One of the central findings of the 9/11 Commission was that the United States government failed to imagine a terrorist attack using airliners to crash into buildings. What other nightmares have we failed to imagine? The Four Faces of Nuclear Terrorism, by Charles D. Ferguson, William C. Potter et al. (Center for Nonproliferation Studies; paperback, $19.95) attempts to fill in the gap for nuclear terrorism and envision how people determined to bring about mass casualties might exploit the world's glut of poorly guarded nuclear materials.

The authors acknowledge that much is unknown about terrorists' behavior. So far, no terrorist group has managed to go nuclear. Why? Terrorists are pragmatic: They don't want to bother with something as difficult as a nuclear device, which requires a high level of expertise and entails huge risks.

This sober study evaluates four paths to nuclear terrorism: theft of an intact bomb; obtaining the fissile material to make a crude device; attacks on a nuclear facility; and fabrication of a "dirty bomb," which uses conventional explosives to disperse radiation. There are obstacles to each, some formidable. But the study notes that a dirty bomb would be relatively easy to build and remains "a viable option for smaller groups with limited financial resources and technical know-how."

The study also underscores the potential dangers of a forgotten area of arms control: Russian and American tactical nuclear weapons, thousands of relatively small and portable bombs for which no legally binding agreements exist. The authors also warn of the continuing risks from the large quantity of highly enriched uranium, enough for thousands of bombs, still spread across more than 50 sites in Russia and poorly secured. The authors implore the U.S. government to put fresh emphasis on consolidating and storing safely this bomb-grade material.

Readers looking for certainty won't find it here. There's just too much mystery about how terrorists function. But as the study suggests, we already know much about the spread of highly enriched uranium, plutonium and other radioactive materials. One way to avoid a catastrophic attack is to just make it hard for terrorists to put their hands on them. There's no failure of imagination here; this volume brims with it. But one can't help but conclude, after reading this account, that imagination is not enough.

-- David E. Hoffman

(Washington Post staff writer)