I've never heard about it in any of the serious discussions of Three Mile Island, and like you can find no actual support for it, including in any of the databases or books I have access to, which are many. The Netflix show, in my view, is sort of a joke — they let a bunch of dedicated, life-long anti-nuclear activists dictate the whole thing, and only brought in essentially two external experts. One was the NRC guy who they spent the entire time then trying to discredit, and the other was Michio Kaku who is a physicist who apparently (judging from his discussion on the show) either knows very little about nuclear power or Three Mile Island, or they edited him in a way that makes him seem that way.

All of which is to say, there is a lot about that show which I would not take seriously without serious checking as to its accuracy, and there was a ton on it that I could not (when I tried to) check on its accuracy. I am not saying it is all necessarily bullshit, but I am saying it is pretty indistinguishable from bullshit at the moment. I concluded while watching it that it was done in incredibly bad faith.

Three Mile Island was a very complex event and one can have many different (pro- or anti- or neither) reads of nuclear power from it. The Netflix documentary was an anti-nuclear screed blended with a conspiracy theory and a lot of dramatic music. The only voices it really showcased were those who had, for reasons good and bad, developed a deep distrust of not only the plant owners, but essentially every possible external source of knowledge about the accident. These included a group of people who consider themselves "survivors" of the accident, and essentially attribute any possible health condition that an individual in the area might have had afterwards with the accident, which is not how epidemiology works (people get cancer, etc., all the time — for an accident, you're looking for excess change, and while at least one study of the TMI area showed an uptick in certain excess cancers, it was a very tiny uptick, and many other studies have shown no such increase, so even if there was an effect, it is one that is so small that is only barely detectable; some studies have shown less cancer near TMI, which I don't offer up as being any inherently more likely, just a further example of how this data is messy).

I put this all out there just as context. Do these people believe this anecdote about the girl? I am sure they do. But it has the feel of the kind of Facebook urban legend that people tell about a shared traumatic experience (and TMI was traumatic, even if it didn't cause direct medical harm from radiation), the sort of thing we have all heard ourselves over the last few years with COVID, a story that gets amplified and elaborated in the telling. What kind of lesions? Where was she riding? When did the "lesions" develop? Did she get them treated? What did the doctor say? Surely if this had happened, one would imagine, her parents would have gotten this checked out and documented and screened? Why is there absolutely no news account of this from the time? (And "the newspapers were in on it" is not really a viable answer — the newspapers, if anything, were amplifying radiation scares.) And given how many people were in the area, and how many news organizations and scientists were monitoring it, what are the odds that somehow this one girl picked up enough radiation to cause lesions (which is a LOT) but nobody else did, and nobody else noticed it? Even if we imagine that this girl is not entirely imagined or misremembered or fabricated, and that these lesions were somehow a real condition, it could be an entirely coincidental event, in the same way that people no doubt stubbed their toes and choked on food during the accident period as well. It takes real, careful observation to establish connections between events, and when you are talking about possibly quite isolated occurrences — which again, nothing has established as actually real — you cannot just go by "anecdata." (I mentioned Facebook earlier, which might sound anachronistic, but the people who do this kind of "survivor" work, and many modern anti-nuclear activists, do use Facebook to collect their stories, and suffer from the same kind of misinformation, error, and uncritical recirculation of dubious information that everyone else does in this area. I have seen much of this kind of thing first hand!)

So yeah. I would discount this story — and really pretty much everything else in the documentary — without a stronger backing to it. If you are interested in a solid, even-handed take on Three Mile Island, J. Samuel Walker's Three Mile Island: A Nuclear Crisis in Historical Perspective (University of California Press, 2004), is excellent. It is critical where it needs to be critical, and sympathetic where it needs to be sympathetic, and technical where it needs to be technical.

One last thing. Anyone who has followed my writings on nuclear power on here knows that I am not uncritical of nuclear power, that I am not in any strong way pro- or anti-nuclear, that I am quite sympathetic to the fears of people who do not trust the "experts" to adequately represent their interests, and have no love whatsoever for the private nuclear power lobby (which I see as an industrial lobby of not much difference, in terms of its commitment to anything other than profits, than any other industrial lobby). I am not a nuclear booster; I regard nuclear power as a complicated technology that has (like all power solutions) both benefits and risks to it. I certainly think the Three Mile Island accident was a serious one, and believe that the pro-nuclear people do themselves a disservice when they fail to acknowledge that. But it is serious primarily in what it revealed about underlying errors, assumptions, methods, and institutions — not in its health effects. It is serious in what could have happened had they not gotten it under control, and what it said about the limits of control in general. (On this, see Charles Perrow, Normal Accidents.) I watched the Netflix show with great disgust. It starts off as being a bit overdramatic and oversimplifying (but it's a Netflix documentary, so there are some allowances for that), but then slides into unvarnished and unsourced anti-nuclear propaganda. I was very disappointed. I was interviewed about the show after it came out, and you might find the overall video somewhat interesting. The person interviewing me is definitely in the pro-nuclear camp, but he did a good job of contacting people (like me!) who are not associated with the nuclear industry, who are not pro-nuclear activists, who are trying to be nuanced about the whole thing.

TLDR;, it's a bad show, and Netflix should feel bad. It has high production values, but really low epistemic values.