Three Mile Island Meltdown of 1979: Causes and Aftermath

Introduction

Early in the morning on March 28, 1979, America's worst fears were realized when a nuclear plant in Pennsylvania suffered an accident that was later classified as a meltdown. Since that time, the nuclear construction program has been halted, and people have remained fearful that a similar event could occur at any of the hundreds of nuclear power plants across the country. The accident was handled without adverse health impact on surrounding communities, and the investigation that followed was intensive. Nevertheless, what happened that day could easily happen anywhere, and it raised urgent questions about where and how nuclear power plants should be built and operated in the future.

Nuclear accidents are among the nation's greatest fears. Films depict what might happen if a meltdown occurs, and few can forget the horror of Chernobyl. During the early morning hours of March 28, 1979, the most serious commercial nuclear power plant accident in United States history occurred at approximately 4:00 a.m. According to the Nuclear Regulatory Commission, "even though it led to no deaths or injuries to plant workers or members of the nearby community, it brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations."

History of the Accident

During that fateful morning, workers were performing their regular duties and nothing appeared to be out of place. The accident originated in an area called the secondary non-nuclear section — notably, in a part of the plant not classified as nuclear, which underscores the wide range of hazards inherent in nuclear power plant operations.

Subsequent investigations revealed precisely how the accident and consequential meltdown occurred. At approximately 4:00 a.m., the main feedwater pump that supplied water to the secondary section stopped working. Whether the cause was electrical or mechanical failure remains unclear, but the pump ceased to function. This pump was responsible for feeding water to key plant components and enabling the steam generators to remove heat. When it stopped, heat removal was no longer possible.

"First the turbine, then the reactor automatically shut down. Immediately, the pressure in the primary system — the nuclear portion of the plant — began to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve, located at the top of the pressurizer, opened."

Under normal operating conditions, the valve is supposed to close once pressure decreases by a measurable amount. In this case, however, the valve failed to close. That was the first problem leading toward meltdown. Compounding matters, the signal that should have alerted workers to the valve's failure to close also malfunctioned, so no one was notified of the malfunction. As a result, cooling water began pouring out through the stuck-open valve, and the reactor core started to overheat.

"As coolant flowed from the core through the pressurizer, the instruments available to reactor operators provided confusing information. There was no instrument that showed the level of coolant in the core. Instead, the operators judged the level of water in the core by the level in the pressurizer, and since it was high, they assumed that the core was properly covered with coolant."

With mechanical and human error working together to produce a mistaken conclusion, no one moved to reverse the situation, and the reactor continued to heat up. "As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident. They took a series of actions that made conditions worse by simply reducing the flow of coolant through the core."

Nuclear fuel depends on adequate cooling, and when that failed, the fuel overheated to the point that a process began which caused the fuel rod cladding to rupture. The cladding consists of long metal tubes that contain the nuclear pellets. When this cladding melts and the pellets are no longer properly contained, the potential for serious and long-term adverse effects on residents for many miles around the plant becomes very real.

Later investigation determined that roughly one half of the cladding tubes had melted during the initial moments of the accident. The investigation also confirmed that the accident represented the most serious type of core meltdown that can occur in a commercial nuclear power plant — one that could easily have placed hundreds of miles of surrounding residents at risk had it not been discovered and addressed as quickly as it was.

"Although the TMI-2 plant suffered a severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the worst-case consequences that reactor experts had long feared. In a worst-case accident, the melting of nuclear fuel would lead to a breach of the walls of the containment building and the release of massive quantities of radiation into the environment. But this did not occur as a result of the Three Mile Island accident."

The Fallout and Public Response

When the accident occurred, several difficult decisions had to be made. The governor of Pennsylvania faced an enormous dilemma: he could evacuate the immediate area as a precautionary measure, or he could wait and hope that things would be brought under control before the public was placed at risk. Waiting and later discovering adverse health consequences would make him responsible for those outcomes — something no public official wants on their conscience. Conversely, ordering an immediate evacuation risked triggering mass panic, with millions of people rushing to leave the area, potentially causing more deaths and injuries than waiting it out would have. He ultimately made the difficult call to evacuate pregnant women and close area schools.

While the reactor was brought under control and shut down by the evening of March 28, concerns remained about radiation that had leaked into the atmosphere and was dispersing across surrounding residential and commercial areas. "A significant release of radiation from the plant's auxiliary building, performed to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core, caused a great deal of confusion and consternation. In an atmosphere of growing uncertainty about the condition of the plant, the governor of Pennsylvania, Richard L. Thornburgh, consulted with the NRC about evacuating the population near the plant."

With experts advising him on both the potential dangers of not evacuating and the risk of mass panic if he did, Governor Thornburgh made the difficult decision to evacuate those most vulnerable to radiation exposure. Pregnant women and preschool-age children were identified as the highest-risk populations. A five-mile radius around the power plant was designated, and those who were pregnant or of preschool age living within that zone were instructed to evacuate immediately.

This announcement sparked a near-panic among several thousand people, who hastily packed belongings and flooded the roads in an effort to leave as quickly as possible. Compounding the crisis, it was discovered that a hydrogen bubble had formed in the dome of the pressure vessel. This raised fresh concerns: if the bubble exploded, it might cause the containment wall to collapse and release extremely dangerous quantities of radiation into the air.

Conclusion

The accident at Three Mile Island, later classified as a core meltdown, quickly dispelled the previously held belief among many Americans that nuclear power plants on U.S. soil posed no serious danger. President Carter and his advisors sent representatives to the site, and he personally appeared there with his wife to demonstrate that the area was safe five days after the accident. Residents were evacuated based on age and pregnancy status but were quickly allowed to return to their homes once the problem was confirmed to be halted. Later studies found that radiation exposure for those in surrounding areas did not exceed the equivalent of a single X-ray.