Three Mile Island: A Timeline of the Worst Nuclear Accident in US History | History Hit

Three Mile Island: A Timeline of the Worst Nuclear Accident in US History

Harry Sherrin

15 Mar 2022
President Jimmy Carter leaving Three Mile Island for Middletown, Pennsylvania, days after the incident.
Image Credit: Tango Images / Alamy Stock Photo

In late March 1979, the Three Mile Island nuclear generating station in Pennsylvania witnessed the worst nuclear incident in American history.

In Unit 2 of the plant, a valve surrounding a reactor core failed to close, leaking thousands of litres of contaminated coolant into the surrounding building and allowing the core’s temperature to rise. A series of human errors and technical complications then exacerbated the issue, with operators shutting off the reactor’s emergency cooling systems in the confusion.

The pressure and temperature of the core reached dangerously high levels, nearing meltdown, but catastrophe was ultimately averted. Low levels of radiation leaked from the plant into the atmosphere, however, leading to widescale panic and the partial evacuation of the surrounding area.

Here’s a timeline of the worst nuclear accident in US history.

28 March 1979

4 am

In unit 2 of Three Mile Island, an increase in reactor temperature and pressure led to a pressure valve opening, just as it was designed to do. The reactor then ‘scrammed’, meaning its control rods were lowered to stop the nuclear fission reaction. As the pressure levels dropped, the valve should have closed. It didn’t.

Cooling water began leaking from the open valve. This had two key outcomes: the surrounding tank began to fill with contaminated water, and the temperature of the nuclear core kept rising.

With coolant leaking from the valve, the unit’s emergency cooling system kicked into action. But in the control room, the unit’s human operators either misinterpreted their readings or received contradictory reports, and shut down the backup cooling system.

The reactor’s temperature continued to rise due to residual heat from the nuclear reaction.

Aerial photograph of the Three Mile Island nuclear plant.

4:15 am

The leaking, contaminated water ruptured its tank and begin to spill into the surrounding building.

5 am

By 5 am, the leaking water had released radioactive gas into the plant and out into the atmosphere through vents. The degree of contamination was relatively low – not enough to kill – but it highlighted the rising threat posed by the incident.

As the rising radiation levels were spotted, efforts were taken to protect workers at the plant. While doing so, the core’s temperature continued to rise.

5:20 am

Two pumps around the reactor core were turned off, contributing to the buildup of a bubble of hydrogen in the reactor that would later exacerbate fears of a possible explosion.

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6:00 am

A reaction in the overheating reactor core damaged the fuel rod cladding and the nuclear fuel.

An operator, arriving for the start of their shift, noticed the irregular temperature of one of the valves, so used a backup valve to prevent any further leakage of coolant. By this point, more than 100,000 litres of coolant had leaked.

6:45 am

Radiation alarms started ringing due to detectors finally registering the contaminated water.

6:56 am

A site-wide emergency was declared.

A Three Mile Island employee has their hand checked for radioactive contamination. 1979.

Image Credit: via Wikimedia Commons / Public Domain

8 am

News of the incident had leaked beyond the plant by this point. The Federal Emergency Management Agency had started to put an evacuation plan into action but had cancelled it by around 8:10 am.

The state governor, Dick Thornburgh, also contemplated ordering an evacuation.

9 am

Journalists and news crews started arriving at the scene.

10:30 am

By half 10, Three Mile Island’s owners, the company Metropolitan Edison (MetEd), had released a statement insisting that radiation had not yet been detected off-site.

5 pm

From 11 am to roughly 5 pm, MetEd consultants vented radioactive steam from the plant.

8 pm

The plant’s pumps were turned back on, and water was passed around the reactors again, lowering the temperature and easing the pressure levels. The reactor was brought back from the brink of total meltdown: at its most volatile, the core had reached 4,000°c, meaning it was 1,000°c – or about an hour of continued temperature rise – from meltdown.

The core was partially destroyed, but it hadn’t ruptured and didn’t appear to be leaking radiation.

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29 March 1979

8 am

As the cooldown operation continued, more radioactive gas was vented from the plant. A nearby plane, monitoring the incident, detected contaminants in the atmosphere.

10:30 am

Governor Thornburgh’s staff insisted local residents didn’t need to evacuate but said they should shut their windows and remain indoors.

30 March 1979

11:45 am

A press conference was held in Middletown, in which officials suggested that a bubble of potentially volatile hydrogen gas had been detected in the pressure vessel of the plant.

12:30 pm

Governor Thornburgh advised that pre-school children and pregnant women evacuate the area, closing various local schools. This, amongst other warnings and rumours, triggered widescale panic. In the following days, some 100,000 people evacuated the region.

1 pm

Schools started to close and evacuate students from within a 5-mile radius of the plant.

1 April 1979

Operators realised there was no oxygen in the pressure vessel, so the likelihood of the hydrogen bubble exploding was very slim: the bubble was vented and reduced, and the threat of meltdown or a serious radiation leak was brought under control.

President Jimmy Carter, in a bid to abate the public’s fears, visited Three Mile Island and toured the control room.


A huge cleanup operation of Unit 2 was carried out over the course of 11 years, only finishing in 1990. In 1985, while the clean-up continued nearby, Unit 1 started operating again.

Three Mile Island personnel clean up radioactive contamination in the auxiliary building. 1979.


Three Mile Island operated continuously for 680 days, breaking a global record for nuclear plants at the time. But in that same year, the plant witnessed another accident as a fire broke out on the site and caused hundreds of thousands of dollars worth of damage.


The plant was shut down on 20 September 2019, having failed to turn a substantial profit for several years.

Harry Sherrin