Three Mile Island: A History of America’s Worst Nuclear Accident

In 1970, nuclear power was assumed to be the bright future of humanity, providing us with nearly limitless quantities of cheap clean energy for the foreseeable future. Instead, by 1980, nuclear power in the US was dead, and no new plants were being built. Part of the reason was the accident at Three Mile Island in Pennsylvania, which remains the worst nuclear accident in the US and the third-worst in the world.


Three Mile Island nuclear power plant                    Photo from US Dept of Energy

On March 28, 1979, the managers at the Three Mile Island nuclear plant, located on an island in the Susquehanna River near Harrisburg, Pennsylvania, had a minor but annoying problem.

Like most nuclear power plants, the Three Mile Island facility was built in two basic parts. The “primary loop” was the reactor itself, consisting of the uranium fuel pellets, coated in zirconium, which are configured in long thin fuel rods. The reactor also contains “control rods” made with cadmium, an element that absorbs neutrons, which are used to control the energy level of the reactor and, in an emergency, shut it down. The whole reactor is bathed in constantly circulating water, which serves both to slow down the neutrons inside the reactor so they can split the uranium atoms to produce energy, and to carry away the huge amounts of heat generated by the fission process. At the top of the reactor is a pressurizer, which is designed to equalize the pressure inside the water coolant system and to prevent “water hammers” caused by shock waves that can travel hydraulically through the pipes. The reactor itself is sealed inside a thick steel “pressure vessel”, and the entire primary system is in turn located inside a thick concrete “containment building”. These are safety measures: the pressure vessel is designed to seal off the highly radioactive reactor core from the outside world, and the containment building is designed to hold in any releases of radiation from an accident.

In the most basic terms, the job of a nuclear reactor is to produce heat. This heat is then used to boil water to make steam. As a safety measure, the steam is not actually produced inside the primary reactor loop itself, since the water inside the primary loop is highly radioactive; instead, heated water from the primary loop runs through a “heat exchanger” and heats water to produce steam in an entirely separate non-radioactive water loop, the “secondary”. In the secondary loop, the hot steam is piped to a generator turbine, where it spins the turbine blades and produces electricity. Once it exits the generator, the water goes to a series of filters to be “polished”, a process that cleans out all the small particles and solid deposits that might otherwise build up inside the delicate turbines and damage them. The polishing is done by running the water through a number of large tanks containing thousands of tiny plastic resin beads, which trap and absorb all the impurities like an enormous fish-tank filter.

Every so often, these beads have to be replaced with clean ones, and this was the task that was being carried out on Unit Two at Three Mile Island on March 28. Normally the old beads were flushed out of the polishing tanks with water. Occasionally, though, the beads would get compacted together at the bottom of the tank, and then a compressed air hose could be used to blow them out. But today, that step failed too, and the beads remained stubbornly stuck. The reactor was running at 97% capacity, and it could not be shut down for such a minor problem. Some solution had to be found.

The plant supervisors and workers improvised. They connected an air hose to the more powerful air system that controlled the valves in the turbine loop, hoping it would be strong enough to dislodge the beads. But unknown to everyone, the workers on the previous shift had already tried this–and had accidentally left one of the valves open. This now allowed water from the secondary loop to move up through the air line towards the open valve. At 3:58 am on the morning of March 28, the water reached the valve. It slammed shut, and the other valves on the other tanks all slammed shut automatically too. The turbines, sensing the drop in pressure, automatically shut themselves off–which in turn triggered an automatic shutdown in the reactor eight seconds later.

In the control room, the warning panels suddenly lit up like Christmas trees and alarms started blaring, but at this point, the situation was not serious. “Turbine trips” had happened before, and so far, all of the emergency systems had acted as they should, the reactor shutdown was a routine safety response–and since the secondary loop was non-radioactive, there was no real safety issue involved. But then another problem entered the picture . . .

With the turbines offline and the secondary loop shut down, the water in the secondary was no longer carrying away any heat from the reactor, and as this heat stayed inside the reactor core, it caused the pressure to begin to rise. As it should, the relief valve in the pressurizer opened to lower the pressure. But as the pressure dropped to the normal level, the valve stuck open and did not close as it should have–but the indicator light on the control panel showed it as closed. This seemingly insignificant malfunction would destroy the reactor and cause the worst nuclear accident in US history.

In the control room, the operators had no instruments that could read the actual level of coolant water in the reactor; their instruments only read the water level in the pressurizer above it. Because they did not realize that the relief valve was stuck open, they assumed that their readings indicating that the pressurizer was full of water meant that the reactor levels were normal too, and when the emergency coolant pumps started up automatically, the operators shut them down, mistakenly thinking that they would overpressurize the system. In reality, the water supply inside the primary coolant loop was now boiling itself away, pushing hot water and steam up into the pressurizer and then out through the open relief valve, finally falling onto the floor of the containment building. As the water level inside the reactor fell, the top of the core was exposed, the fuel pellets began to glow cherry-red, the zirconium coatings melted, and fission products escaped from the now-exposed fuel pellets and were carried away in the escaping steam, to contaminate the pool of water gathering in the floor of the building. Radiation alarms began to sound inside the containment building. The top half of the reactor core, meanwhile, melted itself into a mass of white-hot lava which pooled into the bottom of the reactor. Fortunately, the thick steel pressure vessel did what it was supposed to do–it contained the hot liquid mass and prevented it from melting out through the bottom of the reactor vessel, through the building’s floor, and into the earth below (the so-called “China Syndrome”, which would have resulted in a massive radiation plume that would have contaminated thousands of square miles).

