Could it be Mummer Kiss by Michael Swanwick? I suspect this isn't the story you're thinking about because it's set a hundred years after the Three Mile incident and I think you'd have remember that. However it does match the description pretty well.
As you say, the meltdown had resulted in massive contamination producing an area called The Drift, but it had apparently missed Philadelphia. This story is set a hundred years after the accident and a reporter called Suzette Fletcher has been investigating it. At the end of the story she is murdered but her friend Keith Piotrowicz finds her report showing that Philadelphia had in fact been contaminated:
Keith riffled through thin bundles of paper labeled “Reactorville,” “Mutations/Disease,” “Mutagenic Offspr.” and the like. Halfway through, he hit pay dirt: a bundle labeled “Phila/Drift.” He returned the other papers to their sheath, and began reading.
It’s the best-kept secret in Philadelphia. The infant mortality rate is not a matter of public record. People disappear into the hospitals and the word filters out that they died of “pneumonia” or “flu” or “superflu.” Not a person in a thousand suspects that Philadelphia lies within the Drift.
The name refers to a group called The Mummers who came to control the city in the chaos following the meltdown. The Mummer's Kiss means you've been targeted for assassination on the grounds you're a danger to the city.