A leading U.S. research institute and a former U.S. senator have used a computer exercise to try to convince European officials that Europe could be threatened by a terrorist attack with nuclear weapons, if western countries do not tighten efforts to secure nuclear facilities in the former Soviet Union.

The Center for Strategic and International Studies, a Washington-based policy research organization, and former Senator Sam Nunn, showed during a brainstorming session with top European Union and NATO officials what could happen, if the al-Qaida terrorist group were able to acquire highly enriched uranium from civilian research reactors in the former Soviet Union.

How about an attack with a crude nuclear bomb at NATO headquarters in Brussels, which would immediately kill 40,000 people, overwhelm hospitals with hundreds-of-thousands of injured, spread panic throughout Europe and plunge the world economy into turmoil?

The scenario is fictitious, but is based on documented evidence of al-Qaida efforts to get its hands on highly enriched uranium, and of contacts between the organization's operatives and Pakistani weapons scientists.

Mr. Nunn said preventing al-Qaida from obtaining weapons-grade nuclear material is the best way to stop the group from building such a bomb. "It is well within al-Qaida's operational capabilities to recruit the technical expertise needed to build a crude nuclear device. The hard part is getting the nuclear material, but we do not make it hard enough," he said.

Mr. Nunn, who sponsored a $10 billon program in the U.S. Senate to destroy and safeguard weapons of mass destruction in Russia and other former Soviet republics in 1991, says at least 60 percent of those facilities still need to be adequately secured.

He wants the Group of Eight rich countries to fulfill pledges they made two years ago to commit a further $20 billion to that program over 10 years. And, he said, European leaders should push President Bush and Russian President Vladimir Putin into taking action to protect sites where weapons-grade uranium and plutonium are stored. "The key here, the priority, is securing the material where it is. That takes cooperation. It takes focus. It takes leadership. It takes President Bush and President Putin not simply having a summit conference, but declaring when they leave it that they're going to cut through every bureaucratic obstacle to get this job done. Unless those two leaders cut through their bureaucracies, then it's not going to get done," he said.

Rolf Ekeus, a former chief United Nations weapons inspector in Iraq, says the 50 or so officials who attended the session found that Europe is now as much of a target as the United States for terrorists armed with unconventional weapons. That, he said, is partly due to the fact that terrorists can move easily across European borders, and have set up cells in nearly every European country. "It is clear that Europe has become the breeding ground, the base and the place where planning for terrorist actions is taking place," he said.

Mr. Nunn, Mr. Ekeus and other organizers of the exercise admit that taking measures now to prevent weapons-grade nuclear material from falling into the hands of terrorists would be costly. But they stress that coping with the consequences of an attack like the one in their simulation would be, as one put it, phenomenally expensive.