the irradiated territories (the ones with that black and yellow sign on them) give you -1 army/turn for every one you hold, BUT if you hold more than one of them you start getting a bonus.
So basically you hold a radiation nexus whenever you have more than one irradiated territory. You can check how many armies you get from them if you look at the map, lower left corner, in that big triangle. It's quite easy to spot.