Nuclear-weapons experts are familiar with the book, a transcription of talks delivered at the onset of the Manhattan Project. Theodore B. Taylor, a retired U.S. nuclear-weapons designer who also served as the deputy director of the Pentagon's nuclear agency in the 1960s, knows the title well. In fact, Taylor says he would fail any student assigned to find open-literature publications about nuclear weapons if the student didn't come up with Los Alamos Primer.

"There is a lot more information in the public domain that was not there 10 years ago," acknowledges Terry Hawkins, deputy director of the nonproliferation and international security division at Los Alamos National Laboratory. Declassification and open-literature publication have driven this trend, he notes. "There is so much information floating around," adds his colleague Robert Kelley, who manages the Los Alamos nuclear emergency and proliferation response effort, "and it offers a person so many choices that he is often confused."

Although the U.S. government has been worried about nuclear terrorism and extortion for years, the ready availability of information has recently heightened fears. So has the availability of fissionable material. As the former Soviet empire continues to crumble, its nuclear materials have become tempting targets for smugglers and organized crime.

A third factor that piques concern is the emergence of religiously motivated terrorist groups, bent on causing mass death without regard to political considerations. This marks a change from the days when politically motivated terrorist groups such as the Red Brigade engaged in murder and kidnapping but rarely tried to inflict a large number of casualties.

During the Cold War, the difficulty and expense of obtaining nuclear materials provided a barrier against nuclear terrorism. "That barrier is not as formidable as it once was," says Hawkins. "If a terrorist group or rogue state gets ahold of such material from smugglers, they solve the single most difficult problem in building a bomb."

From a security standpoint, the good news is that the Russian military is believed to be keeping a tight grip on strategic and tactical nuclear warheads. But it is also in the process of dismantling many of these weapons, creating stockpiles of bomb-grade plutonium and highly enriched uranium. The bad news is that at civilian laboratories fissionable material is not as closely guarded as it is in the West.

This has led to a series of smuggling incidents involving plutonium, highly enriched uranium and other bomb-related materials. In any one case, the police have not found enough material to make a bomb. In some instances, including the well-publicized Munich arrests of August 1994, the smugglers were simply falling for a sting operation and selling wares to undercover agents.

The U.S. Department of Energy national laboratories are working closely with their Russian counterparts to improve security, mainly by transferring commercially available Western technology to former Soviet facilities. But progress is slow. "One kilogram under control is only one kilogram under control," says Hawkins. "We have to work one kilogram at a time."

It will take Russia years to properly account for all the material it has--much less determine if any is missing. That raises concern now, because it takes only 30 to 50 pounds of highly enriched uranium or 10 to 20 pounds of plutonium to make a bomb.

According to physicist David Albright, president of the Institute for Science and International Security in Washington, D.C., there are more than 1700 tons of highly enriched uranium and 1100 tons of plutonium in the world. Only 22% of the plutonium is under military control, while the rest is spent fuel from reactors or plutonium already separated from that spent fuel. Despite claims to the contrary, reactor-grade plutonium can be used to make a bomb. In fact, in 1962 the Defense Department set off a bomb (yielding just under 20 kilotons) made of reactor-grade plutonium--just to see if it could be done.

Experts disagree on how difficult it would be for a terrorist group to fabricate a crude nuclear device--one able to deliver at least a one-kiloton blast (1000 times the impact of the World Trade Center bomb). Taylor believes a single individual working alone could do it, given access to enough fissionable material. Taylor, who designed one of the largest as well as one of the smallest nuclear weapons ever developed by the United States, doesn't believe that industrial-grade machine tools would be required.

Another weapons specialist points out that many of the components needed to build a nuclear device are on a list of limited-access items, adding that "you can't buy a fast-firing fuse set by just walking into Sears." But this expert concedes that while the threat of someone building a nuclear bomb in a basement is low, it isn't nonexistent.

On the other hand, J. Carson Mark, former head of nuclear-weapons development at Los Alamos, doesn't think a lone individual could build a bomb unless that person could master several disciplines. Mark believes it would take a team of specialists about a year. The group would have to include a nuclear physicist, a mechanical engineer, a chemist, an explosives expert, a mathematician and others.

Although it might be possible to develop a bomb in a year, history suggests otherwise. Kelley points out that South Africa spent four years creating a gun-type device that used highly enriched uranium. A small team of 10 to 20 people designed the weapon, and a large infrastructure was required to produce the highly enriched uranium.