On March 28, 1979, Unit 2 at the Three Mile Island Nuclear Power Plant near Middletown, Pennsylvania, experienced a partial meltdown in what became known as the nation’s worst commercial nuclear accident.

The incident sparked national protests, prompted more stringent safety standards for nuclear plants across the country, and sapped the industry’s momentum for decades.

Nuclear energy is making a comeback of sorts given ambitious goals by environmental groups and Democratic presidential candidates seeking carbon-free alternatives to fossil fuels. But the stigma of Three Mile Island still looms large more than four decades after the accident.

At noon on Friday, TMI’s remaining reactor will generate its last kilowatt of energy and close down.

Here’s how the accident unfolded some 40 years ago:

At approximately 4 a.m., valves in the condensate polisher system in the turbine building closed, preventing pumps from sending water to steam generators, which cool the nuclear reactor with circulating water. The water in the reactor is pumped under high pressure to keep it from boiling.

Sudden increase in temperature

Within seconds, automatic systems sense a sudden increase in temperature and pressure inside the reactor and shut down the reactor with an emergency shutdown. The control rods drop into the reactor vessel to halt the chain reaction.

Read more: Three Mile Island, where a meltdown forever changed nuclear energy in America, shuts down Friday

A relief valve at the top of the pressurizer – a water tank that maintains reactor system pressure – opens to reduce stress on the system.

The valve should close as pressure falls to normal levels, but it becomes stuck open, allowing coolant water from the reactor to escape. This loss of coolant would ultimately lead to the partial meltdown. Operators do not know water is being lost. They believe the relief valve is shut since a “close” command was sent to the device. However, there is no way to tell if the valve is actually closed.

The Three Mile Island Unit 2 Control Room bustles during the crisis in 1979. NRC

Shutting down the pumps

High temperatures at that valve provided a clue that water was escaping there, but operators had been operating for weeks with a small leak at that valve and became accustomed to the abnormal high temperature.

Emergency cooling water is pumped into the reactor. Operators, believing there’s too much water in the system, reduce the flow, dropping water level.

Steam voids in the system cause other reactor cooling pumps to vibrate. Fearing damage, operators shut down the pumps.

The hot reactor core boils away water, exposing part of the core.

The reactor overheats and partially melts down. A chemical reaction forms a bubble of hydrogen gas inside the reactor.

Operators reduce the bubble’s size through periodic venting to the atmosphere through April 1. The plant enters a “cold shutdown” on April 27.

Aftermath

Unit 2 was closed permanently. The plant’s other nuclear reactor, Unit 1, returned to service in October 1985. It was licensed to operate until 2034, but is being closed due to financial losses.

The accident forced drastic changes in nuclear power plant operations and safety measures.

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SOURCE Nuclear Regulatory Commission; World Nuclear Association; American Nuclear Society; Reuters; Three Mile Island Alert; Associated Press; smithsonian.com