While most people were still asleep just before 4 a.m. on Wednesday, March 28, 1979, a malfunctioning valve set off what would become “the most serious accident in U.S. commercial nuclear power plant operating history.”
The incident at the Three Mile Island nuclear power station in Londonderry Township, Dauphin County, according to the Nuclear Regulatory Commission, happened when “a combination of equipment malfunctions, design-related problems and worker errors led to TMI-2’s partial meltdown and very small off-site releases of radioactivity.”
TMI was de-activated in 2019 after 45 years, but is on track to reopen by 2028 with a new name.
The accident began 46 years ago tomorrow. The Patriot-News printed this chronology a few weeks after the dust had settled:
March 28
3:53 a.m. – A malfunctioning valve causes the shutdown of two pumps in the secondary feedwater system, a separate unit that cools the water circulating through the reactor core, and triggers a chain reaction of mechanical and human errors.
3:53:09 – The reactor scrams, or stops the nuclear fission process by automatically dropping 67 control rods to mesh with the tubes of enriched uranium that form the reactor core.
3:53:12 – A relief valve on the reactor coolant pressurizer fails to close, eventually causing about 250,000 gallons of radioactive water to spill onto the floor of the reactor containment building.
3:53:30 – Three auxiliary pumps kick in but fail to provide cooling water to supplement the shut-down feedwater system because the valves that should have been open were closed.
3:55 – The Emergency Core Coolant System dumps tons of water into the reactor core to reduce the intense heat that should have been diminished with the assistance of the auxiliary feedwater system.
3:57:30 – A control room technician shuts down the first pump on the ECCS after either misreading or getting false readings from his instruments. A second pump was tripped about six minutes later, completely halting the last line of defense against overheating.
4:00:03 – A sump pump in the containment building activates and transfers water into a nearby auxiliary building. Radioactive steam from the water eventually finds its way into the atmosphere through a venting system.
4:01 – Technicians realize the auxiliary cooling system is not functioning and open the closed valves in an attempt to bring down the temperatures in the reactor.
5:22 – The plant operators shut down two other pumps controlling the flow of water around the reactor core, further contributing to the fuel damage and the rise of the hydrogen bubble at the top of the reactor vessel that was to deter cold shutdown steps into the next week.
6:50 – Plant officials first begin to realize the seriousness of the situation and that radiation may have escaped into the atmosphere. A site emergency is declared.
7:00 – Clarence Deller of the state Emergency Management Agency office is notified of the emergency and begins to launch an “alert for evacuation plan” for Dauphin, York and Lancaster counties.
An air view shows the Three Mile Island nuclear power plant near Harrisburg, Pa., March 30, 1979. The small dome at center is where the "incident" occurred. (AP Photo/Barry Thumma, file)ASSOCIATED PRESS
7:37 – Donald “Butch” Ryan, a fire-police dispatcher in Middletown, receives word of an emergency at the plant and is told to stand by.
8:10 – The Emergency Management Agency cancels its alert.
8:30 – Cumberland County emergency preparedness officials are notified of the accident.
8:55 – A Patriot-News reporter learns from county and state officials that an emergency has been declared at TMI. Initial reports indicated there was a radiation leak due to a “failure to fuel.” The first reporter is dispatched to the scene.
10:20 – Bill Gross, a Metropolitan Edison Co. public relations representative, reads a statement from Gary Miller, the station manager, reporting on the problem in technical terms and saying that radioactive monitoring teams have found nothing on or off-site.
11:00 – Between 11 a.m. and 1:30 p.m., MetEd engineers vent radioactive steam from the plant. During the same period, Lt. Gov. William Scranton III is holding the first of three news conferences, saying everything is “under control” and there appears to be “no danger to public health or safety.”
From left, President Jimmy Carter; Gov. Richard Thornburgh; and Lt. Gov. Bill Scranton, April 2, 1979. (The Patriot-News, file)
1:15 p.m. – John G. Herbein, MetEd vice president of generation, greets the news media for the first time at the TMI Observation Center, noting the “plant’s in safe condition.” He denies any delay in notifying authorities and contends radiation releases are very small. He indicates there may have been 0.1% or 0.5% damage to the 36,000 fuel rods.
4:30 – Gov. Scranton says the utility has ended “venting contaminated steam” into the air but accused MetEd of earlier giving the state “conflicting information.”
5 – The plant again vents radioactive steam.
March 29
10 a.m. – MetEd holds a press conference at the Hershey Convention Center where company president Walter M. Creitz says the cooldown is occurring at a “more rapid rate.” The radiation leaked from the plant has been in the low 5- to 7-millirems per hour level, according to company monitoring teams. The incident is nothing out of the “ordinary realm.”
12:15 p.m. – A news conference is held at the Friends Meeting House in Harrisburg by Dr. George Wald, a 1967 Nobel prize winner, and Dr. Ernest J. Sternglass, director of radiological physics for the University of Pittsburgh. Sternglass calls for the plant to be shut down and Wald says any radiation can cause harm.
3 p.m. – MetEd and Nuclear Regulatory Commission officials hold briefings for four senators and five congressmen. Sen. Gary Hart, D-Colo., labels the mishap the most serious accident in the history of nuclear power and calls for congressional hearings. Sen. George W. Gekas, R-Harrisburg, says he will introduce legislation to require immediate notification of officials in the event of any incident at the plant.
