Updated 3/18/2011 at bottom
According to Scientific American, about twenty percent of the electricity manufactured in the United States comes from nuclear energy. While nuclear power falls behind coal and natural gas as the most-used source of electric power, Japan and other countries produce nuclear power and use it at a higher rate than the United States.
Given the delicate nature of nuclear reactors and power plants, an emergency at a nuclear power plant can escalate into a meltdown if the proper precautions are not taken at all times, not just in emergency situations.
How Nuclear Reactors Work
Scientific American describes nuclear reactors as "essentially high-tech kettles that efficiently boil water to produce electricity."
At the center of the reactor is a core of radioactive material. In most reactors, this core is made of enriched uranium, or different uranium isotopes that have been mixed together to foster the fission process. As the two types of isotopes, uranium 235 and uranium 238, interact with one another, the process of fission, or splitting of atoms, begins to occur. Atoms that have undergone the fission process release neutrons, or subatomic particles that carry no electric charge, that are absorbed by other atoms. These atoms become unstable due to excess neutrons, and the fission process begins again.
Inside the nuclear reactor is a rod, usually made of cadmium or similar element, that serves to catch stray neutrons that are released during the fission process. While the reactor is in operation, the rod is kept out of the nuclear core to allow the natural fission process to continue without interruption. If there is an instance in which the reaction needs to be slowed or stopped, says Scientific American, then the rod is placed into the nuclear fuel to absorb the neutrons and stop the reaction.
The continuous fission process produces a large amount of heat, which causes the water surrounding the nuclear core to boil, and the steam from the water is used to power turbines that produce electricity. The water, however, also helps to keep the reactor at a lower temperature to maintain a safe operating temperature.
Pumps continue to supply the reactor with cool water into the vessel, which is brought to a boil by the heat from the reaction, and as the water is turned to steam and powers the turbines to produce electricity, the water is then cooled back to liquid form and pumped back into the reactor. If this mechanism malfunctions, the temperatures inside the reactor continue to rise and create unsafe conditions, which could lead to a nuclear meltdown.
What Causes A Nuclear Meltdown?
According to CNN, the nuclear processes inside a reactor will continue as normal as long as the control rod can keep the reactions under control. If the rod is, for some reason, unable to control the reaction, then the temperature inside the reactor begins to rise from the safe level of 1,400 degrees Fahrenheit.
Once the temperature inside the reactor reaches or exceeds 2,200 degrees Fahrenheit for a prolonged period of time, then the fuel rods begin to sustain damage. As the temperature continues to rise beyond the critical 2,200 degrees Fahrenheit mark, it is also possible that the nuclear core can vaporize and leak radioactive material into the surrounding area. The control rods may also melt and cause the reaction inside the reactor to spiral out of control.
If a reactor is kept at dangerously high temperatures over a period of time, it is also possible for the containment vessel, usually made of concrete and steel, surrounding the nuclear fuel, containment rods, and water to become damaged. Once the containment vessel is damaged, the contents of the reactor are exposed to the surrounding environment and can escape into the surrounding air, ground, and water.
Nuclear meltdown occurs when workers and scientists are no longer able to control the uranium reaction and the containment vessel has been damaged, allowing the fuel to escape into the surrounding environment.
Measuring the Severity of Nuclear Meltdowns
The International Atomic Energy Association (IAEA) has a seven point scale to measure the severity of nuclear emergencies called the International Nuclear Emergency Scale (INES.) INES is used by nuclear scientists to communicate the severity of unnatural nuclear phenomena to the surrounding community and the world. Natural phenomena that are a result of the fission process are not included on the INES. As the IAEA assigns a higher number to a particular nuclear emergency, the more severe the situation is determined to be.
Anything with a INES ranking of four or higher is considered an accident, while occurrences ranked as a one, two, or three are known as incidents. Only one nuclear emergency, the Chernobyl explosion in 1986, has been assigned a rank of seven. The 1979 incident at the Three Mile Island nuclear plant in Pennsylvania was ranked as a five, or an "accident with wider consequences" according to the INES pyramid scale.
The nuclear crisis at the Fukushima Daiichi plant following the March 11, 2011, earthquake and tsunami off the coast of Japan has not officially been assigned a ranking by the IAEA, but Japanese officials have given it a preliminary ranking of a four, or an "accident with local consequences." As the situation continues to unfold at Fukushima Daiichi, however, it is possible that the IAEA or Japanese officials may upgrade the rating based on the effects of the earthquake-damaged reactors and the amount of radioactive material that is exposed to the surrounding area.
While nuclear power may be an alternative way to produce electricity, there are dangers to producing nuclear power that can result in a nuclear meltdown. Under normal circumstances, nuclear meltdowns are prevented by a number of scientific and safety precautions, allowing meltdown to occur only when all safety controls have failed.
UPDATED 3/18/2011: According to a CNN report, the IAEA has raised the severity of the Japanese nuclear crisis from a 4 on the INES graph to a 5, making it an "accident with wider consequences." This makes the ongoing nuclear incident that followed the March 11, 2011, earthquake and tsunami the equivalent to the Three Mile Island incident in 1979.
Sources
- "Agency: Japanese nuclear crisis on par with 3 Mile Island" by the CNN Wire Staff, CNN.com, March 18, 2011
- "Japan's nuclear concerns explained" CNN Interactive, CNN.com
- "International Nuclear Events Scale" International Atomic Energy Association, IAEA..org, copyright 1998-2011
- "What Happens During a Nuclear Meltdown?" by John Matson, Scientific American, published March 15, 2011
Ashley Anderson - Ashley is an Ohio-based writer and editor with specializations in higher education, writing, and social activism.
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