The Three Mile Island nuclear disaster

Accidents happen, but when they upset nature's delicate equilibrium or result in significant human suffering, they turn into disasters. Here I am going to write about one of the biggest catastrophes brought on by human activities in American history, "The Three Mile Island nuclear disaster".

The Three Mile Island power plant, located close to Harrisburg, Pennsylvania, had the worst nuclear reactor disaster in American history on March 28, 1979. When coolant (the liquid that keeps a machine cool) escaped from the reactor core owing to a mix of mechanical failure and human mistake, no one was killed and very little radiation was spilled into the air.

About the Nuclear plant

The Three Mile Island (TMI) nuclear power plant is located in Pennsylvania, not far from Harrisburg. Two pressurized water reactors were present. Until its closure in 2019, TMI-1, an 880 MWe (or 819 MWe net) PWR, was one of the best-performing units in the USA. It went into operation in 1974. At the time of the disaster, TMI-2 had a 959 MWe (880 MWe net) capacity and was essentially brand new.

What happened that day?

A number of water pumps in the TMI-2 unit "tripped" at 4:00 in the morning of March 28, 1979. When the pumps failed, the water supply to the steam generators ceased, which led to an increase in the reactor coolant's temperature. Water that was rapidly heating up expanded as a result of the rising pressure. The pressurizer's top valve opened as it was intended to, but the pressure kept building. Just as it was intended to, the reactor "scrammed," and the control rods descended into the core to halt the nuclear fission reaction. When the pressure gradually returned to normal levels, the valve ought to have closed, but it didn't.

What might merely have been a minor annoyance was made worse by a confluence of mechanical and human faults. Because they feared the core "turning solid"—having too much water and losing control of pressure—they seized physical management of the water system. Alarms were sounding but no valuable information was given to the operator. Due to measuring devices sending erroneous data to the control room; technicians started keeping an eye on rising radiation readings at around 5:00 a.m. Around 6:30 a.m., an on-site emergency was declared. The facility remained in crisis for a few days after that, and eventually, radiation was purposely released into the atmosphere to release pressure within the system and prevent the potential of a hydrogen bubble explosion, which was then suspected but later disproved. 

Chain of Events

The reactor cooling system's pilot-operated relief valve (PORV) opened as it was meant to shortly after the shutdown. It ought to have shut down after about 10 seconds. But it remained open, dripping essential reactor coolant water into the drain tank for the coolant. Instruments showed the operators that a "close" signal was transmitted to the relief valve, which led them to believe the valve had closed. However, they lacked a tool for determining the valve's precise location. High-pressure injection pumps automatically injected replacement water into the reactor system in response to the loss of cooling water. Cooling water gushed into the pressurizer, boosting the water level while water and steam escaped through the relief valve.

In response, operators decreased the flow of replacement water. Their training had taught them that the only reliable indicator of the amount of cooling water in the system was the level of water in the pressurizer. They believed the reactor system was overloaded with water since they noticed an increase in the pressurizer level. According to their training, employees should use every effort to prevent the pressurizer from becoming flooded. If it filled, they wouldn't be able to control the cooling system's pressure, and it might even burst. The reactor's primary cooling system then started to produce steam. The pumps employed in the reactor cooling system vibrated when pumping a steam-and-water mixture. Operators turned off the pumps because the high vibrations may have destroyed them and rendered them useless. This put a stop to the reactor core's forced cooling. Because the pressurizer level remained high, the operators continued to believe the system was almost full of water. The fuel core of the reactor was exposed and heated up considerably more when the reactor cooling water boiled away. Due to the damage to the fuel rods, radioactive material was released into the cooling water.

