From the comments from the developers playing during the "letsallbymayor" events, it takes a really long time for it to dissipate, and then it only transforms into heavy ground pollution rather then just going away.
Have you had the dream again? A black goat with seven eyes that watches from the outside.
Better to start a new city if you don't want radiation. Also, if you don't want meltdowns to happen, read the nuclear power plant's description, it says it needs skilled workers to operate safely. Better to use non- nuclear power until you get higher skilled workers. :)
The thing that happened was I had high skilled workers. I used the education map and there was virtually no unskilled labor however my plant still said not enough skilled workers. It was almost like a glitch. I couldn't figure it out. I had school bus stops everywhere and a college not over crowded. It was weird because for years it said safe and all of a sudden went to dangerous. My population was over 60k with good education levels.