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Three Mile Island accident

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Three Mile Island Nuclear Generating Station consists of two nuclear
   power plants, each with its own containment building and cooling
   towers. TMI-2, which suffered a partial meltdown, is in the background.
   Enlarge
   Three Mile Island Nuclear Generating Station consists of two nuclear
   power plants, each with its own containment building and cooling
   towers. TMI-2, which suffered a partial meltdown, is in the background.

   Three Mile Island Nuclear Generating Station sits on an island of area
   of 3.29 km² (814 acres) in the Susquehanna River in Dauphin County,
   Pennsylvania, near Harrisburg. On March 28, 1979, the Unit 2 nuclear
   power plant (a pressurized water reactor manufactured by Babcock &
   Wilcox) on Three Mile Island suffered a partial core meltdown. This was
   the worst accident in US commercial nuclear power generating history.
   Approximately 25,000 people lived within five miles of the island at
   the time of the accident.

   The accident unfolded over the course of five tense days, as a number
   of agencies at local, state and federal level tried to diagnose the
   problem and decide whether or not the on-going accident required a full
   emergency evacuation of the population. The full details of the
   accident were not discovered until much later. In the end, the reactor
   was brought under control. No identifiable injuries due to radiation
   occurred, and a government report concluded that "the projected number
   of excess fatal cancers due to the accident... is approximately one".
   But the accident had serious economic and public relations
   consequences, and the cleanup process was slow and costly. It also
   furthered a major decline in the public popularity of nuclear power,
   exemplifying for many the worst fears about nuclear technology and,
   until the Chernobyl accident seven years later, it was considered the
   world's worst civilian nuclear accident.

Three Mile Island nuclear accident

   The accident occurred in the TMI-2 reactor. The plant had two reactors;
   TMI-1 was at the time shut down for refueling. The accident began when
   the plant's main feedwater pumps in the secondary non-nuclear cooling
   system failed at exactly 4:00 a.m. on March 28, 1979. This was
   "to-the-minute" the first anniversary of start up. The exact cause of
   the failure was never ascertained, although it was speculated that
   water entered a pneumatic airline that controlled the flow of water
   through a water filter known as the condensate polishers. In fact, the
   NRC stated it did not need to know how the accident started since it
   could have been prevented at many points along the way. The stoppage of
   water flow through the condensate system caused the steam generators to
   cease removing heat. First the turbine, then the nuclear reactor
   automatically shut down. Immediately, the pressure in the primary
   system, the nuclear portion of the plant, began to increase.

   In order to prevent that pressure from becoming excessive, the
   pressurizer relief valve, located at the top of the pressurizer,
   opened. The valve should have closed again when the excess pressure had
   been released but it did not do so. The indication to the plant's
   operators that the signal to close the valve had been sent was, in the
   absence of any indication to the contrary, taken by them to mean that
   the valve had closed. (A "positive feedback" lamp in the control room
   indicating the true position of the valve was eliminated in original
   construction to save time but has now been backfitted to all other
   similar plants.) As a result of this design error, the valve allowed to
   remain open and caused the pressure to continue to decrease in the
   system.

   The operators did not initially recognize the accident as a Loss of
   Coolant Accident (LOCA) since they had only ambiguous reactor water
   level indication and failed to properly interpret other indications of
   a LOCA.

   Meanwhile, another problem appeared elsewhere in the plant with the
   emergency feedwater system, the main feedwater system's backup. Three
   emergency feedwater pumps started automatically following the loss of
   the main feed pumps, but two valves on the emergency feedwater lines
   were shut, preventing the feedwater from reaching the steam generators.
   The emergency feedwater system had been tested 42 hours prior to the
   accident. As part of the test, these valves were closed but should then
   have been reopened at the end of the test. But on this occasion it
   appeared that through either an administrative or human error, the
   valves were not reopened. The President's Commission requested that the
   FBI investigate whether these valves were closed by an act of sabotage.
   The valves were discovered closed about eight minutes into the
   accident. Once they were reopened, emergency feedwater was restored to
   the steam generators. This lack of emergency feed water for eight
   minutes did not have a significant effect on the outcome of the
   accident but did add to the confusion faced by the operators. The
   restoration of feedwater did not return cooling capability, however, as
   steam voids that had formed in the primary loop prevented heat transfer
   from the reactor to the secondary loop via the steam generator. This
   problem hampered efforts to cool the reactor for days.

