PRAGUE, Sept. 27 - The special police unit arrived in the darkness, carrying submachine guns and taking positions around an unmarked cargo truck parked beside a nuclear reactor. The doors to the reactor swung open and a forklift hurried three large steel casks onto the truck. Each container held several fuel rods of highly enriched uranium, potent enough for use in a nuclear bomb. As the city slept, the truck and its armed escorts slipped away from the reactor, at a Czech Technical University campus on the outskirts of the city, and passed through deserted streets, stopping at last...