Finally, two hours and eighteen minutes after the emergency shutdown, there was a shift change, and the new crew of operators realized that the relief valve was stuck open and closed a secondary valve to seal the primary loop. It was too late. The core was already a melted puddle and the reactor was already destroyed (though nobody knew the actual extent of the damage until years later, when camera probes were finally able to enter the radioactive wreckage). Sixteen hours after the shutdown, the operators finally turned the emergency coolant pumps back on and poured water into the core. The temperature began to decrease.

But the problems were not over yet. The containment building was flooded with 32,000 gallons of radioactive water (the exposed fuel pellets had contaminated the coolant water with 300 times the normal level of radiation) which had boiled out through the stuck valve. Then, somehow, radioactive water also began entering the auxiliary building next door, which was not secured against the escape of radiation (it was later discovered that one of the pumps had been fitted with the wrong pipe). Radiation alarms began ringing all over the plant, including inside the control room. At 7 am, the plant’s owner, Metropolitan Edison Electric Company, declared a site emergency, then half an hour later declared a general emergency, indicating a potential release of radiation to the surrounding area. The Nuclear Regulatory Commission and Pennsylvania Governor Dick Thornberg were notified of the accident. President Jimmy Carter, a former US Navy submariner with training in nuclear physics, also inspected the plant.

Three days after the shutdown, however, a new problem appeared. The tremendous heat and the chemical reactions inside the melted core had caused some of the remaining coolant water to separate into oxygen and hydrogen, and a large bubble, 20,000 cubic feet, had formed in the top of the reactor’s pressure vessel. This mixture was explosive–measurements indicated that there had already been at least one small explosion inside the reactor vessel on the first afternoon of the accident, as the hydrogen bubble was just beginning to form (an event that Met Ed had not announced publicly and had not informed the Governor or the NRC about). Now, with the bubble much bigger, an explosion would be enough to tear open the pressure vessel and possibly damage the containment building as well–leading to a large release of radiation directly into the atmosphere. Gov. Thornberg issued an advisory for the voluntary evacuation from the nearby city of Harrisburg of pregnant women and children, first within a five mile radius, and then extended to a 20-mile radius. Something of a panic resulted, with some 50% of the nearby population evacuating. (During this time, the diarist was living in Allentown, Pennsylvania, about 80 miles east of Three Mile Island and directly in the path of any wind-driven radioactive atmospheric plume that would have been released.)

Met Ed, meanwhile, vented the hydrogen bubble the only way it could–by opening the relief valve again and letting the radioactive steam and gases inside the pressure vessel empty out into the atmosphere. Plant operators also pumped out some of the radioactive water in the auxiliary building, draining it into the Susquehanna River. The NRC later concluded that the total radiation exposure for anyone in the area from all the known sources of radiation release totaled about 14 micro-sieverts (roughly half the amount received from a chest x-ray). Most of this was in the form of gaseous iodine-131, krypton-85, and xenon-133 that had escaped through the pressure valve (either when it was stuck open or when the operators vented the hydrogen bubble through it), though other radio-nuclides were carried out when the radioactive water was pumped into the river. Some claims were made that there was a series of unreported hydrogen explosions that released larger amounts of radiation, but this has never been confirmed by any data, and there were no known explosive breaches of either the pressure vessel or the containment building.

Within days of the accident, the EPA began taking samples at the site, and found radiation levels only slightly above normal background; nearby Dickinson College, which had its own radiation monitoring equipment, found similar results. In the decades after the accident, the populations near Three Mile Island have been monitored for health effects. It was reported that in the five years after the accident there were 19 local cases of cancer instead of the 2.6 cases that would be statistically expected, and this led to a study conducted by Columbia University in 1990, which studied the entire population within a ten-mile radius of the plant. The study found that while there had been 1722 cases of cancer in that area from 1975 to 1979, there were 2,831 between 1981 and 1985–a 64% increase. But the study pointed out that it was not known how many of these cancers were due simply to better methods of cancer detection. The rate of childhood leukemia in the area around TMI, the study noted, was actually lower than average. The study was not able to establish a correlation between radiation levels during the accident and subsequent cancer diagnoses. Another study by the University of Pittsburgh in 2003 also concluded that while there seemed to be a slight increase in cancers and mortality in the years after the accident, peaking in 1983, it was not possible to establish a definite connection between exposure to radiation and subsequent health problems. And a 2008 study focusing on thyroid cancers found a slightly higher rate in the area as compared to the surrounding counties, but was also not able to positively link radiation doses to the incidence of thyroid cancers. So while it is possible that the radiation releases at TMI did produce some number of cancers, we will never know–there is no way to definitively tie any particular cancer to the radiation exposure.

After the accident, the Unit One reactor, which had been down for refueling, went back into service and is still functioning today. The destroyed Unit Two was allowed to sit inside its containment building for decades until the radiation levels had dropped, then the nuclear core was removed and the generator unit was salvaged.


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