The threat of nuclear terrorism has long been the stuff of Hollywood blockbusters. In these films, the terrorists acquire or steal a ready-made device, usually from a remote part of the former Soviet Union. They then arm the device and threaten to detonate it in a Western city. These films play fast and loose with the extremely complex safeguards which states incorporate into warheads to prevent such a possibility. If one goes missing and is then sourced by nuclear forensics after terrorist misuse the response is likely to be devastating. They also ignore data collected by the International Atomic Energy Agency, which shows a marked fall in cases of smuggled fissile material, as distinct from the increasing disappearance, and illicit circulation, of industrial or medical radioactive materials.
Security experts are less troubled by the standard Hollywood scenario than by the probability of a terrorist group building a dirty bomb — or a radiological dispersal device (RDD). These do not involve a nuclear reaction like the plutonium and uranium bombs which exploded above Hiroshima and Nagasaki. The difference between these and a dirty bomb is roughly the same as that between the power of a lightning bolt and a light-bulb. A Chernobyl-style event, which contaminated a huge area, requires something to go radically wrong at a major nuclear reactor; thankfully, British reactors are heavily guarded.
RDDs rely upon conventional high explosives dispersing a radioactive isotope, the most commonly commercially available being cobalt-60 and cesium-137. As with a conventional car or truck bomb, the initial explosion is likely to kill or maim more people than the dispersed radiological materials. This also depends on whether these materials are in milled or solid form, as well as on ambient physical and environmental factors. In other words, such weapons are instruments designed to cause immediate mass panic, followed by longer-term economic dislocation to business or tourism, because of what would be a protracted decontamination process. Clean-up costs alone could run into hundreds of millions.