Before commercial airliners were used as guided missiles, few experts viewed x-ray materials as potential terrorist weapons.
But now, in the wake of Sept. 11, experts are warning that not just nuclear weapons, but other radioactive materials - widely used in medicine, agriculture, industry and research - as well as civilian nuclear installations, could become weapons in the hands of terrorists.
Experts meeting in Vienna Friday at a conference on nuclear terrorism called on the international community to act quickly to impose better safeguards for nuclear and radioactive materials. The security of nuclear arsenals in Pakistan and the countries of the former Soviet Union must also be ensured, they said.
The ruthlessness of the Sept. 11 attacks makes it clear that the risks of a nuclear terrorist act are higher than previously thought, says Mohamed ElBaradei, Director General of the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, which organized the conference.
Commonplace radioactive materials - such as those used in radiotherapy or for the preservation of foodstuffs - could be fashioned into a crude "dirty" bomb that could be detonated with conventional explosives, the experts note. "There are few security precautions on radiotherapy equipment, and a large source could be removed quite easily, especially if those involved had no regard for their own health," says Abel Gonzalez, the IAEA's director of radiation and waste safety.
A "dirty" bomb would not necessarily result in a high number of casualties. "But contamination in even small quantities could have major psychological and economic effects," Gonzalez says.
Similarly, an attack on a nuclear power plant would not automatically mean a disaster on the scale of Chernobyl. Most nuclear power plants were built to withstand natural disasters and accidental crashes of small aircraft.
But the design of most reactors did not take into account the potential for an attack like that on the World Trade Center, and IAEA experts acknowledge that they are not sure how bad the fallout would be if a fully fueled jetliner crashed into a nuclear reactor. An attack by well-trained terrorists is "a much larger threat than civilian nuclear security systems are generally designed to deal with," says George Brunn of Stanford University.
Security procedures at nuclear power plants are already under review in the US and across Europe. A number of countries have restricted airspace around plants and posted additional guards. France has deployed antiaircraft missiles near its plant for spent nuclear fuel in La Hague.
Russia has announced that it is stepping up training of personnel at its facilities. But experts warned that the vast nuclear arsenal of the former Soviet Union remains dangerously vulnerable - less to attack than to theft.
Osama bin Laden's Al Qaeda network has in the past attempted to buy nuclear material stolen from the former Soviet states. There have been frequent reports that not all of the former USSR's nuclear weapons can be accounted for, although the Russian government has denied these claims. Pakistan's nuclear weapons are also seen as a security threat, because of the country's current political instability and former close ties with Afghanistan's Taliban regime. Mr. ElBaradei said that safeguards for Pakistan's arsenal - which is not under the purview of the IAEA - appear to be sufficient. But there are fears that the country's nuclear weapons could end up in the hands of pro-Taliban forces if the government were destabilized.
Despite the dangers, conference speakers say that the chances of terrorists setting off a major nuclear bomb remain small, due to the difficulty of acquiring the necessary high-grade materials and expertise.
The experts agreed that safeguarding all nuclear materials should be a counterterror priority In the short term, the IAEA estimates that it needs between $30 million and $50 million annually in additional resources. The IAEA has been underfunded in the face of an ever-increasing workload, and earlier this year the Bush administration had moved to cut funds that help Russia dismantle its nuclear arsenal. But governments may now be more willing to pay the price for greater security.