   As the pressure in the primary system continued to decrease, voids,
   areas where there is no water present, began to form in portions of the
   system other than the pressurizer, most notably in the reactor vessel.
   Because of these voids, the water in the system was redistributed and
   the pressurizer water level rose while overall system water inventory
   decreased. Thus the pressurizer level indicator, which tells the
   operator the amount of coolant capable of heat removal, incorrectly
   indicated that system water level was rising. This caused the operators
   to stop adding water by turning off the Emergency Core Cooling pumps,
   which had automatically started following the initial pressure
   decrease, due to fears the system was being overfilled. They were
   unaware that, because of the voids forming in the reactor vessel, the
   indicator could, and in this instance did, provide false readings. This
   erroneous indication blinded operators to the fact that water level was
   dropping and the reactor core was becoming uncovered.

   With the PORV still open, the quench tank that collected the discharge
   from the PORV overfilled, causing the containment building sump to fill
   and sound an alarm at 4:11 a.m. This alarm, along with higher than
   normal temperatures on the PORV discharge line and unusually high
   containment building temperatures and pressures, were clear indications
   that the pressurizer relief valve was stuck open, but these indications
   were initially ignored by operators. At 4:15 a.m. the quench tank
   relief diaphragm ruptured, and radioactive coolant began to leak out
   into the general containment building. This radioactive coolant was
   pumped from the containment building sump to an auxiliary building,
   outside the main containment, until the sump pumps were stopped at 4:39
   AM.
   Simplified Schematic Diagram of the TMI-2 plant, from the NRC Fact
   Sheet.
   Enlarge
   Simplified Schematic Diagram of the TMI-2 plant, from the NRC Fact
   Sheet.

   After almost 80 minutes of slow temperature rise, the primary loop
   pumps began to cavitate as steam rather than water began to pass
   through them. The pumps were shut down, and it was believed that
   natural circulation would continue the water movement. Steam in the
   system locked the primary loop, and as the water stopped circulating it
   was converted to steam in increasing amounts. About 130 minutes after
   the first malfunction, the top of the reactor core was exposed and the
   intense heat caused a reaction to occur between the steam forming in
   the reactor core and the zirconium nuclear fuel rod cladding. This
   reaction damaged the nuclear fuel rod cladding, which released more
   radioactivity to the reactor coolant, and produced hydrogen gas which
   is believed to have caused a small explosion in the containment
   building later that afternoon. At 6 a.m. there was a shift change in
   the control room. A new arrival noticed that the temperature in the
   holding tanks was excessive and used a backup valve called a block
   valve to shut off the coolant venting, but around 950 m³ (250,000 US
   gallons) of coolant had already leaked from the primary loop. It was
   not until 165 minutes after the start of the problem that radiation
   alarms activated as contaminated water reached detectors — by that time
   the radiation levels in the primary coolant water were around 300 times
   expected levels, and the plant was seriously contaminated.

   At 7:00 a.m. a "Site Area Emergency" was declared and at 7:24 a.m. the
   incident was upgraded to a "General Emergency". Harrisburg radio
   station WKBO announced a problem with the plant at 8:25 a.m. and the
   Associated Press announced the general emergency at 9:00 a.m.

   It was still not clear to the control room staff that the primary loop
   water levels were low and that over half of the core was exposed: a
   Loss Of Coolant Accident. A group of workers took manual readings from
   the thermocouples and obtained a sample of primary loop water. Around
   seven hours into the emergency, new water was pumped into the primary
   loop. The backup relief valve was opened to reduce pressure. At around
   nine hours unnoticed. After almost sixteen hours the primary loop pumps
   were turned back on and the core temperature began to fall. A large
   part of the core had melted, and the system was still dangerously
   radioactive. Over the next week the steam and hydrogen were removed
   from the reactor using a recombiner and, more controversially, by
   venting straight to the atmosphere. It is estimated that a maximum of
   13 million curies (480 petabecquerels) of radioactive noble gases were
   released by the event, though very little of the hazardous iodine-131
   was released.

   The molten fuel did not break through in a " China Syndrome" (see
   below). "Despite melting of about one-third of the fuel, the reactor
   vessel itself maintained its integrity and contained the damaged fuel."

