By the water of babylon by stephen vincent benet

THe radiation is supposed to kill anyone but priests. I don't think they knew it was actually radiation but the people created a taboo around these lower regions to explain certain fears. Priests are really not immune but their belief system must give "powers" to certain people. It is the hierarchy of human nature,

"The north and the west and the south are good hunting ground, but it is forbidden to go east. It is forbidden to go to any of the Dead Places except to search for metal, and then he who touches the metal must be a priest or the son of a priest. Afterward, both the man and the metal must be..."