>>(Besides, if the dome were so toxic that you couldn't get close to it, there wouldn't be vines growing on the dome and trees next to it.)

I mean, I wouldn't be so sure about it. There was a whole document by IAA posted here about the recovery of radioactive material in some former Soviet republic, and the pictures clearly show the piece of metal in between some trees and greenery, even though it was giving off a lethal dose of radiation every few hours(the person who found it died because they spent few hours next to it, the other person had to spend over a year in hospital). My point is that even doses that are lethal to humans don't necessarily affect plant life as much.

Supposedly, it's PU238 underneath, more alpha than gamma danger. Concrete would block alpha radiation pretty well. It's also a heavy metal, so it would be poisonous without the radiation.