If the U.S. attacks Iran, the following 4 major nuclear sites will be highest on the 'strike list':
1) the uranium enrichment facilities at Natanz and at Fordo;
2) the heavy water water reactor at Arak; and,
3) the yellow-cake conversion plant at Isfaham.
In order to hit them would involve the use of at least 100 planes (including tankers for mid-air fueling) to hit multiple targets a thousand miles away. They'd most likely fly over Iraq because Iraq has no air defenses and the United States no longer has the obligation to defend Iraqi air space. Penetrating the deeply buried Natanz and Fordo sites might be the biggest hurdle unless the U.S. provides Israel with the 30,000 pound "Massive Ordinance Penetrator" that was specifically designed for use against Iran and North Korea.