Over the past decade Iran has been cautiously, but steadily, putting in place all the elements it needs to construct a nuclear weapon in short order. But as James R. Clapper, director of National Intelligence, told the U.S. Senate in January, while the Iranians are “moving on that path … we don't believe they have actually made the decision to go ahead with a nuclear weapon.” For several years experts have debated the possibility of a “breakout” scenario in which Iran makes a mad dash to complete and test its first bomb before other nations can act to stop it. That would require doing as much as possible to prepare for bomb making without tripping the alarms of the International Atomic Energy Agency (IAEA), the source of most good intelligence about Iran's declared program. From that point, Iran would then race to conduct a test quickly, perhaps in as little as several weeks. How close is Iran to achieving such an option?
Allison, Graham. "Slinking Toward the Bomb." Scientific American. June 2012.