Well, warpstone is not quite the same as radioactive material (wich doesn't actually exist in the warhammer world). Of course it leads to mutations, because warpstone is solid magic, and magic in warhammer is by nature chaotic. Skaven are somewhat resistant to the effects as a racial trait.
I think Warpstone takes some time to affect you, at least alaric had the time to make the crown out of it (and runic items take a lot of time to be made) before it affected him (assuming that it was the crown that made him mad, there is no confirmed information that it affected him at all).