As concerns grow that terrorists might attack a major American city with a nuclear bomb, a high-level group of government and military officials has been quietly preparing an emergency survival program that would include the building of bomb shelters, steps to prevent panicked evacuations and the possible suspension of some civil liberties. Many experts say the likelihood of al Qaeda or some other terrorist group producing a working nuclear weapon with illicitly obtained weapons-grade fuel is not large, but such a strike would be far more lethal, frightening and disruptive than the attacks of Sept. 11, 2001. Not only could...