That is disputed. See https://en.wikipedia.org/wiki/Three_Mile_Island_accident#Rel... for claims from critics that official accounts of the exposure were low by a factor of 100 to 1000.

Regardless of which version of the facts you consider more believable, the public was scared witless of the possibility that there was an exposure. As a result millions of people were left with the concern that they could get cancer decades down the road. This public fear is also the most important impact of a dirty nuclear bomb used as a terrorist weapon. Very few of people will get sick, and fewer still will die. But lots will be scared.