International experts in radiation disasters have taken the unusual step of assuming that a terrorist attack with a "dirty" nuclear bomb will take place on a high-profile civilian target, such as the London Underground.
The scientists assess the risk of such disasters in terms of probability, but the unknown nature of the dirty-bomb threat has forced them to assume that an attempted attack is all but inevitable in the future.
An international team of senior nuclear scientists has been established to assess what would happen if terrorists detonated a crude nuclear device or released radioactive material in a densely populated setting.
The group, who met last weekend in Stockholm, is composed of some of the most experienced scientists in the field of radiological contamination who have dealt with emergencies ranging from the Chernobyl nuclear disaster to an incident in Brazil, where 120,000 people were at risk of radiation exposure from hospital waste.
The scientists are part of a working party set up by the International Commission on Radiological Protection (ICRP), which is charged with the task of advising governments on how to protect the public against radiation hazards.
A senior diplomat and nuclear scientist who is close to the ICRP's working party told The Independent that the sort of dirty-bomb scenarios being considered are attacks on "soft" targets, such as the London Underground and the Olympic Games in Greece next year. The scientist, who asked to remain anonymous, said that dirty bombs can range from a simple canister of radioactive caesium chloride stolen from a hospital radiological department to a crude atomic bomb.
"You don't need to make a bomb to have a dirty bomb," he said. Caesium chloride, for example, has the constituency of talcum powder and is easily dispersed. "The Tube in London for instance is a fantastic dispersion device. When the train is coming it is like a piston. You just open the canister and I would say that after two or three hours you'll have caesium all over the Tube in London," the scientist said. "Nothing will happen from the health point of view but people will be so afraid that no one will use it ... I know the London Underground has a working party looking at this."
Asked whether governments are doing enough to address the threat, he replied: "I feel we haven't done enough ... If this happened today, I believe we are not prepared to confront it."
When scientists carry out risk assessments of disasters in nuclear power plants they assign probability estimates to different accident scenarios, but this is impossible when it comes to assessing the risk of a dirty bomb attack, he said.
"The problem with this sort of thing is that you cannot assign probabilities. Moreover, the probabilities change compared to the level of preparation of a government," he said.
The ICRP, which meets next month to make recommendations on dirty bombs to its 100 member states, works in parallel to the UN's International Atomic Energy Agency (IAEA), which is responsible for the nuclear safety standards that cover radiological material that could be made into dirty bombs. When the IAEA held a conference on dirty bombs in 1998, most governments dismissed the possibility as too remote.