I think it plausible that they would allow some faction or 'terrorist group' get ahold of a dirty bomb to deploy against the 'infidels'.
You have to keep in mind how Iran operates, they do not send out an army, they will not fire a rocket into Israel or American... why would they do that, when they can provide the means for some group unattached to them to do their work for them?
The Iranians want to rid the world of Israel, yet they do not start a war against them to accomplish this, they fund the Hamas, the PLO, and anyone else that pursues that end.
This is how Iran fights the war, with religious fanatics and power hungry psychopaths that want to destroy the Western civilization- Israel, America, Britain, and Europe.
Weaken them from within, destroy their cultures, their society, their beliefs... until you can destroy them from without, or take them over from within.
Its a war they will win, because it is a war we are unable or unwilling to recognize and address with the needed determination and force.