Nukes should cause some (like 5-10%) damage to all units in the targetted province, a complete loss of organization (to represent the destruction of their supply chain and the chaos of civilians fleeing the suddenly destroyed city), the loss of most of the industry, infrastructure, and resources (75% or so), but allow it to be rebuilt. It shouldn't regenerate, just as strategically bombed cities should not regenerate, but it should be capoable of being rebuilt.
I also agree with a huge dissent increase for the province holder after a nuke goes off. No automatic surrender directly, but if dissent goes too high...
Steele