Aftermath

   The full damage to the reactor core was not known for many years after
   the accident.
   Enlarge
   The full damage to the reactor core was not known for many years after
   the accident.

   The scientific community continues to be split into two camps over the
   aftermath of the Three Mile Island Accident. One camp believes that no
   member of the public was injured by the accident. "The average
   radiation dose to people living within ten miles of the plant was eight
   millirem, and no more than 100 millirem to any single individual. Eight
   millirem is about equal to a chest X-ray, and 100 millirem is about a
   third of the average background level of radiation received by US
   residents in a year."

   The other focus cites evidence observed by locals of a massive
   radiation release from the plant . However, "to this day [late 2004],
   Wing's article remains the only one to present original health data
   supporting a link between Three Mile Island radiation exposure and
   cancer." A baby teeth study is to be done. This camp also notes that
   hundreds of out-of-court settlements have been reached with victims.

   Three Mile Island has been of interest to human factors engineers as an
   example of how groups of people react and make decisions under stress.
   There is consensus that the accident was exacerbated by wrong decisions
   made because the operators were overwhelmed with information, much of
   it irrelevant, misleading or incorrect. As a result of the TMI-2
   incident, nuclear reactor operator training has been improved. Before
   the incident it focused on diagnosing the underlying problem;
   afterwards, it focused on reacting to the emergency by going through a
   standardized checklist to ensure that the core is receiving enough
   coolant under sufficient pressure.

   In addition to the improved operating training, improvements in quality
   assurance, engineering, operational surveillance and emergency planning
   have been instituted. Improvements in control room habitability, "sight
   lines" to instruments, ambiguous indications and even the placement of
   "trouble" tags were made; some trouble tags were covering important
   instrument indications during the accident. Improved surveillance of
   critical systems, structures and components required for cooling the
   plant and mitigating the escape of radionuclides during an emergency
   were also implemented. In addition, each nuclear site must now have an
   approved emergency plan to direct the evacuation of the public within a
   ten mile Emergency Planning Zone (EPZ) and to facilitate rapid
   notification and evacuation. This plan is periodically rehearsed with
   federal and local authorities to ensure that all groups work together
   quickly and efficiently.

   US President Jimmy Carter ordered a full investigation of the TMI
   incident. The Pennsylvania House of Representatives conducted its own
   investigation, which focused on the need to improve evacuation
   procedures. House investigators also visited the Three Mile Island
   site, including the Control Room. Many of the instruments viewed were
   marked with "error tags," explaining how the instrument was supposed to
   work, and how it actually worked. A member of the investigating
   committee, State Rep. Mark B. Cohen of Philadelphia, said it would be
   "virtually impossible" for any Control Room operator to keep track of
   the many variations between the equipment's intended and actual
   functioning.

   There was also the psychological effect on the nation. Before the
   accident approximately seventy percent of the general public approved
   of nuclear power. After it, support for nuclear power across the
   country fell to about fifty percent, where it remained for decades.
   Recently public support for nuclear power has been on the rise, and the
   George W. Bush administration has been particularly supportive of
   nuclear power, encouraging power companies to begin considering plans
   for the first new nuclear reactors in the United States in decades.

   The 1979 TMI accident did not, however, initiate the demise of the US
   nuclear power industry. As a result of post-oil-shock analysis and
   conclusions of overcapacity, 40 planned nuclear power plants had
   already been canceled between 1973 and 1979. No US nuclear power plant
   had been authorized to begin construction since the year before TMI.
   Nonetheless, TMI added a clearly strong impact on this demise: of 129
   plants approved at the time of TMI just 53 of those not already
   operating were ever completed. Federal requirements became more
   stringent, local opposition became more strident and construction times
   were vastly lengthened. The two shortest nuclear power plant
   construction projects were in this same volatile period, River Bend and
   St. Lucie-2.
   Viewed from the west, Three Mile Island currently uses only one nuclear
   generating station, TMI-1, which is on the left. TMI-2, to the right,
   has not been used since the accident.
   Enlarge
   Viewed from the west, Three Mile Island currently uses only one nuclear
   generating station, TMI-1, which is on the left. TMI-2, to the right,
   has not been used since the accident.

