For the possible effects we have to extrapolate from the few instances where people have been unwittingly contaminated by radioactive materials. In 1987, thieves in Goiânia in Brazil stole a radiological capsule from a defunct radiotherapy machine. Their friends and neighbours were tantalised by the blue glow it emitted, with some unwisely deciding to break the capsule and smear the contents on their bodies. A thousand people were contaminated and four of them died. In 2001, three Georgian woodcutters stole a nuclear-fuelled generator used to power a remote lighthouse with a view to employing it as a heat source on chilly nights. They all suffered fatal radiation sickness. Earlier this year, panic ensued after a scrap dealer in a slum near New Delhi decided to dismantle equipment containing cobalt-60. No one has died yet. 

While security agencies should certainly countenance worst-case scenarios — which is as likely to involve river-borne gunmen debouching from the Thames by the London Eye as it is a dirty bomb — historical experience suggests this is a threat with which we can deal. Although al-Qaeda would like to get its hands on such weapons, in practice only Chechen Islamist separatists have easy access to sloppily guarded radioactive materials. In 1995, Chechen terrorists drew the attention of the Russian media to a small amount of cesium-137 they had buried in a Moscow park. Three years later, they planted a landmine sheathed in radioactive materials next to a railway line used to ferry Russian troops to the Caucasus. Neither device was detonated. In 1999, two Chechen terrorists attempted to steal radioactive materials from a plant in Grozny. They both collapsed (and one of them died) within a few minutes of carrying the container. 

Even if such a device were successfully detonated, any casualties would result mainly from the explosion itself and the ensuing mass panic, rather than from the effects of radiation poisoning. People need to be educated about the realities of such an attack, with clear instructions about covering one's face and closing any doors and windows. The government needs to account precisely for all disused or orphan radioactive materials and to inspect every conceivable target, with a view to understanding how contamination could be contained and neutralised. In addition to ensuring the availability of personnel to deal with the crisis, it needs to rehearse the means needed to reassure what will be a hysterical public reaction. As we are discovering with the financial crisis, sharing bad news with the people is one hallmark of a mature and resilient democracy.