Nuclear power plants are safe and a major nuclear accident is essentially impossible.

By 1977, the promise of commercial nuclear power was well established. The United States had 65 operating nuclear reactors contributing about 12 percent of the nation's electricity. France was further along, building toward a nuclear fleet that would eventually supply 75 percent of its power. The technology was depicted in school curricula as the responsible, clean-energy path forward, a settled engineering achievement rather than an ongoing experiment.

The safety argument rested partly on design and partly on track record. Light-water reactors, the American standard, were engineered with multiple redundant cooling systems. The Nuclear Regulatory Commission had established rigorous licensing requirements. The operating record through the mid-1970s was genuinely good; no American commercial reactor had experienced a major accident. Within this frame, concluding that a serious accident was extremely unlikely was reasonable.

What the frame excluded was the possibility of simultaneous, interacting failures that no single redundant system was designed to address. On March 28, 1979, a pump failure at Three Mile Island Unit 2 in Pennsylvania triggered an automatic reactor shutdown. The emergency cooling system engaged as designed. But a pressure-relief valve stuck open while displaying a "closed" reading on the control room instrumentation. Operators, interpreting the panel as showing normal conditions, took actions that inadvertently made the situation worse. The core was partially uncovered for several hours. Nearly a third of the fuel melted.

The accident released only small amounts of radiation into the environment and killed no one immediately. But it destroyed the claim that a serious accident was essentially impossible. The regulatory ratcheting that followed made nuclear power economically noncompetitive with alternatives. No new reactors were ordered in the United States after Three Mile Island. The lesson wasn't that nuclear power was necessarily unsafe; it was that "essentially impossible" had been doing a great deal of work in a claim that deserved more careful examination.