Companion piece · 2026-10-26
By Morgan Miller
On March 28, 1979, a nuclear reactor at Three Mile Island, Pennsylvania, experienced a partial core meltdown.
The accident began with an equipment malfunction and developed through a combination of misleading instrumentation, inadequate procedures, and operator decisions.
Radioactive gases were released. Residents received conflicting information about the reactor's condition, and more than 100,000 people eventually left the surrounding area.
No deaths were established as a result of radiation exposure.
The accident nevertheless became the most serious in the history of American commercial nuclear power.
Part of the explanation lies in the reactor's control room.
Another part lies in what the public was told.
A Reactor Less Than Three Months Old
Three Mile Island was located on the Susquehanna River, approximately ten miles southeast of Harrisburg, Pennsylvania.
The station contained two pressurized-water reactors operated by Metropolitan Edison, a subsidiary of General Public Utilities.
Unit 1 entered commercial service in 1974. Unit 2 began commercial operation on December 30, 1978.
It had been operating commercially for fewer than three months when the accident occurred.
The reactor was designed by Babcock & Wilcox and used pressurized water to carry heat from its nuclear fuel to steam generators.
The arrangement depended upon maintaining adequate coolant circulation, even after the nuclear chain reaction stopped.
Stopping a reactor does not immediately eliminate the heat produced by radioactive decay.
That distinction became important at approximately 4:00 a.m. on March 28.
March 28: A Misleading Indicator
A malfunction interrupted the supply of feedwater to the steam generators.
The turbine and reactor automatically shut down.
Pressure increased in the reactor's primary cooling system, opening a pilot-operated relief valve.
The valve was supposed to close when pressure decreased.
It remained stuck open.
In the control room, an indicator showed that the electrical signal commanding the valve to close had been sent. It did not indicate the valve's actual position.
Operators believed the valve was closed while reactor coolant continued escaping.
Emergency cooling systems activated automatically, but the operators reduced their output.
Their attention was focused on the pressurizer's indicated water level, which they incorrectly interpreted as evidence that the reactor contained sufficient coolant.
As pressure fell, the reactor coolant pumps began vibrating and were eventually stopped.
The core became partially uncovered and overheated.
A substantial portion of the nuclear fuel melted.
The reactor pressure vessel remained intact, preventing the molten fuel from escaping into the surrounding environment.
The operators did not initially understand the extent of the damage.
Neither did the people responsible for explaining it.
March 28–29: The First Reassurances
Metropolitan Edison initially presented the event as a manageable technical incident.
Pennsylvania Lieutenant Governor William Scranton repeated company assurances that no significant radiation had escaped.
He subsequently learned that a radioactive release had occurred.
The company's principal public spokesman was John G. Herbein, commonly known as Jack Herbein, its vice president for power generation.
Herbein was an engineer, not an experienced public-relations representative.
He became responsible for answering questions from an expanding group of journalists, residents, and public officials.
At a March 29 press conference, he rejected the suggestion that the reactor had faced a serious danger of complete meltdown.
He also emphasized that the accident had not killed anyone.
The statement concerning fatalities was accurate.
His broader characterization of the reactor's condition was considerably more reassuring than the available information justified.
Metropolitan Edison was attempting to explain an accident whose severity its own personnel had not yet established.
March 30–31: Conflicting Authorities
On March 30, Pennsylvania Governor Richard Thornburgh recommended that pregnant women and preschool children within five miles of the plant leave the area.
The recommendation was precautionary rather than a general evacuation order.
Nevertheless, an estimated 140,000 residents voluntarily departed.
Concern increased when officials discussed a large accumulation of hydrogen inside the reactor vessel.
The possibility of an explosion became a prominent subject of public briefings, although later assessments established that the necessary conditions for a hydrogen explosion inside the vessel were absent.
Harold Denton, director of reactor regulation at the Nuclear Regulatory Commission, was dispatched to Pennsylvania to coordinate the federal technical response.
On March 31, Herbein told reporters that he personally believed the crisis was over.
Approximately one hour later, Denton publicly disagreed, stating that the crisis would continue until the reactor reached a safe, cold shutdown condition.
The company's position and the federal regulator's assessment were plainly inconsistent.
Metropolitan Edison subsequently ended its independent news conferences, leaving federal authorities to conduct the principal briefings.
The communications problem was no longer limited to individual statements.
Residents had been receiving incompatible assessments from organizations responsible for their safety.
April 1: A Presidential Visit
On April 1, President Jimmy Carter and First Lady Rosalynn Carter visited Three Mile Island with Governor Thornburgh.
Carter had trained in nuclear engineering during his naval career.
The visit provided a public demonstration of confidence while technical personnel continued examining the reactor.
By that afternoon, specialists had substantially resolved concerns surrounding the hydrogen bubble.
The immediate crisis gradually subsided.
The damaged Unit 2 reactor never generated electricity again.
Radiation and Public Health
Radiation monitoring and subsequent epidemiological investigations found that environmental releases had been relatively small.
The NRC estimated that approximately two million people living around Three Mile Island received an average additional dose of about one millirem, equivalent to 0.01 millisieverts.
Major health investigations did not establish detectable radiation-related deaths or a measurable increase in illness attributable to the accident.
Some residents and community groups have continued to dispute the official conclusions.
The psychological and social consequences were more readily documented.
Families evacuated, businesses were disrupted, and public confidence in nuclear regulation declined.
For many residents, the uncertainty was itself a substantial consequence.
Investigations and Accountability
On April 11, Carter established a presidential commission chaired by Dartmouth College president John G. Kemeny.
Its October 1979 report identified failures extending beyond individual operators.
Equipment design, training, management practices, regulatory oversight, and emergency communication all required improvement.
The nuclear industry subsequently established the Institute of Nuclear Power Operations, while the NRC strengthened operator training, emergency planning, control-room design requirements, and regulatory oversight.
Cleanup continued for nearly twelve years, costing approximately $973 million.
Fuel removal began in 1985. By 1990, most of the damaged fuel had been removed.
Unit 2 entered monitored storage in 1993.
The Failure of Reassurance
Three Mile Island demonstrated that a technically contained nuclear accident could still become a major institutional crisis.
Herbein did not cause the reactor malfunction. His statements did not produce an established radiation fatality.
However, premature assurances and conflicting official accounts contributed to a serious breakdown in public confidence.
The accident was ultimately controlled through engineering and emergency response.
Restoring confidence proved more difficult.
I'm Morgan Miller. The valve failed to close. The information gap remained open.