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Mechanical Failures Spark Nuclear Melt-Down in Three Mile Island Power Plan


Modern human civilization is built on and continues to be principally dependent on large quantities of energy to sustain it. While fossil fuels and hydroelectric plants have been the primary source of energy for men since the 19th century, these traditional sources have been stretched to the limit and are now unable to satisfy the global energy demands. Nuclear Generating Stations have been proposed and used as a feasible and cost-effective source of energy. Even so, there are major risks associated with nuclear power generation plants. Malfunctions of the systems at nuclear power plants can have dire consequences. These dangers arise from the risk of the radioactive material, which is used to generate the power, becoming exposed to the environment as a result of a breach to the containment unit of the nuclear power plant. This report will set out to analyze the system failure that resulted in the accident at the Three Mile Island (TMI) nuclear generating station on March 28, 1979. The paper will set out to give a detailed report of the mechanical failures that sparked the nuclear melt-down in the Three Mile Island power plant. The paper will begin by describing the structure of the TMI plant and the process that the plant uses to produce electricity. The mechanical failures that occurred at the plant and how they caused the accident at the TMI plant will be revealed. The errors in interpreting warning signs by the operators will also be highlighted. Finally, this report will consider how unclear control indicators resulted in the operators making assumptions that further escalated the situation.

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The Nuclear Reactor

The TMI nuclear plant is composed of two pressurized water reactors that are designated “TMI-1 and TMI-2” (Lilley 2010, p.1). In operation, the nuclear reactors work by heat being generated inside a nuclear core through the splitting of atoms through the process known as nuclear fission. This nuclear fission must be controlled since if it is allowed to go on uninhibited, an uncontrolled chain reaction will occur resulting in a nuclear melt-down. Therefore, control rods are used to regulate nuclear fission and therefore prevent a meltdown. The nuclear fission process results in the generation of vast amounts of heat energy.

The reactor has two main loops which are the primary and secondary loops. Lilley (2010) states that the primary loop pumps water through the core and while in contact with the core, absorbs the heat energy that has been generated through nuclear fission. Because this water is in contact with the core, it is radioactive and stays at very high temperatures. The primary system much be pressurized to prevent the water from turning into steam and for this; a pressurizer that has a pilot-operated relief valve (POV) that opens to release excess pressure is available. The system also has an emergency core coolant system (ECCS) which serves as a fail-safe in case the primary loop should suffer from inadequate coolant availability (Lilley 2010). The secondary loop also contains water which is heated by the primary loop as it passes through the heat exchanger. The water in the secondary loop is not radioactive since it does not come into direct contact with the primary loop water. The heat from the primary loop causes the water in the secondary loop to turn into steam and this steam is used to turn the generator which in turn produces electricity.

The accident at TMI in 1979 began as a result of a mechanical failure in the secondary loop. This failure resulted in the pumps ceasing operation which in turn caused the turbines to shut down. The control rods which regulate the rate of nuclear fusion were automatically lowered into the core to stop the nuclear reaction. Due to the closure of the main pumps, the backup pumps are activated automatically to cool the primary loop (Lilley 2010). However, the valves for the backup pump had been closed during maintenance services hence the backup system failed to pump water to cool the primary loop. As a result of the failure of the backup system, the residual heat built up despite the nuclear reaction having been shut down through the lowering of the control rods. There was no means to remove the heat built up inside the primary loop since the secondary loop and the backup pumps were not functioning. To counter the pressure build-up inside the primary loop, the POV opened to release pressure. However, a second mechanical failure caused the valve not to close even after the pressure had reduced to acceptable levels.

