בס"ד
Iran would never use its nukes (no matter when and why). An Iranian nuclear strike upon Israel would be the end for Iran. Israel has enough nukes to wipe Iran off at least hundreds of times, not even talking about our aerial and ballistic advantages.
we could intercept any rocket, nuclear or not, used by Iran in the sec it gets any nearer to the Israeli borders, not even talking about their old, poor Soviet-made/self-made aircrafts, and our satellites in outerspace, it can find out about any action in Iran, which means that we can know about any operation against Israel immadiately.
Iran uses its nuke as a threat, not as a weapon. Iran doesn't want suicide, it would accomplish none of Iran's goals.