So let's say a plutonium dirty bomb exploded on Wall Street. I have my doubts that the fallout would extend beyond Canal Street or Houston Street. And people in the Wall Street area could be evacuated without taking too much radiation. Now, Lower Manhattan would have to be shut down for a while while the stuff is cleaned up, so it would be impactful economically, but the death toll would probably be modest.
Which is, ironically, why Iran might try that approach. It's like a big escalation without being a turn-your-country-into-glass level escalation.