Three Mile Island: The worst nuclear accident in US history. The most populated areas were threatened with infestation

From a purely statistical point of view, nuclear is one of the safest sources of energy. Even when accidents and malfunctions of nuclear submarines are taken into account, the total number of victims of atomic energy is in the order of thousands. Air pollution from fossil fuels, on the other hand, kills millions of people every year and its influence can be traced to one in five deaths on the planet. Opponents of nuclear power plants have one strong argument after all – the number of nuclear victims is relatively small only because a serious accident occurred only once in history. But several times we narrowly escaped disaster.

The nuclear power plant began to grow on the island surrounded by the Susquehanna River in 1968. The construction is an illustrative example of the optimistic attitude of the public towards nuclear energy in the 1960s. The blocks of the power plant stand literally within sight of the capital of Pennsylvania. In 1974, the first reactor was started, four years later the second, but the cooling towers shrouded in steam in Harrisburg of 50,000 did not worry anyone. Not that we were completely unaware of the potential risk. In March 1979, the movie China Syndrome was even made about him, in which Jane Fonda, Jack Lemmon and Michael Douglas face the accident of a fictitious nuclear power plant near Los Angeles. It caused an uproar in the US, but nuclear scientists appeared in the media with a god-like statement that the probability of such an event in reality is close to zero. Otherwise, we would not have built power plants so close to densely populated cities. Their confidence was to take a serious hit just 22 days after the premiere.

A chain of unfortunate coincidences and mistakes

On March 28th, 1979, at about four in the morning at the Three Mile Island Power Station, the pump supplying water to the steam exchanger of the generator stopped. The interruption of the water supply automatically shut down the generator turbine, the temperature from the core had nowhere to escape, and the pressure in the reactor piping began to rise rapidly. At the recently started reactor, it was an exceptional, but not unexpected event, and the safety mechanisms had anticipated it. Rods absorbing the neutron flux immediately fell into the core and the fission reaction stopped. At the same time, three emergency cooling safety pumps were started, which were supposed to inject cold water into the reactor’s cooling circuit. In two of them, however, the supply valves were inadvertently closed. In the control panel of the power plant, warning lights lit up on the control panel, but the operator did not notice them. The reasons for the oversight are not entirely clear. One indicator light was allegedly inadvertently covered with opaque tape, and the operator was said to have been unable to see the other due to his extraordinarily arched stomach. In any case, not enough cold water flowed into the safety cooling, and the residual heat of the stopped reactor began to overheat the pipes again. Even at this moment, automatic security systems intervened. When the pressure reached a critical point, a relief valve opened and released the steam from the pipe. Unfortunately, it did not fully close afterwards and the contaminated water continued to leak from the circuit. When the containment tank overflowed, it entered the space around the reactor. The operators made the situation worse. They explained the open safety valve by the fact that there is already too much water in the cooling circuit, and they manually turned off the only pump pumping cold water into the reactor.

Crisis and danger

A wrong decision started a chain of events that even the most sophisticated security systems could not cope with. The core temperature rose rapidly and the fuel began to melt. At the same time, the radioactivity of the water escaping through the unclosed overpressure valve was increasing, and a radiation alarm was declared at the power plant. A few minutes later, a weak radiation leak was detected outside the power plant, and the alarm was extended to its surroundings. It was only after five hours that the specialists of the state supervision of nuclear facilities arrived at the site, discovered the defect and restarted the emergency cooling. Both temperature and pressure began to drop. By then, news of the accident had already leaked to the media, and the governor of Pennsylvania was moments away from declaring a mass evacuation.

During the five hours of feverish effort, the situation at the power plant looked very serious. However, the true extent of the impending disaster was revealed only afterwards. Two days after the accident, the emergency team discovered that a pocket of hydrogen had formed in the reactor. The gas was created by the reaction of escaped superheated steam with a partially molten core and already caused several small explosions during the crisis in the morning hours. If the entire hydrogen bubble were to explode, the entire reactor and building would most likely be torn apart. The amount of radioactive materials that would then leak into the environment is difficult to estimate. But when the completely cooled reactor was opened years later, inspectors found that half of the uranium fuel had melted. The consequences of its dispersal along the densely populated East Coast of the US would therefore be catastrophic.

While the experts present argued whether there was enough oxygen for the explosion of the hydrogen bubble in the reactor, Governor Thornburg could not stand to wait any longer. He appeared on television and recommended that pregnant women and children stay away from a ten-kilometer radius around the power plant. Over 100,000 residents fled Harrisburg and the surrounding area in the following hours. Only President Jimmy Carter, who was a nuclear engineer by training and worked with atomic reactors during his service in the Navy, helped to quell the panic. When he personally visited the power plant on April 1, the fear subsided somewhat and people began to return to their abandoned homes. The hydrogen bubble dissipated due to the cooling of the reactor and the danger passed.

The end of the atomic age

Reports of how much radiation leaked into the vicinity of the plant vary. There are reports of residents experiencing burning eyes, nausea, vomiting and a metallic taste in the mouth within days of the crash. However, according to official measurements, they were only exposed to a small dose of radiation roughly equivalent to a visit to a radiologist. There were no immediate deaths from the radiation, and none of the power plant workers were killed either. The incident dealt a literal death blow to confidence in atomic energy. The symbol of a new era of humanity has turned into an invisible threat in the eyes of the public.

After the accident, 67 nuclear power plant projects were halted. The drastic tightening of regulations contributed to the overall safety of nuclear energy, but made the construction of new installations very expensive, and in the USA their construction was limited for a long time. A change of mood also affected Three Mile Island. The first block of the power plant, which was shut down for maintenance during the accident, was only started up again in 1985. It then changed owners several times and produced energy until 2019, when it was closed. The second unit was never repaired due to extensive damage. The clearing work lasted almost 20 years, the complete demolition will probably only be possible in 2052.