The leak of Nuclear Materials

The operators who were monitoring the progress at the nuclear plant did not notice that the POV valve remained open even after the pressure and heat reduced to normal levels. Water, therefore, started to flow from the primary loop through the open POV and the pressure inside the loop decreased up to the point where it could no longer prevent the highly heated water from turning into steam. This steam resulted in an increase in pressure in the primary loop despite the open POV. The control panels showed high-pressure readings which the operators assumed was caused by the POV valve being closed (Lilley 2010). A conclusion was therefore reached that the core was overflowing which caused the operators to shut off the ECCS. The pressure build-up from the steam in the core pushed more radioactive water through the POV into the adjacent tank and the tank overflowed and eventually burst. As it is, nuclear power plants are constructed such that there is minimal risk of the radioactive material escaping from the containment unit. The radioactive water, therefore, flowed into the containment building.

As time proceeded, the reactor core became exposed to the heat and steam that had been accumulating in the primary loop. This resulted in the fuel rods reacting with steam resulting in its melting as well as the release of more radioactive material to the coolant. This reaction produced hydrogen bubbles which prevented water from freely flowing in the core. A new shift of operators took over the operation of the power plant and they duly noted that the temperatures in the POV were excessive. They reacted by taking corrective action due to the realization that the system was experiencing a loss of coolant. However, their efforts were too late since as Lilley (2010, p.2) notes, “32,000 gallons of radioactive water had already spilled out of the primary loop”. 30 minutes later the radiation alarms were sounded which signaled that radiation levels in the containment system were excessive. It was approximated that radiation levels had reached 300 times the expected values. Since the problem had been correctly identified at this point, plant personnel began to work on how to cool the system.

Proximate Causes

The TMI accident was caused by the malfunctioning of the pumps in the secondary loop of the nuclear reactor system. Due to this malfunction, the heat from the primary loop could no longer be removed. When the POV was opened to relieve the pressure and heat that had built up in the primary loop, a mechanical failure prevented the POV from closing once pressure levels had dropped to normal levels (Lilley 2010). Radioactive water, therefore, flowed out of the open valve into a tank which broke down releasing contaminated water into the containment unit. The unpressurized water in the core vaporized therefore reacting with the core which further increased the radioactivity levels in the primary loop.

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Once the initial mechanical failure of the secondary loop occurred, several backup pumps were designed to take over operation in the event of main pumps failure. The isolation valves for the backup pumps were closed during a maintenance check which prevented the pumps from operating well. The plant operators did not know this since they used a POV indicator to tell if the valve was open or closed. The POV indicator light was linked to the power line that powered the POV and not the POV itself (Lilley 2010). As such, the light indicated the status of the power source (On/off) and not the status of the POV itself (open/close). Another problem was that the plant lacked a direct means through which to assess the level of water at the core. The operators, therefore, relied on other indicators such as temperature and pressure gauge to approximate the water level in the core.


This paper set out to give detailed documentation of the mechanical failures that sparked a nuclear melt-down in the Three Mile Island power plant. The paper began by giving a brief overview of the nuclear reactor whose key components include the primary loop which pumps water through the core absorbing heat generated by the nuclear fission process, and the secondary loop which pumps steam into turbines, therefore, turning them and generating electricity. The TMI plant experienced mechanical failures which caused water to stop circulating in the secondary loop. This resulted in an increase in the heat and pressure in the primary loop. Backup pumps failed to take over when they should have, heat in the primary loop increased causing the POV to open. The POV failed to close which resulted in a reduction of pressure which caused water in the primary loop to turn into steam and overflow. These failures resulted in significant damages the most serious of which was the spilling of 32000 gallons of radioactive water into the containment area.

Nuclear power plants are indispensable to satisfy the growing demands for energy in the country. This is because nuclear fusion is capable of producing electricity on a large scale and therefore offers a solution to the increasing energy needs. Even so, accidents in nuclear power generating plants can have dire consequences. From the report on the TMI accident, it is clear that system failure at the nuclear generating station can have catastrophic consequences. As such, care should be taken to ensure that these power generating stations are error-free to avoid the damages that an accident can cause.


Lilley, S 2010, “System Failure Case Studies: Island Fever”, NASA Safety Center, Vol 4, Issue 3.

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