Well sensible people shut nuclear reactors down once there is even a risk someone might shoot it up. Not that the reactors are a problem. Iran has a couple of Russian-designed Light Water reactors which use slightly enriched fuel. They do not pose a proliferation risk.
I agree with your statement but I am afraid Ahmadenijad fancies himself such a populist hero that, in the imminence of a "decomissioning strike" on radioactive sites/reactors, he would not shut anything down. His rhetoric in the past 6 months has been based upon a textbook nationalism-cum-"for the glory of our people, Iran", the extension of which would have him win the ultimate victim status if said attacks polluted his fatherland with clouds of radioactivity... "see what they did? Those American terrorists..." You get the idea.
What America may have trouble hitting is the nuclear enrichment plants. Iran has, I gather, bought centrifuges from Pakistan which stole the technology from Belgium. These are safe things to bomb because they are not in any way radioactive. They are also hugely power-hungry. You could not build a generator large enough to run a full-scale plant without someone noticing. I assume that Iran has, or will, try to excavate tunnels and bury their plants, but even so the power lines would be vunerable.
Although he has an axe to grind the size of Paul Bunyan's, Sy Hersch's news breaking piece in today's New Yorker confirms what you say here about the underground facilities. From hazy memory, I believe he asserts the existence of an underground facility boasting floor space/power capability to run 50,000 (yes, 50 thousand!) enrichment centrifuges. Buried some 75 feet in the ground, and protected by massive 4' to 8' thick ceilings, this seems to be the technical reason for US "mention" of employing Tactical Nukes. And I am sure the Iranians are busy digging as I write. The question your point raises is "how many of these things are there, and where?".
Well there is a lot of doubt as it happens. Iran could not more threaten the US than Belgium could. Actually Belgium is more of a threat. At best Iran might be able to invade Iraq. Compared to the Soviet Union, or Germany, or even Japan and Italy, Iran is no threat to anyone but its neighbors.
I agree that Iran couldn't directly threaten the USA on its own soil (but actually, a somewhat paranoid case could be made of the length of Iran's reach via the strong arm of Hezbollah et al...worth considering at least). But it is US
interests which are mainly the consideration here. I have written in another thread of Iran's threat in/to the Middle East in recent history. This is not to be taken lightly. With growing sectarian violence, border transcending terrorism/insurgency/banditry, this is very much more a problem of the middle east than anyone else's, which makes it a global problem seeing at to the extent to which the world economy runs off of the availability of ME crude.
And a further consideration I am too dumb to speculate upon: What does a nuclear Iran do to certain "balances of power"? What does this scenario do to the Pakistan/India arms race? Saudi Arabia has expressed keen interest in, if not advanced efforts to, develop its own capability to offset an Iranian nuke capability. And Israel? And Egypt? And UAE?
