The reaction to Three Mile Island was large because of several factors. One is because it had the potential to break down more catastrophically than it did. There were real fears that a hydrogen bubble had formed and could detonate at any moment and break containment (this is what happened at Fukushima that caused the contamination, ultimately). This was a real fear, and took several days to get under control. So it is important to keep in mind that while Three Mile Island was not catastrophic, it did have the real potential to be.

The second is because for several decades the nuclear industry, the Nuclear Regulatory Commission, and the Atomic Energy Commission and its successor organization had assured people that nuclear reactors were entirely safe, that the chance of a catastrophic accident was essentially zero, and that people who worried about such things were off their rocker. There were legitimate concerns about nuclear safety raised prior to Three Mile Island, by such groups like the Union of Concerned Scientists (which was not made up of NIMBYs or hippies, but people with PhDs in physics and related subjects). There were concerns also about the "regulatory capture" of the nuclear industry — that there was a revolving door between the regulators and the industry they were supposed to regulate. When the accident happened, it appeared to vindicate these fears in a major way, and served to discredit those sources of official authority that had for years downplayed the risks. (The film The China Syndrome, which by chance debuted less than two weeks before the accident, was reflective of these kinds of concerns.)

Lastly, in retrospect it is easy to say, "oh, we know what happened and it turned out OK." At the time it was not at all clear what was happening, not only inside the reactor, but in terms of policy guidance. It turned out that there were insufficient guidelines in place to dictate who was in charge in the event of such an emergency — the plant operator, the state of Pennsylvania, the Nuclear Regulatory Commission, and the federal government all appeared to have overlapping responsibilities and duties. They each gave out conflicting information about what was happening and what ought to be done. Pregnant women and children were evacuated from 20 miles around the reactor — not exactly a confidence-building exercise. Ultimately the public takeaway was that all of the people who said that they were in control were patently not, and that this supposedly well-run industry was in fact not. Indeed, it took years to actually find out what had truly happened inside the reactor, and there were significant inquiries afterwards that documented all of the many failures — technical, human, and systemic. It also turned out that there had been a very similar issue at another plant (Browns' Ferry) only a little while beforehand.

All of which is to say, the public takeaway from Three Mile Island was that nuclear reactors were far more dangerous than they had been led to believe, that the authorities were at best ignorant and at worst liars, and that they had barely missed out on a major catastrophe.

Now, one can, with the benefit of hindsight, say that some of these fears were overblown, or at least that they did not anticipate that significant reforms could be made. We have not had anything similar in the United States since Three Mile Island. There were significant changes made to how reactors were operated to avoid similar kinds of accidents happening again. The case of Fukushima though gives some considerable pause, because these were similar reactors to the ones we operate in this country, and run by a similar sort of organization and regulations, and came up once again with a failure of imagination that led to an extremely dangerous situation. (Chernobyl is less applicable given its Soviet context and reactor design.)

I like the formulation that J. Samuel Walker, the former official historian of the NRC, gives of Three Mile Island:

Yet even though the core of the TMI-2 plant was destroyed, the accident did not release large amounts of hazardous radiation into the surrounding countryside. It did not cause a failure of the reactor’s pressure vessel or approach a breach of containment. The concept of defense-in-depth, the basic philosophy that guided the regulatory decisions of the AEC and the NRC, was tested as never before. In the face of a massive core meltdown, it worked. Although the serial equipment malfunctions and operator errors that occurred at Three Mile Island had never been anticipated, they did not trigger the China syndrome. The consequences of the accident could conceivably have been worse if more of the core had melted and the pressure vessel had failed, but reactor experts concluded that, even in that event, there was "little chance" that containment would have been breached in a way that allowed an uncontrolled release of radiation. The applicability of the Three Mile Island accident to other plants under different conditions was uncertain. But the results suggested that nuclear proponents had underestimated the risks of a major accident at a nuclear plant in the United States, and that nuclear critics had overstated the likely consequences.

The last sentence is I think an important one. The reaction to Three Mile Island was perhaps overblown. But so, proportionately, was the over-confidence that the nuclear proponents had about reactor safety prior to the accident. A loss of faith in expertise seems proportionate, to some degree, to the amount of hubris said expertise had prior to the loss.

I am not sure, ultimately, if the response to TMI was roughly the same as Chernobyl. Chernobyl and Fukushima both resulted in major reactor shutdowns. TMI did not. It did contribute to a hostile environment for nuclear power in the United States, but new orders for reactors had already gone to zero before the accident, for economic reasons. TMI exacerbated those, adding decreased public interest and higher chance of lawsuits. But many new plants were constructed after TMI, and the US still has the largest nuclear fleet in the world. So I would not say it is really exactly the same.

Anyway, the most balanced history of TMI is J. Samuel Walker, Three Mile Island: A Nuclear Crisis in Historical Perspective (Berkeley: University of California Press, 2004) — definitely worth checking out!