Three Mile Island

From Chempedia

On March 28, 1979 the Three Mile Island nuclear power plant in Pennsylvania experienced a partial meltdown in the reactor core of unit 2 (TMI-2). This introduced many fears about the safety of nuclear power in the United States. While the results of the event are still being debated, the consequences of Three Mile Island are still prevalent. The reputation of nuclear power in the United States has never fully recovered.

In the early hours on the 28^th, at about 4:00 am, a pump that provided water to the steam generator malfunctioned for an unknown reason. Without this pump, heat could not be removed from the reactor. Control rods automatically dropped into the reactor, halting the fission process, when the pump stopped. However, the products of fission were still giving off heat so the pressure began to rise. The valve on the pressurizer opened to reduce the excess pressure. Up to this point, everything operated as expected. Unfortunately, a series of mechanical failures, human error, and flawed design caused what was the most serious accident in U.S. commercial nuclear power plant operating history.^[1]

When the pressure was sufficiently reduced, the steam valve should have closed, but it did not. This caused steam to continue pouring out and severely reduced the level of water in the system. The operators at the plant did not know that the valve was still open, and since the level of water in the system was designed for measuring by the level in the pressurizer, they assumed everything was fine. When the main pump failed, backup pumps turned on, but a valve was closed and prevented the pump from supplying water to the system. With these malfunctions, heat continued to accumulate in the reactor core and steam collected around it, pushing more water into the pressurizer. In this situation of coolant (water) loss, an emergency water supply was supposed to enter the system, but the operators prevented this because they thought the system was full. Since no water was available to remove the he at, the reactor core started to break down and melt into the remaining water.

Finally, more than two hours later, an operator found out that the steam valve had not closed, and water was immediately introduced into the system. Unfortunately, by this time a large portion of the fuel had already melted. It was found later that a small amount of radioactive water was released into the surrounding environment during the meltdown. The actual amount of radiation released is unknown, but estimated at about 25-50 milli-rem (A rem is a measurement unit for absorbed radiation doses in humans. The average member of the U.S. population receives about 0.36 rems of radiation annually from naturally occurring radiation, medical uses of radiation and consumer products).^[2]

The excessive heat created another hazard; the overheated zirconium started to react with the water and produced hydrogen gas. This was not realized until 2:00 pm when something caused the gas to ignite. The hydrogen was not addressed until 2 days later. The area was then evacuated for fear of an explosion releasing radioactive material. The nuclear reactor used by Three Mile Island is a Pressurized Water Reactor (PWR). This refers to the type of heating/cooling system used to turn the turbines to generate electricity. There are three separate cooling systems in the PWR. The first is the Reactor Coolant System, this is the only system that is radioactive. It is connected to the reactor and steam generator. As pressurized water passes by the reactor, it is heated and transferred to the steam generator. In the steam generator, the Reactor Coolant System meets with the Secondary Coolant System. As steam is generated from the Reactor Coolant System it is used to turn the turbines. Water from the Condenser Cooling Water system is pumped from the Susquehanna River in Pennsylvania^[3] and is used to cool the steam into water from the Secondary Coolant System.^[4]

The overall reaction that drove TMI-2 and every current commercial nuclear plant in the United States is called nuclear fission. The process starts by colliding a slow moving neutron into an isotope known as Uranium-235. This reaction decays the Uranium-235 into Barium, Krypton, three neutrons, and energy.

This reaction is important because it releases ten times as much energy as normal nuclear decay. It also releases three neutrons which can be collided into more uranium-235 isotopes. Now if this reaction were to go on without intervention it would become highly unstable and dangerous to harness energy from it. In turn, substances are needed to slow down the neutrons, which are called moderators. The neutrons collide into these atoms which lowers the kinetic energy of the neutrons, therefore slowing the reaction. Water can be used as a moderator but it often absorbs the neutrons which stops the reaction. That is why either graphite or heavy water (Deuterium, which is an isotope of hydrogen that has a proton and neutron in the nucleus) is used instead of water.^[5]

