Three Mile Island - America's Chernobyl

In March 1979, the Three Mile Island nuclear power plant experienced a "loss-of-coolant" accident and a partial core meltdown, the effects of which could have been catastrophic.
Marcia Wendorf

On March 16, 1979, the movie entitled The China Syndrome starring Jane Fonda, Michael Douglas and Jack Lemmon premiered. The film concerned an accident at a Southern California nuclear power plant that could have caused a meltdown of the reactor core, which would have then melted through the reactor building floor until it hit groundwater.

The resulting steam explosion would have spread radioactivity throughout the region and killed an untold number of people.

The movie was met with a backlash from the nuclear power industry who claimed it was "sheer fiction" and a "character assassination of an entire industry."

In the film, a nuclear engineer named Dr. Lowell reviews a secret tape made of the accident:
Dr. Lowell: It looks serious. In the control room, these lights are concerned with core water level. They might have come close to exposing the core. If that's true, we came very close to the China Syndrome.
Kimberly Wells: The what?
Dr. Lowell: If the core is exposed, the fuel heats up in a matter of minutes. Nothing stops it. It melts through the ground, theoretically to China. But when it hits ground water, it explodes into radioactive clouds. The number killed depends on the wind. Render an area the size of Pennsylvania uninhabitable, with many cancer cases later.

The name "China Syndrome" refers to the melted core's ability to burn all the way through the earth until emerging out the other side, or, in more specific terms: to China.

In reality, a core could not penetrate the several-kilometer thickness of the earth's crust, and it certainly could not travel back upwards against the pull of gravity. Also, China is not the antipode of any landmass in North America.

A Case of Life Imitating Art

The timing of the film could not have been more propitious, nor could Dr. Lowell's statement has been more portentous: "Render an area the size of Pennsylvania uninhabitable" because just 10 days later, on March 27, 1979, workers at the Three Mile Island nuclear power plant in Londonderry Township, Pennsylvania were dealing with something every homeowner knows well — hard water.

TMI-2, Three Mile Island reactor number 2, was running at 97% of the power while its sister reactor, TMI-1, was shut down for refueling. On that Monday afternoon, workers were attempting to remove a blockage in one of eight resin condensate polishers, which were filters that cleaned secondary loop water, stopping minerals and impurities from accumulating in the steam generators.

Diagram of TMI-2
Diagram of TMI-2. Source: U.S. Nuclear Regulatory Commission/Wikimedia Commons

That is how a nuclear reactor works, the nuclear fuel heats water that turns into steam, which drives a turbine which creates electricity.

When compressed air failed to dislodge the stuck resin, the operators blew compressed air directly into the water, which forced a small amount of water past a stuck-open check valve, and into an instrument air line. Hours later, this would cause an emergency shutdown of the turbine or a turbine trip.

4:00 a.m. Wednesday, March 28, 1979

With the turbine shut down, heat and pressure immediately increased in the reactor coolant system, and the reactor performed an emergency shutdown or SCRAM. SCRAM is supposedly an acronym for "Safety Control Rod Axe Man," and it was coined by the creator of the world's first nuclear reactor, Enrico Fermi.

The first nuclear reactor was built under the spectator stands at the University of Chicago's Stagg Field. On December 2, 1942, when the first sustained chain reaction occurred, Fermi created the acronym SCRAM for Norman Hilberry, whose job it was to kill a possible runaway reaction by using an axe to cut a rope that held the control rods, thus allowing them to drop into the pile.


At Three Mile Island, the SCRAM caused the control rods to be inserted automatically into the core to halt the nuclear chain reaction, but the reactor continued to generate residual decay heat. Because steam was not being used by the turbine, heat was not being removed from the reactor's primary water loop.

Three auxiliary pumps should have activated automatically, but their valves had been closed for routine maintenance. This was a clear violation of a key Nuclear Regulatory Commission (NRC) rule which stated that the reactor should have been shut down if all the auxiliary feed pumps were closed for maintenance.

Pressure in the primary loop increased, triggering the pilot-operated relief valve at the top of the pressurizer to open automatically. Once the excess pressure had been released, this valve should have closed, but it remained stuck open, permitting the cooling water to escape from the system.

In the control room, a light on the control panel indicated that the solenoid that controlled the value had power, not that the valve was closed. This caused operators to miss the problem entirely, and it was not until a new shift of workers arrived that the problem was correctly diagnosed.

