History Channel did an excellent job covering this and several other questions that would arise in a situation like this in their special Life After People. I saw it a couple of years ago so I don't remember what they said specifically about Nuclear Reactors but I do know they covered them. The series focused on what would happen to the world if human kind disappeared from the face of the Earth while the show avoided discussing a specific cause. So it was an in depth look at what would happen after shit hit the fan and we failed to survive as a species. Check out the link, I'm sure you can find a free version to watch online some where.