After the TMI partial meltdown, there were various health and environmental concerns in the surrounding public because of the released radiation. It was certain that radiation had been given off from the accident, but to what extent was unknown. Various researchers and study groups tested the area to see what kind of damage had been done. They found that the majority of the radioactive material that was released into air was noble gases Xenon and Krypton, which cannot be absorbed by the human body. There were also small amounts of radioactive iodine and other elements that were released into the air. After taking close to 10,000 samples of the air, water, meat, fruit, fish, and soil it was found that these elements were in such small amounts that they wouldn\’t be detectable in the human body. "According to a report by the U.S. Nuclear Regulatory Commission (NRC), the average dose of radiation received by the two million people in the surrounding area was 0.0014 rems.The highest estimated individual exposure resulting from the release was 0.075 rems.". A study was concluded in 1997 after an 18-year run to see the long term effects of the radiation because there was no abnormal behavior in the average number of cancer deaths.^{[6] [7]}

With new technology and safety regulations coming into play, we are now not as concerned with the possibilities of nuclear meltdowns as we are with the storage of the nuclear waste. Many possibilities were considered for the storage, but Congress finally settled on Yucca Mountain in Nevada. The waste will be stored in glass rods surrounded by a metal with low long term corrosion properties. It is apparently still unknown if this will be the final location for this project because of the questionable stability of the Yucca Mountain. ^[8]

Along with the enviromental aftermath, the accident at Three Mile Island had significant political backlash detrimental to the further implementation of nuclear power plants. U.S. President Jimmy Carter ordered a full and thorough investigation of the accident at Three Mile Island. The initial report would most likely have shut down all nuclear plants in the nation, according to Admiral Hyman G. Rickover, a key figure in the development of nuclear power plants. The final release version was not as harsh. The Pennsylvania House of Representatives also conducted its own investigation. Their investigation focused more on response and evacuation methods. Ultimately this incident has caused about a 58% reduction in the capacity of all nuclear power plants, and to this day, there is only about a 50% acceptance of them by the general population. It was fortunate that it was only a near disaster however it brought light to the possibilities of using nuclear power plants. Since then, an enormous increase in safety has occurred. "Improved operating training, improvements in quality assurance, engineering, operational surveillance and emergency planning have been instituted." There are still 103 active nuclear power plants in the United States and these precautions and improvements are still utilized today. ^{[9] [10] [11]}

Today, TMI-2 has been fully decontaminated and decommisioned. The entire plant will be shut down when TMI-1 runs out its license. While this has been the most disasterous nuclear accident in the United States, Chernoybl remains as the worst accident in world history. If we contrast these two incidents, we can see that even though nuclear power in the United States had flaws, we were able to prevent an even larger catastrophe such as Chernoybl with the over-construction of nuclear power plants. With careful monitering and design, nuclear power can still be beneficial to the United States power infrastructure. With recent thoughts of alternative energies, nuclear power can still compete with oil, coal, and other natural resources, even after the events of Three Mile Island.

Footnotes

1. ^  NRC: Fact Sheet on the Accident at Three Mile Island, March 31, 2005 http://www.nrc.gov/reading-rm/doc-ollections/fact-sheets/3mile-isle.html, (accessed September 2005)
2. ^  Dickenson College: What Went Wrong? 2004 http://www.threemileisland.org/science/what_went_wrong/index.html, (accessed September 2005)
3. ^  Three Mile Island Alert homepage. http://www.tmia.com/, (accessed September 2005)
4. ^  The Virtual Nuclear Tourist: Pressurized Water Reactor, 2005 http://www.nucleartourist.com/type/pwr.htm, (accessed September 2005)
5. ^  Heath Physics, Dr. Philip C. Eastman, 1992, pg 786-800
6. ^  Uranium Information Centre: Three Mile Island: 1979, 2005 http://www.uic.com.au/nip48.htm, (accessed September 2005)
7. ^  Wikipedia: Three Mile Island, September 24, 2005 http://en.wikipedia.org/wiki/Three_Mile_Island, (accessed Septmeber 2005)
8. ^  NRC: Fact Sheet on the Accident at Three Mile Island, March 31, 2005 http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html, (accessed September 2005)
9. ^  Milloy, Steven J. Three Mile Island, 1996 http://www.junkscience.com/news/three-mile-island.html (acccessed Septemeber 2005)
10. ^  TMI Nuclear Incident, 1997 http://home.paonline.com/hurst/tmi/TMI.HTM (accessed September 2005)
11. ^  Gonyeau, Joseph 1996, 2005 http://www.nucleartourist.com/basics/environ1.htm (accessed September 2005)

Researched and written by:

• Jeff Anderson
• Margaret Anderson
• Matt Boylan
• Stephanie Howie