A TMI business task force meeting. Gov. Dick Thornburgh is at the podium. George Gekas is in the front from, seated next to the empty seat, June 7, 1979. (Allied Pix for The Patriot-News, file)
11 – An NRC official in Washington says the damage to the fuel rods is more serious than originally thought and indicates it will be a long time before the clean-up is finished.
March 30
7:15 a.m. – Between 7:15 and 8:15 a.m. an “uncontrolled” burst of radioactive gas is vented from the containment building into the auxiliary building and a plane carrying detection equipment measures radiation levels of between 300 and 1,200 millirems per hour.
9 – York County Civil Defense director Leslie Jackson puts phase 1 of the nuclear disaster plan for the county into operation, primarily to “stop the panic,” caused by unconfimed reports of evacuation.
The evacuation center at the Hershey Arena after the 1979 accident at Three Mile Island. (Photo by John Jones, file)
9:30 – E.C. McCabe, an NRC official, confirms the release of radioactivity to reporters outside the NRC trailer at the TMI Observation Center.
9:45 – Herbein tells a Patriot-News reporter that “some venting” was done in the evening and possibly radiation levels did climb “momentarily.” Asked if an area alert was in progress, he replied he was “not sure.”
10 – County Civil Defense organizations and the Pennsylvania National Guard are placed on standby for possible evacuation. The Associated Press moves an “urgent” story staying the governor is considering the evacuation of four counties.
10:35 – Gov. Dick Thornburgh’s press secretary Paul Critchlow indicates there is “no need to evacuate” but says the governor has warned people within a 10-mile radius of the plan to stay indoors and close their windows.
11 – A second release of radioactive gases occurs.
11:15 – President Jimmy Carter telephones Thornburgh and offers his assistance. A siren goes off in downtown Harrisburg, causing some panic.
11:45 – Herbein holds a news conference in Middletown where he tells reporters he did not see the need to warn Civil Defense officials of the latest release and, for the first time, considerable discussion centers on a bubble building in the reactor vessel.
John Jones of Hummelstown took this photo of a news crew covering the partial meltdown of the Unit 2 reactor at Three Mile Island in March 1979.
12:30 p.m. – Thornburgh issues his “advisory” for pregnant women and preschool-age children to evacuate from a five-mile radius of the plant. He also orders 23 schools in the area to close.
1 – Three West Shore School District schools begin evacuation.
4 – The Harrisburg East Mall closes early.
7 – Evacuees from the Middletown area line up for their first meal at Hersheypark Arena.
10 – Thornburgh holds a news conference in the Capitol News Center and introduces Harold Denton, the NRC official personally sent by Carter to supervise the operation. Thornburgh says that the indoor restriction will be lifted at midnight for residents within the 10-mile radius of the plant.
Gov. Dick Thornburgh, left, wears a glum expression as Harold Denton, right, director of operations for the Nuclear Regulatory Commission, briefs the media at the state Capitol in Harrisburg after the partial meltdown at TMI, March 30, 1979. Lt. Gov. William Scranton III is in the center. (The Associated Press, file)PN
March 31
4 a.m. – Evacuation of the residents of Frey Village nursing home in Middletown takes place as an independent decision of the home operators. Officials at the Odd Fellows Home of Pennsylvania, another nursing home in the Middletown area, make a similar decision somewhat later.
11 – MetEd holds its final news conference in Middletown and the responsibility for the emergency operations at the plant shifts to the NRC. MetEd and the NRC disagree over the size of the bubble. Herbein says he believes the “crisis is over.”
8:27 – An AP story marked “urgent” quotes an NRC source in Washington as saying the hydrogen bubble is so volatile there is a chance of an explosion.
10:10 – Denton and Thornburgh are conferring in the governor’s office when they are interrupted by a call from presidential press secretary Jody Powell informing them that President Carter would be visiting Three Mile Island.
11:50 – Denton at a news conference says he “sees no possibility” of an explosion in the reactor vessel or containment building. The Thornburgh advisory remains in effect. Mass evacuation is again discounted.
April 1
10 a.m. - Middletown Borough officials receive word that Carter is coming. Workers tear down a temporary press center set up for visiting reporters to permit Carter to hold his news conference in the borough hall.
2 p.m. – Carter dons yellow plastic booties and tours the control room at TMI-2.
It takes a TMI visit by then-President of the United States Jimmy Carter -- and more than a little luck – to finally calm public fears. (file photo)
3 – A grave Carter reads a brief statement to the press while his wife, Rosalynn, greets the crowd gathered outside the borough hall.
3:15 – Denton briefs reporters on the bubble and warns that the “next few days are critical” in reducing the danger. He raises the possibility of mass evacuation if the bubble is to be removed by increasing coolant pressure or blasting it out with high-pressure steam.
April 2
10:00 – MetEd engineers obtain a reading of about 175 cubic feet for the bubble. Estimates had placed the size of the sphere at about 850 cubic feet a few days prior.
11:30 – A “dramatic” decrease in the size of the bubble has been realized. Denton tells reporters during a news conference in the Middletown Borough building. An almost audible sigh of relief seems to emanate from the community as the crisis passes.
The times represented in this chronology in some cases are approximate.
A version of this story originally ran on PennLive in 2023.
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