 strikes another nucleus, it causes it to split, thus leading to the chain reaction that is at the heart of nuclear power and atomic explosions.

Less than one percent of naturally-occurring uranium is 235; the rest is in the form of the non-fissile 238 isotope. By separating out the two isotopes, scientists can therefore boost the concentration of 235 in their material, a process called enrichment.

Different activities need different levels of enrichment. For nuclear power, enrichment - that is, the proportion of uranium-235 - is around three percent.

For nuclear bombs, where an uncontrolled chain reaction is needed, enrichment can go as high as 95 percent, although some estimates say 20 percent is enough to build a crude device.

The "Little Boy" bomb that destroyed Hiroshima in August 1945 was a uranium bomb.

Nearly 40,000 metric tons of uranium was mined worldwide in 2006, according to the World Nuclear Association, a body which represents uranium miners and producers.

More than half of the world’s production of uranium comes from mines in Canada, Australia and Kazakhstan. Russia, Namibia and Niger are also major producers.

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