Iran will use the nukes to guarantee that there will never be an invasion of Iran, and probably no large direct attacks, while they continue to advance their position through terrorist proxies. They know they will be annihilated if they use nuclear weapons. The regime leaders are evil not crazy. They are willing to send idiot martyrs to die for them, but they want long lives of power themselves.
That would be easier to believe if they didn't have Twelvers in their midst.