The Three Mile Island nuclear power plant sits upon the Susquehanna River, just south of Harrisburg, Pennsylvania. The plant was state of the art during its day, equipped with the most modern computerized control and safety systems. The public was told that an accident was impossible as the plant’s safety systems had, ‘backups to backups’.
At the time, nuclear power was an exciting prospect. In 1979, America was still reeling from an oil crisis, and the idea of cheap domestic energy got a lot of attention. Nuclear power was sold to the public as the energy of the future.
Construction of the plant began in 1968, with Reactor Unit 1 commissioned in 1974. Work on Reactor Unit 2 began in 1969, but wasn’t completed until December 1978. By 1979, both units were operational, and steam could be seen rising from all four of the plant’s massive cooling towers.
HOW IT WORKS
The process of generating electricity from nuclear fission is relatively simple in concept. The goal of the process is to harness the power of atoms to create usable energy, such as heat. The reactor itself made up of three distinct parts: The fuel rods, control rods, and moderator.
The fuel rods contain enriched uranium; a refined radioactive element. Uranium atoms are radioactive because they sheds neutrons, which travel at alarming speeds. These subatomic artillery shells fly around with enough force to actually split other uranium atoms, generating an enormous amount of heat and energy. This is the magic behind nuclear fission. Uranium ore is mined, processed, and placed into long metal tubes. These tubes, or fuel rods, are placed in close proximity inside the sealed reactor vessel.
The tricky part of this process is achieving a sustained, continuous chain reaction. This is where the moderator comes in. The moderator (water, in this case, or graphite in Soviet RBMK reactors like Chernobyl) slows down flying neutrons, causing the reaction to take place at a sustainable rate.
Finally, control rods are placed in channels between the fuel rods. Control rods are made of a dense element (such as boron) which absorbs flying neutrons, effectively stopping the reaction. Control rods are the throttle and the brakes of the reactor.
During a sustained reaction, heat is harnessed in the form of steam, which is piped out of the reactor into a massive set of steam turbines. The turbines turn a shaft, which turns a generator and produces electricity. After the steam leaves the turbines, it flows through pipes into giant cooling towers. The steam is not released into the atmosphere; instead, it remains in a closed circuit, and is condensed by water from the river, which passes over the pipes. The steam rising from the giant towers is steam from river water used to cool the pipes. The cool, condensed steam is then piped back into the reactor, and the process begins again.
In the early morning of March 28th, 1979, workers at the newly commissioned TMI Unit 2 reactor were attempting to unclog an air filter. To do so, the workers shot compressed air down a pipe, hoping to dislodge the contaminants inside. Somehow, a small amount of water entered the air pipe, and reached a sensor. The sensor alerted the plant’s central computer system, which reacted as though the air system was full of water. The air system was vital for actuating heavy valves and pumps, and water in the system would be disastrous. That night, there was not enough water to damage the system, but the computers were not designed to differentiate, only to detect its presence. As a result, the computers shut down the pumps, and deprived the reactor of its vital cooling water.
In a moment, the instrument panels in the reactor control room went into a frenzy. Operators scrambled to make sense of the situation, but the data presented to them was contradictory and confusing. At this point, the computer triggered a reactor shutdown, and the control rods descended into the core, stopping the reaction. However, even with the reaction stopped, enormous heat and pressure continued to build.
Soon after the shutdown, even as operators restarted the pumps, heat and pressure inside the reactor vessel caused a relief valve to automatically open. The relief valve, called a ‘PORV’, illuminated a light on the control panel, alerting the operators. Unfortunately, the PORV was defective, and failed to close again after the pressure within the vessel normalized. However, the PORV indicator light on the control panel went out, and the operators believed the valve had closed. Nobody ever informed the operators that the light was only designed to indicate that the order to close had been sent. The control order met no response when it reached the valve, and nobody in the control room had any idea.
Soon after, another major miscommunication between man and machine occurred. At the time, there was no way to directly measure how much coolant was in the reactor. The instruments only measured pressure, which was increasing due to coolant levels being too low. The operators now believed the reactor was in danger of ‘going solid’, or filling with water, when in fact the reactor was rapidly losing water. They made the decision to shut off the pumps.
With the pumps shut off, the remaining coolant started to vaporize and escape through the stuck valve. At this point, the reactor operators noticed the temperature continuing to increase, and realized that the instruments could no longer be trusted. To make matters worse, word began to spread that there was a problem at the plant. The control room had only one phone line, which was now constantly tied up by reporters and anxious residents of the nearby towns. Engineers from Babcock & Wilcox, who designed the reactor, were unable to reach the control room operators.
Eventually, engineers from Babcock & Wilcox managed to send word to the operators, who (by then) had discovered the stuck valve. They turned on the pumps again, cooling the reactor, preventing a complete meltdown. By this time, it was too late for the reactor; cameras placed inside eventually discovered that the core did indeed begin to melt.
If the operators waited just a little longer to restart the pumps, the result would have been disastrous. If the reactor melted completely, the reaction could have restarted, causing a scalding radioactive lava flow to burn through the concrete floor and into the ground. This phenomenon is called ‘the China syndrome,’ as (in theory) radioactive lava would burn all the way down to China. In reality, the molten nuclear lava would contact ground water and explode, rendering a radius of more than 30 miles permanently uninhabitable.