Nobody really knows what they'll do, but no one with any brains believes with any conviction that they would be have more or less rationally in the way the Soviets did.
The earlier poster who objected to the comment about the Russians loving their children being irrelevant was wrong. Believing that the US would massively retaliate if they used nuclear weapons (even indirectly), they were not willing to see their own families destroyed. The Russians, at least as officially atheist Marxists, did not have any hope of achieving paradise by having the world blown up.
One cannot thus argue concerning the Iranian mullahs!
Let me say it another way. If Iran used a Nuclear weapon they would be destroyed!