To Read about: The Three Mile Island nuclear disaster

And lastly, workers shut a block valve between the relief valve and the pressurizer at 6:22 a.m. By taking this action, the relief valve-related coolant water loss was stopped. However, the core cooling system's water flow was obstructed by superheated steam and gases. Operators tried to pump more water into the reactor system throughout the morning in an effort to condense steam bubbles that they thought were obstructing the flow of cooling water. Operators tried to lower the pressure in the reactor system throughout the afternoon so that a low-pressure cooling system could be employed and emergency water supplies could be added to the system.

Finally, at about 8 o'clock, plant managers recognized they needed to restart the pumps in order to get water flowing through the core once more. Pressure in the reactor decreased as the temperature started to drop. Less than an hour separated the reactor from total meltdown. Although more than half of the core was damaged or in the process of melting, the core's protective shell was intact, and no radiation was leaking out. It appeared that the problem was over.

But on March 30, two days later, a bubble of extremely flammable hydrogen gas was found inside the reactor structure. When exposed core materials reacted with extremely hot steam two days prior, a bubble of gas resulted. Some of this gas had erupted on March 28 and some radiation had been dispersed into the environment. Plant staff members were not aware of the explosion at the time because it sounded like a ventilation door closing. Upon learning of the radioactive leak on March 30, locals were told to stay inside. As a precaution, Governor Thornburgh recommended: "pregnant women and pre-school age children to evacuate the region within a five-mile radius of the Three Mile Island facility until further notice." Experts were unsure whether the hydrogen bubble would cause further melting or even a massive explosion. As a result, the governor's attempt to prevent panic was successful; within days, more than 100,000 residents had left the nearby towns.

40 Years Later TMI Shut down on September 20, 2019

TMI-2 had a difficult time recovering from the mishap. Since then, TMI-1 has continued to function normally. The power plant started consistently and dependably losing money since it was initially built to run two cores, TMI-1 and TMI-2. It was revealed that the power plant would finally shut down in 2017 after the current owner, Exelon, was unable to persuade Pennsylvania state legislators to appropriate the required cash to maintain the power plant's competitiveness versus less expensive energy sources, like natural gas. The TMI-1 program ended formally on September 20, 2019. On the day of the closure, Exelon issued a statement in which it expressed sadness that "state legislation does not permit the ongoing operation of this safe and reliable source of carbon-free power" at a time when "our communities are seeking more clean energy to address climate change." Decades will pass throughout the decommissioning procedure, which is expected to cost at least $1 billion.

Health issues:

The acute health consequences described by certain locals and documented in two books cannot be explained by the official figures because they call for exposure to at least 100,000 millirems (100 rems) to the full body, which is 1000 times higher than the official estimates. Although there are many other possible reasons, the documented health impacts are consistent with high doses of radiation and analogous to the experiences of cancer patients receiving radiotherapy. Metal taste, erythema, nausea, vomiting, diarrhea, hair loss, farm, and wild animal deaths, and plant damage were some of the side effects. In Dauphin County, where the Three Mile Island plant is located, the death rate among infants under one year represented a 28 percent increase over that of 1978, and among infants under one month, the death rate increased by 54 percent. These local statistics demonstrated dramatic one-year changes among the most vulnerable. These figures were included in the 1981 version of physicist Ernest Sternglass' book Secret Fallout: low-level radiation from Hiroshima to Three-Mile Island. Sternglass is an expert in low-level radiation. The Pennsylvania Department of Health concluded that the TMI-2 disaster did not contribute to any local newborn or fetus deaths in its final 1981 report after looking at death rates in the 10-mile radius around TMI for the six months following the event.

As the Kemeny Commission had determined that this was the only effect on public health, scientific research continued in the 1980s but concentrated mainly on the mental health effects of stress. The TMI Public Health Fund eventually reviewed the data and supported a thorough epidemiological study by a team at Columbia University after a 1984 survey of 450 local residents by a local psychologist revealed acute radiation health effects (as well as 19 cancers among the residents in 1980–84 against an expected 2.6.

Other Chemical Disaster: Chernobyl Tragedy (Nuclear Annihilation )