A Loss-of-Coolant Accident

Within the nuclear core, steam voids formed as the water boiled, and these bubbles blocked the flow of coolant, causing the temperature of the nuclear fuel to rise. Operators thought there was an overabundance of water in the core when the opposite was actually true. They were now heading for a "loss-of-coolant accident."

At 4:11 a.m., an alarm sounded, and at 4:15 a.m., the relief diaphragm of the pressurizer relief tank ruptured, allowing radioactive coolant to leak into the containment building. As the remaining water in the reactor was converted into steam, the top of the reactor core was exposed.

The intense heat caused a chemical reaction with the fuel rod cladding, which created zirconium dioxide and hydrogen gas. Just like at Chernobyl, this hydrogen gas produced an explosion. By now, radiation levels in the primary coolant were 300 times higher than normal, and the containment building was contaminated with radiation. Just like at Chernobyl, the melted fuel had formed the lava-like substance corium.

A "site area emergency" was declared at 6:57 a.m., and a "general emergency" was declared 30 minutes later. This second emergency carried with it "potential for serious radiological consequences" to the general public.

Metropolitan Edison (Met Ed) alerted the Pennsylvania Emergency Management Agency and other state agencies. After learning that Met Ed had not informed them before venting radioactive steam, State officials contacted the U.S. Nuclear Regulatory Commission (NRC).

The Evacuation

Two days after the accident, Pennsylvania's lieutenant governor William Scranton III reassured the state that everything was under control, only to reverse that statement later that same day. Schools closed, and residents were asked to stay indoors.

On the advice of the NRC, pregnant women and pre-school age children within a five-mile radius of the plant were evacuated. By Friday, March 30th, the evacuation zone was extended to a 20-mile radius. By April 1979, 98% of those evacuated had returned to their homes.

Three Mile Island accident marker
Three Mile Island accident marker. Source: Z22/Wikimedia Commons

The Aftermath of the Accident

It wasn't until 30 years later, in 2009, when the reactor vessel was physically opened, that the NRC learned that fuel temperatures had been measured "near the melting point," and that "roughly half of the uranium fuel had already melted."

A report following the accident determined that "... approximately 2.5 MCi (93 PBq) of radioactive noble gases and 15 Ci (560 GBq) of radioiodines were released" resulting in an average dose of 1.4 mrem (14 μSv) to two million people living near the plant. For point of reference: a patient receives 3.2 mrem (32 μSv) from a chest X-ray.

An inter-agency analysis concluded that the accident did not raise radioactivity enough to cause even one cancer death, however, measures of beta radiation were not included. The U.S. Environmental Protection Agency (EPA) found no contamination in water, soil, or plant samples.

President Jimmy Carter created a commission headed by John Kemeny, president of Dartmouth College, to study what happened at Three Mile Island. Among the commission's findings was that the same pilot-operated relief valve had previously failed on 11 occasions, nine in the open position, which allowed coolant to escape.

The exact same things that happened at Three Mile Island had happened at another Babock & Wilcox-manufactured reactor at the Davis-Besse Nuclear Power Station in Ohio. The difference was that operators at that plant had identified the valve failure after only 20 minutes, while operators at Three Mile Island had taken 80 minutes to identify the problem. Babcock & Wilcox's engineers had recognized the problem but failed to notify their customers.

The Effect of Three Mile Island

Three Mile Island's effect on the nuclear power industry was swift. Between 1980 and 1984, orders for 51 nuclear reactors were canceled and it wasn't until 2012 that a nuclear power plant was authorized to begin construction in the U.S. Cleanup costs for Three Mile Island totaled 1 billion, and the accident caused $2.4 billion in property damage.

Reduced reactor capacity
Reduced reactor capacity. Source: Global Warming Art/Wikimedia Commons

Three Mile Island is the most significant accident in U.S. commercial nuclear power plant history. On its seven-point International Nuclear Event Scale, Three Mile Island was rated a five. Only two incidents have earned a seven — Chernobyl on April 26, 1986, and Fukushima Daiichi on March 11, 2011.

This past May, it was announced that TMI-2's sister reactor, TMI-1, is scheduled to be shut down by September 30, 2019, a victim of falling wholesale electricity prices and cheap natural gas from hydraulic fracturing.

Add Interesting Engineering to your Google News feed.
Add Interesting Engineering to your Google News feed.
message circleSHOW COMMENT (1)chevron
Job Board