On the 35th anniversary of the most serious accident in U.S. commercial nuclear power plant operating history, it seems fitting to take a look back at the Three Mile Island Unit 2 (TMI-2) meltdown.

One of POWER’s contributing editors—Kennedy Maize—wrote an article on the 30th anniversary that recollected his experience covering the event. It’s still a good read, but there are probably a lot of younger folks who don’t know the specifics of what happened on March 28, 1979.

TMI-2 began commercial operation on Dec. 30, 1978. The industry was forever changed 88 days later, when a failure of the main feedwater pumps began the sequence of events that would result in the meltdown. With no feedwater being sent to the steam generators, the heat sink for the reactor was removed.

The plant operated as designed and automatically shut down the reactor. As typically happens, pressure began to increase in the primary system and a pilot-operated relief valve opened to prevent an overpressure condition. However, the valve failed to close properly once the pressure had returned to a safe level, which allowed pressure to continue decreasing and additional cooling water to escape.

Instruments that should have helped the reactor operators understand the situation provided confusing information, including false indications that the relief valves were closed. As a result, the operators did not understand why pressure was falling and some of their subsequent actions made matters worse.

Operators shut off the reactor coolant pumps because of vibration concerns and reduced emergency cooling water addition to prevent overfilling the pressurizer, but these actions starved the reactor core of coolant causing it to overheat. Eventually, the cladding—the thin‑walled metal tube that forms the outer jacket of a nuclear fuel rod—ruptured and the fuel pellets began to melt. It was later determined that about half of the uranium pellets melted during the early stages of the accident.

The Nuclear Regulatory Commission (NRC) dispatched a team of experts as soon as the news of the accident reached authorities. The Department of Energy (DOE) and the Environmental Protection Agency also mobilized response teams. TMI’s owner—General Public Utilities (GPU)—and the DOE had sampled radioactivity in the atmosphere above the plant by mid-day and the operators had restored cooling to the core by that evening.

Within a short time, however, new worries emerged. Chemical reactions in the melting fuel had created a large hydrogen bubble in the dome of the pressure vessel—the steel container that holds the reactor fuel. Experts worried that the hydrogen bubble might explode, rupturing the pressure vessel and the containment building, and releasing radioactive contamination to the surrounding area. Pennsylvania’s governor advised pregnant women and young children within a five-mile radius of the plant to leave the area.

On Sunday, April 1, experts determined that the hydrogen bubble would not be able to burn or explode due to the lack of oxygen in the pressure vessel. At that point, GPU had succeeded in greatly reducing the size of the bubble and the crisis had been averted.

Although TMI Unit 1 continues running to this day, TMI-2 was never operated again. The reactor coolant system is drained, and the water has been decontaminated and evaporated. Radioactive waste, reactor fuel, and debris from the core have all been shipped offsite.

The NRC made extensive regulatory changes. New requirements aimed at reducing human errors affected plant designs, human performance standards, and fitness for duty programs. The NRC also expanded incident response staffing, emergency planning, and its resident inspector program.

Thousands of environmental samples of air, water, vegetation, soil, and food were collected. Thorough assessments by respected organizations concluded the actual release of radiation was small and had negligible effects on the physical health of individuals and the environment, but the public’s perception of nuclear power would never be the same.

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)