   The TMI cleanup started in August 1979 and officially ended in December
   1993, having cost around US$975 million. From 1985 to 1990 almost 100
   tons of radioactive fuel were removed from the site. However, the
   contaminated cooling water that leaked into the containment building
   had seeped into the building's concrete, leaving the radioactive
   residue impossible to remove. TMI-2 had been online only three months,
   but now had a ruined reactor vessel and a containment building that was
   unsafe to walk in — it has since been permanently closed. Many similar
   Babcock and Wilcox reactors on order were canceled — in total, 51
   American nuclear reactors were canceled from 1980 to 1984.

   In the end, a few simple water level gauges on the reactor vessel might
   have prevented the accident. The operators' focus on a single
   misleading indication, the level in the pressurizer, was a significant
   contributing factor to the meltdown.

   Unit 1, which had had its license temporarily suspended following the
   incident at Unit 2, was permitted to resume operations in 1985. General
   Public Utilities Corporation, the plant's owner, formed General Public
   Utilities Nuclear Corporation (GPUN) as a new subsidiary to own and
   operate the company's fleet of nuclear facilities, including Three Mile
   Island. The plant had previously been operated by Metropolitan Edison
   Company (Met-Ed), one of GPU's regional utility operating companies. In
   1996, General Public Utilities shortened its name to GPU Inc. Three
   Mile Island Unit 1 was sold to AmerGen Energy Corporation, a joint
   venture between Philadelphia Electric Company (PECO), and British
   Energy, in 1998. In 2000, PECO merged with Unicom Corporation to form
   Exelon Corporation, which acquired British Energy's share of AmerGen in
   2003, dissolving the company in the process. TMI Unit 1 is now owned
   and operated by Exelon Nuclear Inc., an Exelon Corp. subsidiary.

   Three Mile Island Unit 2 was too badly damaged and contaminated to
   resume operations. The reactor was gradually deactivated and mothballed
   in a lengthy process completed in 1993. Initially, efforts focused on
   the cleanup and decontamination of the site, especially the defueling
   of the damaged reactor. In 1988, the Nuclear Regulatory Commission
   announced that, although it was possible to further decontaminate the
   Unit 2 site, the remaining radioactivity had been sufficiently
   contained as to pose no threat to public health and safety.
   Accordingly, further cleanup efforts were deemed unnecessary. The
   defueling process was completed in 1990, and the damaged reactor was
   removed and disposed of in 1993. General Public Utilities was legally
   obliged to continue to maintain and monitor the site, and therefore
   retained ownership of Unit 2 when Unit 1 was sold to AmerGen in 1998.
   GPU Inc. was acquired by First Energy Corporation in 2001, and
   subsequently dissolved. First Energy then contracted out the
   maintenance and administration of Unit 2 to AmerGen. Unit 2 has been
   administered by Exelon Nuclear since 2003, when Exelon Nuclear's parent
   company, Exelon, bought out the remaining shares of AmerGen, inheriting
   First Energy's maintenance contract.

The China Syndrome

   The accident at the plant occurred a few days after the release of the
   movie The China Syndrome, which featured Jane Fonda as a news anchor at
   a California TV station. In the film, a nuclear accident almost happens
   while Fonda's character and her cameraman are at a plant doing a series
   on nuclear power. She goes on to raise awareness of how unsafe the
   plant was. Coincidentally, there is a scene in which Fonda's character
   speaks with a nuclear safety expert who says that a meltdown could
   force an area "the size of Pennsylvania" to be evacuated. Also, the
   fictional near-accident in the movie stems from plant operators
   misunderstanding the amount of water within the core.

   Soon after the release of the film, Fonda began lobbying against
   nuclear power — the only actor in the film to do so. In an attempt to
   counter her efforts, the nuclear physicist Edward Teller, "father of
   the hydrogen bomb" and long-time government science advisor, himself
   lobbied in favour of nuclear power, and eventually the 71-year-old
   scientist suffered a heart attack, which he later blamed on Fonda: "You
   might say that I was the only one whose health was affected by that
   reactor near Harrisburg. No, that would be wrong. It was not the
   reactor. It was Jane Fonda. Reactors are not dangerous." Rhetoric based
   on the movie is still used to debate for and against nuclear power.

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