Radioactive Sunflowers? What??

This sounds an excellent idea, provided that the plants can then be disposed of safely (which sounds easy enough). I have known that certain plants can concentrate metals and other minerals very effectively for some time. For example, the common horsetail (a plant that has been on this earth since long before the dinosaurs) is very effective at extracting gold from the spoil heaps of gold mines. Where it would not be economic to process the spoil for the minute amounts of gold it contains, it is sometimes worth while to plant this common weed and then to harvest it and burn it to recover the gold from the ash.

Don't know about sunflowers cleaning up radiation, but I do know that geraniums will clear soil of heavy metals (though it takes a few years).