There has been a lot of debate over reports that the United States is exploring the use of tactical nuclear weapons against Iran. Setting aside the question of whether military action is wise—and there are strong arguments for focusing on nonmilitary options—one thing is clear: the nuclear option makes little sense.
Discussion focuses on Natanz, where Iran is building laboratories to enrich uranium, ostensibly for nuclear energy but also useful for making a nuclear bomb. Those plants are buried underground, leading many to conclude that only a nuclear weapon could destroy them.
That conclusion is wrong. In general, there are three intertwined reasons military planners might consider using nuclear weapons against an underground target: uncertainty about the target’s location, concern that the depth makes conventional weapons impotent, and a need to destroy the target near-instantaneously. None of these apply in the case of Iran.
If an underground lab were bored into a mountain, or involved a labyrinthine tunnel system, its location may not be well known. Military planners might then argue—as some did in considering a tactical nuclear attack on the Libyan chemical weapons facility at Tarhuna in 1996—that only the broad blast of a nuclear weapon could guarantee destruction.
But the precise locations of the underground chambers at Natanz are well known—they were built in open pits, visible to American satellites, before being covered with concrete, rock and dirt. (And the only building at Natanz where we know Iran has enriched uranium thus far is above ground.) If anyone wants to to bomb Natanz, they will know where to aim.
The second concern is that if an underground laboratory is deeply buried, that can also confound conventional weapons. But the depth of the Natanz facility—reports place the ceiling roughly 30 feet underground—is not prohibitive. The American GBU-28 weapon—the so-called bunker buster—can pierce about 23 feet of concrete and 100 feet of soil. Unless the cover over the Natanz lab is almost entirely rock, bunker busters should be able to reach it. That said, some chance remains that a single strike would be unsuccessful.
That leads to the third factor. Advocates of nuclear weapons normally plan on using them in a time-sensitive scenario: an enemy is about to launch an attack on the United States, and the only way to immediately stop it is to employ nuclear arms, taking out the enemy base in a single strike.
This is weak as a generic argument, and it is patently unsound in the case of Iran. Natanz poses no imminent threat—the worst-case prediction is that, in several years, the Iranians might produce enough material for a nuclear bomb, but we do not worry that any weapons there endanger us now. The United States could repeatedly bomb the plant, if it wished, drilling down until it reached the underground chambers. Even if that took days, it would set back the Iranian program just as decisively as a nuclear attack.
In the end, the nuclear option makes little sense—and flirting with it undermines the American stance against nuclear proliferation. Taking nuclear weapons decisively off the table would reinforce the taboo against the bomb, and make American actions to oppose proliferation more effective.