The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant in Pennsylvania on March 28, 1979 was one of the most serious in the history of the U.S. nuclear industry. It not only brought to light the hazards associated with nuclear power, but also forced the industry to take a closer look at the operating procedures used at the time. What makes the TMI-2 accident such an interesting case study is the series of events which led up to the partial meltdown of the reactor core. It was a combination of human error, insufficient training, bad operating procedures and unforeseen equipment...
The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant in Pennsylvania on March 28, 1979 was one of the most serious in the history of the U.S. nuclear industry. It not only brought to light the hazards associated with nuclear power, but also forced the industry to take a closer look at the operating procedures used at the time. What makes the TMI-2 accident such an interesting case study is the series of events which led up to the partial meltdown of the reactor core. It was a combination of human error, insufficient training, bad operating procedures and unforeseen equipment failure that culminated in a nuclear accident that could have easily been prevented.
It is important to understand the basics of how a nuclear reactor produces electricity and what the basic components of the reactor at Three Mile Island were. Three Mile Island was the home to two nuclear plants, TMI-1 and TMI-2. Their combined power generating capacity was 1,700 megawatts or enough electricity to supply 300,000 homes. In a nutshell, nuclear power plants generate electricity by using steam turbines. The function of the nuclear fuel is to heat water and convert it to steam. Now in theory this sounds quite simple, but the hazards associated with nuclear fuels add to the complications. A nuclear reactor such as TMI-2 can be divided into various sub-systems.
In order to understand the accident at TMI it is essential that we have a rudimentary understanding of how these systems function.
A nuclear reactor generates heat by harnessing atomic energy. The core of an atom, which is also called the nucleus, can be split by bombarding it with neutrons. When a free neutron strikes the core it splits the atom into two smaller atom fragments, giving off energy in the form of kinetic energy of the fragments, plus 2 or three neutrons -and likely some gamma rays-. One of the neutrons produced by this reaction strikes another atom, which in turn produces more energy and more neutrons. This chain reaction continues from one atom to another producing nuclear fission energy 1.
The TMI-2 Reactor Core held 100 tons of uranium to fuel the nuclear reaction. This was in the form of cylindrical uranium oxide pellets that were one inch tall and roughly one half inch wide. These pellets were stacked inside 12-foot long fuel rods made of a zirconium alloy (Zircaloy-4). The entire reactor consisted of 36,816 such fuel rods along with 69 control rods, 52 instrument tubes and spaces between them for the cooling water to flow.
Control rods are used to control the amount of power produced by the reactor, or to completely halt the reaction in the case of an emergency. The control rods are able to control the rate of nuclear fission taking place by absorbing a portion of the free neutrons produced. The rate varies depending on the number of rods and the length of the rod inserted into the core of the reactor. TMI-2 used control rods made of 80% Silver, 15% indium and 5% cadmium.
Control rods are designed such that in the event of an emergency the magnetic clamps holding them release the rods into the core of the reactor thus completely halting the reaction.
The Instrument Tubes contain instruments used to measure such things as the temperature within the reactor.
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There are three basic closed loop Water Systems in a nuclear power plant such as the one at TMI.
The first water system is the Primary Loop or the Reactor Coolant System. This is a closed loop system that circulates water through the nuclear core and serves two purposes. Firstly, by coming in contact with the hot fuel rods the water is heated. At the same time the water flowing through the core cools down the fuel rods preventing them from melting. It is important that the water coming in contact with the reactor core remain in liquid form so that it can effectively cool the rods. In order to prevent the water from turning into steam it is pressurized to about 2,155 psi. In the event of an accident either an increase in temperature or a decrease in pressure can convert the heated water to steam, this is undesirable but not dangerous because the steam too can help cool the reactor core.
Once the steam from the Feedwater System has driven the turbines it goes to the condenser where it is cooled and converted back to water. The final water system is the Condenser Water that is cooled in the cooling towers. This is the water used to cool the steam from the turbines and is also a closed loop water system. This ensures that the radioactive water from the reactor cannot contaminate the water in the cooling towers or the water vapor that is released from the cooling towers to the atmosphere.
Under normal operating conditions the primary loop is the only system which contains traces of radiation.
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Heated water from the reactor goes to the Steam Generator where it is passed through corrosion resistant tubes. At the same time water from the Secondary Loop or Feedwater System is passed through the steam generator around the tubes containing hot water from the reactor. This converts the Secondary Loop water into steam and also cools down the reactor coolant water. It is important to note that at no point does water from the two separate systems mix with each other.
The steam is then sent to the Steam Turbines where it runs the electricity-producing generator while the relatively cooler water of the Reactor Coolant System is sent back into the reactor core to pick up more heat and repeat the whole cycle.
The Pressurizer ensures that the reactor coolant water is kept at the correct pressure. If for some reason the water pressure increases above safe levels the Pilot Operated Relief Valve (PORV) is supposed to open, thus relieving excess pressure by releasing water/steam to the drain tank. While this solves the immediate problem and prevents the reactor vessel from getting damaged, it reduces the level of water in nuclear core.
A reduction in the reactor core water level would lead to the core being uncovered. This is undesirable because it would allow the core temperature to increase beyond safe levels. If the core temperature were to reach around 2,200 degrees Fahrenheit the water would begin to react with the cladding producing hydrogen. This could potentially explode. If the temperature were to reach 5,200 degrees Fahrenheit the uranium could potentially melt, releasing far more radioactive materials into the surroundings.
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