YET another terrorist threat has been thrust into the media spotlight in recent weeks: 鈥渄irty bombs鈥�, conventional explosives spiced with radioactive material.
But why is this particular weapon suddenly causing so much concern? And how worried should we really be? What鈥檚 changed is the realisation that not only are terrorists interested in exploding dirty bombs, the material is out there to build one. And while the devices might not kill many people, they could still do exactly what terrorists want: force the long-term evacuation of central London, downtown Manhattan or any other urban area attackers manage to hit.
Government and UN agencies have just admitted that there鈥檚 enough missing caesium-137 and other lethal leftovers around to do it. This week the race is on to find and recover them.
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The US has released $25 million of emergency funding to help comb the former Soviet Union, by far the world鈥檚 richest source of abandoned radioactive material. Meanwhile Russia and the US have agreed to collaborate in searching other former Soviet states.
Experts used to dismiss the possibility that terrorists might try to deploy dirty bombs, which would scatter radioactive material over a large area. No group has ever used one. And apart from the blast itself, dirty bombs kill relatively few people, although the radiation can cause cancers years later.
Instead, counter-terrorism measures have focused on materials that might be turned into a nuclear bomb. 鈥淲e never really thought of other radioactive material as much of a threat before,鈥� says Jack Caravelli of the US Department of Energy.
That鈥檚 no longer the case, especially since the arrest in the US this month of Al-Qaida sympathiser Jose Padilla. It鈥檚 not clear how close he was to making a dirty bomb, but that isn鈥檛 the point, says Michael Levi of the Federation of American 快猫短视频s. 鈥淭here were two questions about dirty bombs. One was, are terrorists interested? Now we know they are.鈥�
The other question was, can they get the ingredients? Last week that was answered spectacularly, when the IAEA admitted that an unknown number of lead-lined cases from the former Soviet Union, which each originally contained a massive 130,000 gigabecquerels (GBq) or 3500 curies of caesium-137, are unaccounted for. By comparison, a caesium source containing only around 10,000 GBq that was lost from a hospital in Goi芒nia, Brazil, in 1987 killed four people who came into contact with it and made dozens of others seriously ill.
Built in Moldova in the 1960s, the Soviet cases each hold 40 grams of caesium. They were housed on trucks and used to irradiate seeds to prevent germination, or induce mutations for crop breeding.
鈥淎fter the fall of the Soviet Union, the trucks were taken,鈥� says Abel Gonzales, head of radiation and waste safety at the IAEA. No one knows what happened to the cases. But given caesium-137鈥檚 half-life of about 30 years, they still contain at least 52,000 GBq each. Worse, the caesium was powdered, making it easier to pack into a dirty bomb than solid material.
And there are plenty of other radioactive stashes at large. Vials containing 13,000 GBq of caesium were stolen in Russia last year. Officials are also searching for 100 abandoned strontium-powered batteries from the former Soviet Union, each containing up to 1,500,000 GBq.
Levi has worked out how much land would be contaminated by the explosion of a bomb containing 10 kilograms of TNT and a pea-sized 74 GBq of caesium 鈥� the amount in a lost medical gauge that turned up in North Carolina this year. His conclusions, presented to a Senate committee in March, were that airborne caesium would pollute a swathe of city a kilometre and a half long to levels that would produce one extra cancer in every 10,000 people who kept living or working there. At that level, US law requires long-term evacuation or demolition if decontamination is impossible. The psychological and economic impact would be enormous.
But just one of the Soviet caesium sources would be enough to make seven bombs of 7400 GBq each 鈥� 100 times as radioactive (see Map). The abandoned streets would bear lasting witness to the attack.
So the hunt is on. Any radioactive material recovered will be locked up, probably in the Mayak nuclear reprocessing centre in the Ural Mountains.